In the middle of the nineteenth century, chemists could weigh a compound, burn it, and work out how much carbon and hydrogen it held. What they could not say was how those atoms were joined. Two substances would give exactly the same analysis and yet smell different, boil at different temperatures and behave differently in a flask. Something about carbon was not being captured by the numbers.
In 1858 two chemists working separately arrived at the same answer. August Kekulé in Germany and Archibald Scott Couper in Scotland both proposed that a carbon atom has four points of attachment, and — this was the surprising part — that carbon atoms can link directly to one another, over and over, building chains. Suddenly the puzzle made sense. If four carbon atoms can be joined in a straight row or with one carbon branching off the side, then the same set of atoms can be put together in two different arrangements, giving two different substances.
Seven years later Kekulé went further and suggested that in benzene the six carbon atoms close up into a ring. Chemists began drawing structures rather than only counting atoms, and the study of carbon compounds became a subject of its own. Today more than ten million carbon compounds are known, and the number grows every year — all of them built on that one idea about how carbon joins.
Q1. The same four carbon atoms and ten hydrogen atoms can be built into two different substances. How can two compounds be made of exactly the same atoms and still not be the same substance? Q2. Carbon is not the only element whose atoms can join to one another, yet no other element forms chains anywhere near as long. What might make carbon–carbon linking so much more successful? Q3. Common salt melts at a very high temperature, while sugar chars and breaks down in a hot pan. Why should compounds that come from living things behave so differently from those dug out of rocks? Q4. Methane and ethane are hard to persuade into a reaction, but ethene and ethyne react readily. What is present in the second pair, and absent from the first, that could account for this? Q5. Wine left standing open begins to taste sour, like vinegar. What could the air be doing to the alcohol in it?
12.1 Organic Chemistry And Its Beginnings
12.1.1 The Word ‘Organic’ And The Vital Force Theory
The word ‘organic’ means pertaining to life. When chemistry was still a young subject, chemists noticed that certain substances came only from living things. Sugar, starch, protein and acetic acid were all obtained from plants and animals, and never from a rock or a mineral.
Because of where they came from, such substances were called organic compounds — the compounds derived from living organisms — and the branch of chemistry that dealt with them was named organic chemistry. Set against them was a second group of substances. Common salt, blue vitriol and nitrate could be obtained from minerals and other non-living sources, so these were called inorganic compounds, and the study of them was called inorganic chemistry.
That division looked reasonable at the time, but it left one awkward fact unexplained. Chemists could take an inorganic substance apart and put it together again in the laboratory, yet no one had managed to make an organic substance in the same way. Every organic compound then known had been taken straight from a plant or an animal.
The explanation offered was that living things possessed something a laboratory did not. According to the vital force theory, organic compounds could be produced only under the influence of a mysterious ‘vital force’ present in living organisms. On this view, no amount of skill with flasks and burners would ever produce them.
Vital Force The idea was not foolish for its time. Living matter behaves very differently from a mineral, and no one yet knew that the difference lies in how atoms are joined rather than in where the substance came from.
12.1.2 The Fall Of The Vital Force Theory
The vital force theory was discarded because of a single laboratory result. In 1828, the German chemist Friedrich Wöhler was heating ammonium cyanate, an inorganic compound, and found that it changed into urea — a compound that until then had been obtained only from animals.
Wöhler had not extracted urea from anything living. He had made it on a bench, from a starting material with no connection to a living organism. The change is written first in words:
ammonium cyanate → urea
\[\mathrm{NH_4CNO} \xrightarrow{\ \Delta\ } \mathrm{CO(NH_2)_2}\]
Here \(\Delta\) over the arrow simply means that the mixture is heated. Notice that nothing is added and nothing is driven off; the same atoms are present on both sides, rearranged into a different substance. That is why the equation needs no balancing coefficients.
Wöhler’s Letter Wöhler wrote to his former teacher, the Swedish chemist Jöns Jacob Berzelius, that he could now make urea without needing a kidney — his way of saying that the vital force was not required after all.
The Synthesis Continues
One result rarely settles a question by itself, and other chemists soon added to it. In 1845, Kolbe prepared acetic acid from its constituent elements — carbon, hydrogen and oxygen — and in 1856, Berthelot synthesized methane gas, \(\mathrm{CH_4}\).
Three substances once thought to need a living source had now been built in the laboratory. The old idea of organic chemistry was therefore given up. Chemists accepted instead that organic chemistry is essentially the chemistry of carbon compounds, and the origin of a substance ceased to matter.
‘Organic’ In Chemistry And ‘Organic’ In A Shop In a shop, ‘organic’ describes food grown without certain fertilisers or pesticides. In chemistry it has nothing to do with farming — it means a compound of carbon, whatever its source.
12.2 Organic Compounds And Their Sources
12.2.1 What Counts As An Organic Compound
Once the source of a substance stopped mattering, chemists needed a new test for membership. The test they settled on is the presence of carbon. An organic compound is a compound of carbon, and organic chemistry is the study of carbon compounds.
A few carbon-containing substances are kept out of the group by agreement, because they behave like minerals rather than like compounds of living matter. The oxides of carbon, the metallic carbonates, and related compounds such as the metal cyanides and metal carbides are all excluded.
Organic Compounds Organic compounds are the compounds of carbon, and organic chemistry is the study of carbon compounds, excluding oxides of carbon, metallic carbonates and related compounds like metal cyanides and metal carbides.
A Carbon Compound And A Carbonate Both contain carbon. Carbon dioxide and limestone (calcium carbonate) are counted as inorganic by agreement, because they behave like minerals. A carbon compound is organic only if it falls outside that excluded list.
12.2.2 Sources Of Organic Compounds
Organic compounds still come largely from living matter and from what living matter has turned into over long ages. Seven sources account for most of them.
- Plants : Compounds like sugar, starch and cellulose, as well as several drugs, are obtained from plants.
- Animals : Urea, proteins and fats are obtained from animals.
- Coal : Destructive distillation of coal produces benzene, toluene, naphthalene, dyes, drugs and perfumes.
- Petroleum : A large number of organic compounds, such as gasoline, fuel gases, petrol and naphtha, are obtained from petroleum.
- Fermentation : Compounds like ethyl alcohol and acetic acid are obtained by fermentation.
- Wood : Methyl alcohol and acetone are obtained by the destructive distillation of wood.
- Synthetic methods : Most organic compounds are now synthesized in the laboratory.
Two of these terms need a word of explanation. Destructive distillation means strongly heating a solid such as coal or wood out of contact with air; instead of burning, the solid breaks down and gives off a mixture of liquids and gases that can then be separated. Fermentation is the slow breaking down of a substance such as sugar by micro-organisms, which is how alcohol is obtained from grain and fruit.
Sort the following into organic and inorganic compounds, giving your reason in each case: cane sugar, sodium carbonate, cellulose, carbon dioxide, methyl alcohol, potassium cyanide.
12.2.3 Organic Compounds Compared With Inorganic Compounds
Organic and inorganic compounds do not merely come from different places. They differ in almost every property a student can test in a laboratory — how they dissolve, how they melt, whether they burn, and whether they conduct electricity. Table 12.1 sets the two side by side.
Table 12.1 — Organic Compounds Compared With Inorganic Compounds
| Presence of carbon |
Carbon is a necessary element in every organic compound. |
Carbon is not an essential element in inorganic compounds. |
| Solubility in water |
They generally do not dissolve in water. |
They generally dissolve in water. |
| Solubility in organic solvents |
They dissolve in organic solvents like alcohol, benzene and chloroform. |
They generally do not dissolve in organic solvents. |
| Melting and boiling point |
They have low melting and boiling points, and decompose easily on heating. |
They have high melting and boiling points, and usually do not decompose on heating. |
| Combustibility |
They are inflammable, that is, they catch fire easily. |
They do not burn easily. |
| Bonding |
They form covalent bonds. |
Most of them form ionic bonds. |
| Conductivity |
They are non-electrolytes. |
Only those that form ionic bonds are good electrolytes. |
| Isomerism |
They show the phenomenon of isomerism. |
No such phenomenon is shown by inorganic compounds, except some covalent and coordinate compounds. |
| Colour and smell |
Organic compounds have characteristic colour and odour. |
Most of them are colourless and odourless. |
| Reactions |
Molecular reactions are slow, because whole molecules must be broken apart, and often do not go to completion. |
Ionic reactions are fast; covalent reactions are slow. |
Read down the table and one cause runs through it. Organic compounds are built from covalent bonds, so their molecules are held to one another only weakly and separate at low temperatures. Most inorganic compounds are built from ions, which grip each other strongly in a lattice and need great heat to be pulled apart.
The same difference explains the last two rows. Because an ionic compound is already made of ions, its reactions need only bring existing ions together and are over quickly. In an organic reaction, strong covalent bonds inside a molecule must first be broken, which takes time.
Q3. Common salt melts at a very high temperature, while sugar chars and breaks down in a hot pan. Why should compounds that come from living things behave so differently from those dug out of rocks?
A. Sugar is a covalent, organic compound: its molecules pull on each other weakly, so it softens at a low temperature, and the molecule itself falls apart on further heating, leaving carbon behind as the black char. Common salt is ionic, and its ions hold one another so strongly that a very high temperature is needed even to melt it.
12.2.4 Organic Chemistry In Daily Life
Organic compounds are not laboratory curiosities. The soaps and shampoos used while bathing, the powders and perfumes applied to the body, and the clothes worn over it are all organic. So is the food eaten — carbohydrates, proteins, fats and vitamins.
The list runs on through the fuels used at home and in vehicles, natural gas, petroleum products, medicines, explosives, dyes and insecticides. There is hardly any walk of life in which organic compounds are not needed, and that is a large part of why carbon deserves a branch of chemistry to itself.
12.3 The Unique Nature Of The Carbon Atom
Carbon is one element among more than a hundred, yet its compounds outnumber those of all the other elements put together. That is not an accident of history. It follows from two properties of the carbon atom itself — tetravalency and catenation — and from a third consequence of them, isomerism.
12.3.1 Tetravalency Of Carbon
The atomic number of carbon is 6, so its electron arrangement is 2, 4. There are four electrons in the outermost shell, and eight would be needed for a stable octet.
Carbon cannot reach that octet by losing four electrons or by gaining four, since either would leave a highly charged ion. Instead it shares each of its four outer electrons with another atom. Each shared pair is a covalent bond, so a carbon atom forms four covalent bonds, and this is why carbon is described as tetravalent.
Tetravalency Of Carbon The characteristic of the carbon atom by virtue of which it forms four covalent bonds is called the tetravalency of carbon.
In a diagram, the four outer electrons of carbon may be drawn as four crosses around the symbol, or the four bonds may simply be drawn as four short lines. Fig. 12.3-1 shows both, and the two say the same thing.
Four Bonds And Four Neighbours Tetravalency means four bonds, not four attached atoms. In \(\mathrm{CH_2=CH_2}\) each carbon is joined to only three atoms, but the double bond uses two of its valencies, so all four are still satisfied.
12.3.2 Catenation
A carbon atom can use one of its four bonds to hold another carbon atom, which in turn holds a third, and so on. Carbon atoms therefore link together into very long chains, and this self-linking is called catenation.
Catenation The property of self-linking of atoms of an element through covalent bonds, in order to form straight chains, branched chains and cyclic chains of different sizes, is known as catenation.
Catenation is shown by other elements as well, but carbon exhibits it to the maximum extent. There are two reasons. The carbon–carbon bond is unusually strong, so a long chain of them does not fall apart; and because carbon is tetracovalent, each carbon in a chain still has spare valencies for hydrogen or for other atoms.
Q2. Carbon is not the only element whose atoms can join to one another, yet no other element forms chains anywhere near as long. What might make carbon–carbon linking so much more successful?
A. The carbon–carbon bond is far stronger than the corresponding bond in other elements, so a long chain of carbon atoms holds together instead of breaking up. Carbon is also tetracovalent, so every atom in the chain keeps two spare bonds for hydrogen or other atoms.
Silicon Tries Silicon lies directly below carbon and is also tetravalent, so it catenates too. But the silicon–silicon bond is much weaker, so its chains break up after a few atoms and never reach the lengths carbon manages.
Straight, Branched And Cyclic Chains
The combination of carbon atoms with one another gives rise to straight chains, to branched chains, and to cyclic or closed chains. In a straight chain the carbon atoms run one after another in a single line. In a branched chain, a carbon atom part-way along the line carries a further carbon on a side arm. In a cyclic chain the two ends of the chain join up, closing the ring. The three arrangements are shown in Fig. 12.3-2.
12.3.3 Single, Double And Triple Covalent Bonds
Two carbon atoms need not share only one pair of electrons. They may share one, two or three pairs, giving a single bond, a double bond or a triple bond respectively. Each shared pair uses up one valency on each of the two atoms.
Count the valencies and the pattern is clear. In \(\mathrm{-C-C-}\) each carbon uses one bond for its neighbour and has three left over. In \(\mathrm{C=C}\) each uses two and has two left; in \(\mathrm{-C\equiv C-}\) each uses three and has one left.
Carbon forms multiple bonds with other elements as well, not only with itself. A carbon atom can be joined to oxygen by a double bond, as in \(\mathrm{>C=O}\), or to nitrogen by a triple bond, as in \(\mathrm{-C\equiv N}\). Fig. 12.3-3 collects these together.
12.3.4 Isomerism And The Large Number Of Carbon Compounds
The four valencies of carbon point away from each other into the corners of a tetrahedron rather than lying flat. Because of this three-dimensional arrangement, and because chains may be straight or branched, the same set of atoms can often be assembled in more than one way.
Two or more compounds of carbon can therefore have the same molecular formula while differing in structure — that is, in the arrangement of their bonds. Such compounds are called isomers, and the phenomenon is called isomerism. Glucose and fructose are a familiar pair: both have the formula \(\mathrm{C_6H_{12}O_6}\), yet they are different substances with different structures.
Put the three properties together — catenation, tetravalency, and the capacity to form isomers — and the result is an enormous family of compounds. More than 10 million organic compounds are known today, and thousands of new ones are added every year.
A chain of five carbon atoms is joined by single bonds only. How many hydrogen atoms are needed to satisfy every remaining valency? Show your reasoning by counting the spare bonds on each carbon.
12.4 Types Of Organic Compounds
Ten million compounds cannot be studied one at a time, so they are sorted into types. The main types of organic compounds are the hydrocarbons and their derivatives — alcohols, aldehydes, carboxylic acids, ethers, halides and others.
A hydrocarbon holds carbon and hydrogen only. Its derivatives are formed when some hydrogen is replaced by a small group of atoms, and these compounds may contain any number of other elements — hydrogen, nitrogen, oxygen, the halogens, and also phosphorus, silicon and sulphur.
What decides the type is the group of atoms a molecule carries. Every alcohol carries \(\mathrm{-OH}\), every carboxylic acid carries \(\mathrm{-COOH}\), and a chemist reading a formula looks for that group first. Table 12.2 sets out the common types with one example of each.
| Hydrocarbon — alkane |
\(\mathrm{-C-C-}\) bond |
Ethane, \(\mathrm{H_3C-CH_3}\) |
| Hydrocarbon — alkene |
\(\mathrm{-C=C-}\) bond |
Ethene, \(\mathrm{H_2C=CH_2}\) |
| Hydrocarbon — alkyne |
\(\mathrm{-C\equiv C-}\) bond |
Ethyne, \(\mathrm{HC\equiv CH}\) |
| Alcohol |
\(\mathrm{-OH}\) group |
Methanol, \(\mathrm{CH_3OH}\) |
| Aldehyde |
\(\mathrm{-CHO}\) group |
Ethanal, \(\mathrm{CH_3CHO}\) |
| Ketone |
\(\mathrm{>C=O}\) group |
Propanone, \(\mathrm{CH_3COCH_3}\) |
| Carboxylic acid |
\(\mathrm{-COOH}\) group |
Ethanoic acid, \(\mathrm{CH_3COOH}\) |
| Ether |
\(\mathrm{C-O-C}\) group |
Dimethyl ether, \(\mathrm{H_3C-O-CH_3}\) |
| Halide |
\(\mathrm{-X}\) (F, Cl, Br, I) |
Chloroethane, \(\mathrm{C_2H_5Cl}\) |
Table 12.2 : Types of organic compounds and the groups that identify them
Notice that the first three types are told apart by a bond rather than by an added group. An alkane, an alkene and an alkyne all contain carbon and hydrogen only; what separates them is whether the carbon atoms are joined by a single, a double or a triple bond.
Why X? In organic chemistry the letter X is used as a stand-in for any one of the halogens — fluorine, chlorine, bromine or iodine. Writing \(\mathrm{-X}\) saves stating the same rule four times over.
Aldehyde And Ketone Both carry a carbon joined to oxygen by a double bond. In an aldehyde that carbon sits at the end of the chain and still holds a hydrogen, giving \(\mathrm{-CHO}\). In a ketone it sits within the chain, with carbon on both sides.
12.5 Hydrocarbons And Their Classification
Of all the organic compounds, the simplest are those built from two elements only. These are the hydrocarbons, and every other organic compound can be regarded as one of them with something attached in place of a hydrogen atom. For that reason organic chemistry may be defined as the chemistry of hydrocarbons and their derivatives.
Hydrocarbons Hydrocarbons are compounds that are made up only of carbon and hydrogen atoms.
Hydrocarbons are divided into two main groups according to the shape of their carbon skeleton. In aliphatic or open chain compounds the carbon chain has two ends. In cyclic or closed chain compounds the chain has joined up into a ring. The open chain compounds are then sub-divided further, as set out in FC 12.1.
12.5.1 Saturated Compounds
The simplest open chain hydrocarbon is the alkane. Alkanes are represented by the general formula \(\mathrm{C_nH_{2n+2}}\), where \(n\) is a natural number — put \(n = 1\) and the formula gives \(\mathrm{CH_4}\), put \(n = 2\) and it gives \(\mathrm{C_2H_6}\).
An alkane is described as a saturated hydrocarbon because all four valencies of every carbon atom are satisfied by single covalent bonds. There is no room left on the molecule for anything further to be attached; it is full, or saturated.
12.5.2 Unsaturated Compounds
The hydrocarbons that have double or triple covalent bonds between two adjacent carbon atoms are called unsaturated hydrocarbons. They fall into two families. Alkenes are the hydrocarbons with a double bond, and alkynes are the hydrocarbons with a triple bond between two carbon atoms.
The word ‘unsaturated’ is worth taking literally. It means that more hydrogen, or some other atom, can still be added to the hydrocarbon to make it saturated — that is, to leave it with single bonds only. Table 12.3 sets the two kinds side by side.
Table 12.3 — Saturated Compared With Unsaturated Hydrocarbons
| Valencies of carbon |
All four valencies of each carbon atom are satisfied by forming single covalent bonds with carbon and with hydrogen atoms. |
The valencies of at least two carbon atoms are not fully satisfied by hydrogen atoms. |
| Bond between carbon atoms |
Carbon atoms are joined only by a single covalent bond, \(\mathrm{-C-C-}\) |
Carbon atoms are joined by a double covalent bond, \(\mathrm{>C=C<}\), or by a triple covalent bond, \(\mathrm{-C\equiv C-}\) |
| Reactivity |
Less reactive, because the electrons in a single covalent bond are not available; they therefore undergo substitution reactions. |
More reactive, because of the electrons present in the double or the triple bond; they therefore undergo addition reactions. |
Q4. Methane and ethane are hard to persuade into a reaction, but ethene and ethyne react readily. What is present in the second pair, and absent from the first, that could account for this?
A. Ethene and ethyne carry a double and a triple bond, and the extra electrons in those bonds are available to an attacking reagent. In methane and ethane every electron is locked into a single bond, so there is nothing loosely held for a reagent to seize.
12.5.3 Substitution And Addition Reactions
Those last two rows of Table 12.3 name two kinds of reaction, and both are worth defining now because they run through the rest of this chapter.
Substitution Reaction A reaction in which one atom of a molecule is replaced by another atom, or by a group of atoms, is called a substitution reaction.
For example, when methane reacts with chlorine, one hydrogen atom of the methane molecule is replaced by a chlorine atom. In words: methane + chlorine → chloromethane + hydrogen chloride.
\[\mathrm{CH_4} + \mathrm{Cl_2} \rightarrow \mathrm{CH_3Cl} + \mathrm{HCl}\]
Addition Reaction A reaction involving the addition of atoms or molecules to the double or the triple bond of an unsaturated compound, so as to yield a saturated product, is known as an addition reaction.
For example, ethene takes up bromine across its double bond. In words: ethene + bromine → 1,2-dibromoethane.
\[\mathrm{C_2H_4} + \mathrm{Br_2} \rightarrow \mathrm{C_2H_4Br_2}\]
Compare the two equations and the difference is plain. In substitution something leaves the molecule as well as joining it, so a second product — here hydrogen chloride — is always formed. In addition nothing leaves, and there is only one product.
Placing A Hydrocarbon From Its Molecular Formula
1. A hydrocarbon has the molecular formula \(\mathrm{C_4H_6}\). To which family does it belong, and would you expect it to undergo substitution or addition?
Here \(n = 4\), since there are four carbon atoms. Test the three general formulae in turn. For an alkane, \(\mathrm{C_nH_{2n+2}}\) gives \(\mathrm{C_4H_{10}}\). For an alkene, \(\mathrm{C_nH_{2n}}\) gives \(\mathrm{C_4H_8}\). For an alkyne, \(\mathrm{C_nH_{2n-2}}\) gives \(\mathrm{C_4H_6}\).
Only the alkyne formula matches. The compound is an alkyne, so it carries a triple bond, is unsaturated, and will undergo addition reactions.
Classify each of the following as an alkane, an alkene or an alkyne, and state in each case whether the compound is saturated or unsaturated: \(\mathrm{C_3H_8}\), \(\mathrm{C_5H_{10}}\), \(\mathrm{C_3H_4}\), \(\mathrm{C_6H_{14}}\).
12.5.4 Cyclic Compounds: Alicyclic And Aromatic
Cyclic or closed chain compounds contain three or more carbon atoms in their molecules, joined into a ring. Their properties are similar to those of the open chain hydrocarbons, and because the ring itself is built of carbon they are also called carbocyclic compounds. They are of two types.
Alicyclic compounds are ringed carbon compounds containing three or more atoms of carbon arranged in a closed structure. Cyclopropane, \(\mathrm{C_3H_6}\), has a three-carbon ring; cyclopentane, \(\mathrm{C_5H_{10}}\), has a ring of five; and cyclohexene, \(\mathrm{C_6H_{10}}\), has a ring of six carbon atoms carrying one double bond. These are drawn in Fig. 12.5-1.
Aromatic compounds are those whose molecules contain at least one benzene ring. The benzene ring is a specific type of ring structure of six carbon atoms having carbon–carbon single and carbon–carbon double bonds in alternate positions, with one hydrogen atom on each carbon.
Most of these compounds have a pleasant smell, and it is from this that they take their name. Toluene, phenol and naphthalene are further members of the class; all three are shown with benzene in Fig. 12.5-2.
‘Aromatic’ In Chemistry The first aromatic compounds found did smell pleasant, and the name stuck. Today a compound is called aromatic because it contains a benzene ring, not because of its smell — some aromatic compounds smell unpleasant, and some sweet-smelling compounds are not aromatic at all.
12.7 The Alkyl Group
Chains rarely stand alone. In most organic compounds a chain of carbon atoms is attached to something else — to a group of atoms, or to a second chain — and for that a chain with one free bond is needed. Such a fragment is called an alkyl group.
An alkyl group is obtained by removing one atom of hydrogen from an alkane molecule. Since an alkane has the general formula \(\mathrm{C_nH_{2n+2}}\), removing one hydrogen leaves one hydrogen less:
\[\mathrm{C_nH_{2n+2}} \xrightarrow{\ -\mathrm{H}\ } \mathrm{-C_nH_{2n+1}}\]
So the general formula of an alkyl group is \(\mathrm{-C_nH_{2n+1}}\). Removing a hydrogen atom from methane, for instance, leaves the methyl group.
\[\mathrm{CH_4} \xrightarrow{\ -\mathrm{H}\ } \mathrm{-CH_3}\]
An alkyl group is named by replacing the suffix -ane of the alkane with the suffix -yl. Methane gives methyl, ethane gives ethyl, propane gives propyl and butane gives butyl. Table 12.4 lists the first four.
| Methane, \(\mathrm{CH_4}\) |
Methyl |
\(\mathrm{-CH_3}\) |
\(\mathrm{-CH_3}\) |
| Ethane, \(\mathrm{C_2H_6}\) |
Ethyl |
\(\mathrm{-C_2H_5}\) |
\(\mathrm{-CH_2-CH_3}\) |
| Propane, \(\mathrm{C_3H_8}\) |
Propyl |
\(\mathrm{-C_3H_7}\) |
\(\mathrm{-CH_2-CH_2-CH_3}\) |
| Butane, \(\mathrm{C_4H_{10}}\) |
Butyl |
\(\mathrm{-C_4H_9}\) |
\(\mathrm{-CH_2-CH_2-CH_2-CH_3}\) |
Table 12.4 : The first four alkyl groups, each derived from its alkane
Notice the short line written in front of every alkyl formula. When a hydrogen atom is taken away, the electron it was sharing is left behind, so a member of an alkyl group has an unpaired electron. That unpaired electron is indicated by a line with no symbol attached, and it is the bond by which the group joins to the rest of a molecule.
Alkane And Alkyl An alkane is a complete molecule that can be bottled: methane is \(\mathrm{CH_4}\). An alkyl group is a fragment with one bond left free: methyl is \(\mathrm{-CH_3}\). The free line is the whole difference.
Pentane has the molecular formula \(\mathrm{C_5H_{12}}\). Write the name and the molecular formula of the alkyl group obtained from it, and check your answer against the general formula \(\mathrm{-C_nH_{2n+1}}\).
12.8 Functional Groups
Methanol, ethanol and propanol have chains of different lengths, yet all three burn in the same way, all three react with sodium in the same way, and all three smell recognisably alike. Something other than the chain is deciding their chemistry, and that something is the \(\mathrm{-OH}\) group each of them carries.
Functional Group The functional group is defined as an atom, or a group of atoms joined in a specific manner, which is responsible for the characteristic chemical properties of the organic compound.
The hydroxyl or alcohol group \(\mathrm{-OH}\), the formyl or aldehyde group \(\mathrm{-CHO}\), and the carboxyl or carboxylic acid group \(\mathrm{-COOH}\) are three familiar examples. Alkanes do not contain any functional group at all, which is one reason they are so unreactive.
In the unsaturated hydrocarbons the multiple bond itself does the work of a functional group. The functional group of alkenes is considered to be the double bond \(\mathrm{>C=C<}\), while in alkynes it is the triple bond \(\mathrm{-C\equiv C-}\). The common functional groups are collected in Table 12.5, where R stands for an alkyl group, \(\mathrm{-C_nH_{2n+1}}\).
| Halide, \(\mathrm{-X}\) (F, Cl, Br, I) |
\(\mathrm{R-X}\) |
Haloalkanes |
-ane |
\(\mathrm{CH_3Cl}\), methyl chloride / chloromethane; \(\mathrm{C_2H_5Cl}\), ethyl chloride / chloroethane |
| Double bond, \(\mathrm{>C=C<}\) |
\(\mathrm{C_nH_{2n}}\) |
Alkenes |
-ene |
\(\mathrm{H_2C=CH_2}\), ethylene / ethene; \(\mathrm{H_2C=CH-CH_3}\), propylene / propene |
| Triple bond, \(\mathrm{-C\equiv C-}\) |
\(\mathrm{C_nH_{2n-2}}\) |
Alkynes |
-yne |
\(\mathrm{HC\equiv CH}\), acetylene / ethyne; \(\mathrm{HC\equiv C-CH_3}\), methyl acetylene / propyne |
| Hydroxyl, \(\mathrm{-OH}\) |
\(\mathrm{R-OH}\) |
Alcohols |
-ol |
\(\mathrm{CH_3OH}\), methyl alcohol / methanol; \(\mathrm{C_2H_5OH}\), ethyl alcohol / ethanol |
| Aldehyde, \(\mathrm{-CHO}\) |
\(\mathrm{R-CHO}\) |
Aldehydes |
-al |
\(\mathrm{HCHO}\), formaldehyde / methanal; \(\mathrm{CH_3CHO}\), acetaldehyde / ethanal |
| Carboxyl, \(\mathrm{-COOH}\) |
\(\mathrm{R-COOH}\) |
Carboxylic acids |
-oic acid |
\(\mathrm{CH_3COOH}\), acetic acid / ethanoic acid; \(\mathrm{CH_3CH_2COOH}\), propionic acid / propanoic acid |
| Keto, \(\mathrm{>C=O}\) |
\(\mathrm{R-CO-R'}\) |
Ketones |
-one |
\(\mathrm{CH_3COCH_3}\), acetone / propanone; \(\mathrm{C_2H_5COC_2H_5}\), diethyl ketone / pentan-3-one |
| Alkoxy, \(\mathrm{-C-O-C-}\) |
\(\mathrm{R-O-R'}\) |
Ethers |
-oxy- |
\(\mathrm{CH_3-O-CH_3}\), dimethyl ether / methoxymethane; \(\mathrm{CH_3-O-C_2H_5}\), ethyl methyl ether / methoxyethane |
| Ester, \(\mathrm{-COO-}\) |
\(\mathrm{R-COO-R'}\) |
Esters |
-oate |
\(\mathrm{HCOOCH_3}\), methyl formate / methyl methanoate; \(\mathrm{CH_3COOCH_3}\), methyl acetate / methyl ethanoate |
Table 12.5 : Some common functional groups, with the suffix each contributes to the IUPAC name
The suffix column will be needed shortly. When a compound is named, the ending of the name announces which functional group is present — a name ending in -ol belongs to an alcohol, one ending in -oic acid to a carboxylic acid.
12.8.1 Characteristics Of Functional Groups
Three consequences follow from the idea of a functional group, and together they are what make the whole subject manageable.
- The chemical properties of compounds containing the same functional group are similar. Compounds of the same functional group can therefore be identified using the same type of test.
- The physical and chemical properties of compounds of different functional groups are different.
- There exists a series of compounds containing a particular type of functional group. For example, the series of alcohols runs \(\mathrm{CH_3OH}\), \(\mathrm{C_2H_5OH}\), \(\mathrm{C_3H_7OH}\) and onward.
The division of labour inside a molecule is worth stating plainly. In an organic compound the alkyl group mainly settles the physical properties — the boiling point, the density, the solubility — whereas the functional group is responsible for the chemical properties, that is, for how the compound reacts.
Alkyl Group And Functional Group The alkyl part of a molecule is the chain; it decides mostly physical behaviour such as boiling point. The functional group is the reactive part; it decides what the compound reacts with. Ethanol and butanol boil at different temperatures but react alike.
Name the functional group present in each of the following, and state the type of compound: \(\mathrm{C_2H_5OH}\), \(\mathrm{CH_3CHO}\), \(\mathrm{CH_3COOH}\), \(\mathrm{C_2H_5Br}\), \(\mathrm{CH_3COCH_3}\).
12.9 Homologous Series
Millions of compounds would be hopeless to learn one by one. They become manageable because they fall into families whose members are built to a pattern, and each such family is called a homologous series.
Homologous Series A homologous series is a group of organic compounds represented by the same general formula, having a similar structure and similar chemical properties, in which the successive compounds differ by a \(\mathrm{CH_2}\) group.
The \(\mathrm{CH_2}\) unit is known as the methylene group. Adding it to a given hydrocarbon produces the hydrocarbon with one more carbon atom, called the higher homologue. Adding \(\mathrm{CH_2}\) to methane, the first member of the alkanes, gives ethane, the second:
\[\mathrm{CH_4} + \mathrm{CH_2} \rightarrow \mathrm{C_2H_6}\]
Removing a \(\mathrm{CH_2}\) group works the other way and gives the lower homologue, the member with one carbon atom less:
\[\mathrm{C_4H_{10}} \rightarrow \mathrm{C_3H_8} + \mathrm{CH_2}\]
12.9.1 Characteristics Of A Homologous Series
- All members of a homologous series share the same general formula, since they contain the same elements and the same functional group. The general formula for an alkane is \(\mathrm{C_nH_{2n+2}}\), for an alkene \(\mathrm{C_nH_{2n}}\), and for an alkyne \(\mathrm{C_nH_{2n-2}}\).
- Each member differs from the preceding one by the addition of a \(\mathrm{CH_2}\) group, and therefore by a molecular mass of 14 a.m.u.
- The physical properties of the members change gradually — they show a gradation in properties — as the number of carbon atoms per molecule, and hence the molecular mass, increases. Melting point, boiling point and density all increase with increasing molecular mass.
- Members of a homologous series have similar chemical properties. Methane reacts with chlorine to form methyl chloride, and ethane reacts in the same way to form ethyl chloride.
- All the members of a homologous series can be prepared by using the same general method of preparation.
The figure of 14 a.m.u. in the second point is not a fact to be remembered separately. A \(\mathrm{CH_2}\) group holds one carbon atom of mass 12 and two hydrogen atoms of mass 1 each, and \(12 + 2 = 14\).
Molecular Mass And a.m.u. Molecular mass is the mass of a molecule compared with one-twelfth of a carbon-12 atom, expressed in atomic mass units (a.m.u.). It has no unit of its own beyond that comparison. \(\mathrm{CH_2}\) has molecular mass 14 a.m.u.
The similar chemistry of point 4 is easily shown. In each case one hydrogen atom is replaced by chlorine, and hydrogen chloride is released:
\[\mathrm{CH_4} + \mathrm{Cl_2} \rightarrow \mathrm{CH_3Cl} + \mathrm{HCl}\]
\[\mathrm{C_2H_6} + \mathrm{Cl_2} \rightarrow \mathrm{C_2H_5Cl} + \mathrm{HCl}\]
The common method of preparation of point 5 can be shown just as simply. Alcohols are prepared from alkyl halides, using the same reagent and the same conditions whichever member is wanted:
\[\mathrm{CH_3Br} + \mathrm{KOH}\text{(aq)} \xrightarrow{\ \text{boil}\ } \mathrm{CH_3OH} + \mathrm{KBr}\]
\[\mathrm{C_2H_5Br} + \mathrm{KOH}\text{(aq)} \xrightarrow{\ \text{boil}\ } \mathrm{C_2H_5OH} + \mathrm{KBr}\]
Gradation In Physical Properties
The third characteristic is the one that can be seen in a table of data. As the chain grows longer, the molecules attract one another more strongly, so more heat is needed to separate them and the boiling point climbs. Table 12.6 follows the first five alkanes.
| Methane |
16 |
\(-161.4\ ^\circ\text{C}\) |
gas |
| Ethane |
30 |
\(-88.3\ ^\circ\text{C}\) |
gas |
| Propane |
44 |
\(-44.5\ ^\circ\text{C}\) |
gas |
| Butane |
58 |
\(0\ ^\circ\text{C}\) |
gas |
| Pentane |
72 |
\(36.2\ ^\circ\text{C}\) |
liquid |
Table 12.6 : Gradation of boiling point along the alkane series
Read the boiling point column downwards and it rises steadily, each member differing from the last by 14 in molar mass. The change of state in the final column is simply a consequence: at ordinary room temperature the first four members are already above their boiling points and are gases, while pentane is not, and is a liquid.
Homologue And Isomer Two homologues have different molecular formulae, differing by \(\mathrm{CH_2}\) — methane and ethane. Two isomers have the same molecular formula but different structures. Homologues belong to one series; isomers are alternative arrangements of one formula.
12.9.2 Examples Of Homologous Series
Four series are met most often at this level. Table 12.7 gives their members in order, each obtained from the one above it by adding a \(\mathrm{CH_2}\) group.
| 1 |
Methane, \(\mathrm{CH_4}\) |
— |
— |
Methanol, \(\mathrm{CH_3OH}\) |
| 2 |
Ethane, \(\mathrm{C_2H_6}\) |
Ethene, \(\mathrm{C_2H_4}\) |
Ethyne, \(\mathrm{C_2H_2}\) |
Ethanol, \(\mathrm{C_2H_5OH}\) |
| 3 |
Propane, \(\mathrm{C_3H_8}\) |
Propene, \(\mathrm{C_3H_6}\) |
Propyne, \(\mathrm{C_3H_4}\) |
Propanol, \(\mathrm{C_3H_7OH}\) |
| 4 |
Butane, \(\mathrm{C_4H_{10}}\) |
Butene, \(\mathrm{C_4H_8}\) |
Butyne, \(\mathrm{C_4H_6}\) |
Butanol, \(\mathrm{C_4H_9OH}\) |
| 5 |
Pentane, \(\mathrm{C_5H_{12}}\) |
Pentene, \(\mathrm{C_5H_{10}}\) |
Pentyne, \(\mathrm{C_5H_8}\) |
Pentanol, \(\mathrm{C_5H_{11}OH}\) |
| 6 |
Hexane, \(\mathrm{C_6H_{14}}\) |
Hexene, \(\mathrm{C_6H_{12}}\) |
Hexyne, \(\mathrm{C_6H_{10}}\) |
Hexanol, \(\mathrm{C_6H_{13}OH}\) |
| 7 |
Heptane, \(\mathrm{C_7H_{16}}\) |
Heptene, \(\mathrm{C_7H_{14}}\) |
Heptyne, \(\mathrm{C_7H_{12}}\) |
Heptanol, \(\mathrm{C_7H_{15}OH}\) |
| 8 |
Octane, \(\mathrm{C_8H_{18}}\) |
Octene, \(\mathrm{C_8H_{16}}\) |
Octyne, \(\mathrm{C_8H_{14}}\) |
Octanol, \(\mathrm{C_8H_{17}OH}\) |
| 9 |
Nonane, \(\mathrm{C_9H_{20}}\) |
Nonene, \(\mathrm{C_9H_{18}}\) |
— |
— |
| 10 |
Decane, \(\mathrm{C_{10}H_{22}}\) |
Decene, \(\mathrm{C_{10}H_{20}}\) |
— |
— |
Table 12.7 : Members of the alkane, alkene, alkyne and alcohol series
There is no alkene or alkyne with a single carbon atom, and the reason is structural rather than accidental. A double bond needs two carbon atoms to sit between, and a triple bond needs two as well, so both series can only begin at \(n = 2\).
Finding A Higher Homologue
1. Ethene is the first member of the alkene series. Write the molecular formula and the molecular mass of the third member of the series.
The general formula for an alkene is \(\mathrm{C_nH_{2n}}\). Ethene is \(n = 2\), so the third member is \(n = 4\). Substituting, \(\mathrm{C_4H_{2\times4}} = \mathrm{C_4H_8}\).
Its molecular mass is found from the atomic masses, taking C as 12 and H as 1: \((4 \times 12) + (8 \times 1) = 48 + 8 = 56\).
The third member is \(\mathrm{C_4H_8}\), of molecular mass 56 a.m.u. As a check, it should exceed ethene (\(\mathrm{C_2H_4}\), mass 28) by two \(\mathrm{CH_2}\) groups, that is by 28 — and \(28 + 28 = 56\).
12.9.3 Significance Of The Homologous Series
- The nature of any member of a family of compounds can be worked out if the properties of the first few members of the series are known.
- It helps to predict the properties of even those members of a series that are yet to be prepared.
- Knowledge of a homologous series is useful for the systematic study of organic chemistry, since it saves learning time.
Write the molecular formula of the fourth member of the alkyne series and of the next higher homologue of propanol. State the difference in molecular mass between two adjacent members of any homologous series.
12.10 Nomenclature Of Organic Compounds
Nomenclature is the system of assignment of names to organic compounds. With millions of compounds in existence, the name has to be built from the structure by rule, so that any chemist reading the name can draw the molecule back again.
12.10.1 Trivial Names And The IUPAC System
Two systems of nomenclature are in use: the trivial system and the IUPAC system, named after the International Union of Pure and Applied Chemistry.
In the early days the name of a compound depended on the source from which it was obtained. Methane, \(\mathrm{CH_4}\), was called marsh gas because it was collected from marshy places. Acetic acid came from vinegar and took its name from the Latin acetum, meaning vinegar. Methyl alcohol was obtained from wood, and its name was built from Greek words meaning spirit and wood; citric acid was named after the botanical source citrus. Such names are called common names or trivial names.
Trivial names carry no information about structure, and there are far too few of them to go round. The nomenclature was therefore systematized by the chemists of IUPAC in 1957. According to the rules of this system, the name of an organic compound consists of three parts: the root word, the suffix and the prefix.
12.10.2 Root Word, Suffix And Prefix
Root word : The root word depends upon the number of carbon atoms present in the longest carbon chain selected. The roots are given in Table 12.8.
| One carbon atom, \(\mathrm{C_1}\) |
Meth |
| Two carbon atoms, \(\mathrm{C_2}\) |
Eth |
| Three carbon atoms, \(\mathrm{C_3}\) |
Prop |
| Four carbon atoms, \(\mathrm{C_4}\) |
But |
| Five carbon atoms, \(\mathrm{C_5}\) |
Pent |
| Six carbon atoms, \(\mathrm{C_6}\) |
Hex |
| Seven carbon atoms, \(\mathrm{C_7}\) |
Hept |
| Eight carbon atoms, \(\mathrm{C_8}\) |
Oct |
| Nine carbon atoms, \(\mathrm{C_9}\) |
Non |
| Ten carbon atoms, \(\mathrm{C_{10}}\) |
Dec |
Table 12.8 : Root words for chains of one to ten carbon atoms
Suffix : The suffix is a partial word or group of letters placed after the word root. There are two kinds. The primary suffix represents the nature of the bond between the carbon atoms: -ane for a single bond as in alkanes, -ene for a double bond as in alkenes, and -yne for a triple bond as in alkynes.
The secondary suffix indicates the particular functional group linked to the carbon chain. The group \(\mathrm{-OH}\) gives -ol, the group \(\mathrm{-CHO}\) gives -al, and the group \(\mathrm{-COOH}\) gives -oic acid.
Prefix : The prefix is a partial word or group of letters placed before the word root. The primary prefix indicates the nature of the carbon chain, that is, whether it is open or closed — a closed chain takes cyclo-, as in cyclopropane and cyclobutane.
The secondary prefix indicates the presence of other groups or substituents on the carbon chain. The common ones are:
- alkyl groups — methyl for \(\mathrm{-CH_3}\), ethyl for \(\mathrm{-C_2H_5}\), propyl for \(\mathrm{-C_3H_7}\)
- halogens — fluoro for \(\mathrm{-F}\), chloro for \(\mathrm{-Cl}\), bromo for \(\mathrm{-Br}\), iodo for \(\mathrm{-I}\)
When multiple substituents are present, they are listed in alphabetical order. If identical substituent groups occur more than once, the prefixes di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona- and deca- indicate two, three, four, five, six, seven, eight, nine and ten of them respectively.
The complete name is assembled in a fixed order:
Name = Prefix + Word root + Primary suffix + Secondary suffix
Fig. 12.10-1 takes one compound apart to show each piece doing its work.
Follow the four steps in the figure. The parent chain has four carbon atoms, so the word root is ‘but’. All the bonds between carbon atoms are single, so the primary suffix is ‘ane’. The \(\mathrm{-OH}\) group is attached to the second carbon atom, giving the secondary suffix ‘2-ol’, and the methyl group on the third carbon gives the prefix ‘3-methyl’. Assembled, the name is 3-methylbutan-2-ol.
Notice one small piece of spelling. The final ‘e’ of the alkane name is dropped when a secondary suffix beginning with a vowel is added — butane plus 2-ol becomes butan-2-ol, not butane-2-ol.
12.10.3 Rules Of IUPAC Nomenclature
- Longest chain rule : The longest continuous chain of carbon atoms, known as the parent chain, is selected. In \(\mathrm{C-C-C-C-C}\) the longest chain is of five carbon atoms, so the root word is ‘pent’.
- The longest chain need not be drawn as a straight line. A structure may be drawn with a bend in it, and the chain must be traced right through the bend; any carbon atom left outside the chosen chain is then a substituent, or branch.
- Branches are substituents : The branch chains are considered to be substituents, and their positions are indicated by the number of the carbon atom to which they are attached.
- Lowest number to the substituents : The carbon atoms of the longest chain are numbered in such a way that the alkyl groups get the smallest possible number. For \(\mathrm{CH_3-CH(CH_3)-CH_2-CH_2-CH_3}\), numbering from the left gives 2-methyl, which is right; numbering from the right would give 4-methyl, which is wrong.
- The position and the name of the alkyl group are written just before the name of the parent hydrocarbon, as in 3-ethylheptane.
- Multiple alkyl groups are labelled with the numerical prefixes di- for two, tri- for three, tetra- for four and penta- for five. If two alkyl groups are on the same carbon atom, the numeral is repeated, as in 2,2-dimethylbutane and 3-ethyl-3-methylpentane.
- Lowest sum rule : If two or more substituents are present, the numbering is done so that the sum of the position numbers is the minimum, and the substituents are then named alphabetically.
- Smallest number for functional groups : If a functional group is present in the chain, the carbon atoms are numbered so that the functional group gets the smallest possible number.
The Longest Chain And The Longest Straight Line The parent chain is the longest continuous path through the carbon atoms, whichever way the structure happens to be drawn on paper. A chain drawn with a bend in it is still one chain, and it may well be longer than the horizontal row.
Applying The Lowest Sum Rule
Consider \(\mathrm{CH_3-CH(CH_3)-CH_2-CH(C_2H_5)-CH_2-CH_2-CH_3}\), a seven-carbon chain carrying a methyl group and an ethyl group. Counting from the left, the methyl and ethyl groups are at positions 2 and 4, giving a sum of \(2 + 4 = 6\). Counting from the right, they are at positions 6 and 4, giving a sum of \(4 + 6 = 10\).
The smaller sum wins, so counting is done from the left. Naming the two substituents alphabetically, ethyl before methyl, the correct name is 4-ethyl-2-methylheptane, and not 4-ethyl-6-methylheptane. The same rule settles \(\mathrm{CH_3-CH(Br)-CH_2-CH(Cl)-CH_2-CH_3}\), which is 2-bromo-4-chlorohexane and not 5-bromo-3-chlorohexane. Both cases are set out in Fig. 12.10-2.
When a functional group is present, it outranks the substituents in deciding which end to count from. In \(\mathrm{CH_3-CH_2-CH_2-CH(OH)-CH_3}\) the \(\mathrm{-OH}\) group is on the second carbon counting from the right, so the compound is pentan-2-ol; counting from the other end would give pentan-4-ol, which is wrong.
The functional group must also remain part of the parent chain. In \(\mathrm{CH_3-CH(CH_3)-CH_2-CH(C_2H_5)-CH_2OH}\) the ethyl group is left outside the chain, even though including it would give a longer path, because the chain chosen has to carry the \(\mathrm{-OH}\) group. The name is 2-ethyl-4-methylpentan-1-ol.
12.10.4 Naming Esters
An ester is an organic compound formed when a carboxylic acid reacts with an alcohol, with the elimination of water. In words: carboxylic acid + alcohol → ester + water.
\[\mathrm{R\text{-}COOH} + \mathrm{R'\text{-}OH} \xrightarrow[\ \ ]{\ \text{conc. } \mathrm{H_2SO_4}\ } \mathrm{R\text{-}COOR'} + \mathrm{H_2O}\]
Here the \(\mathrm{-OH}\) group of the carboxylic acid is replaced by the \(\mathrm{-OR'}\) group of the alcohol, R and R′ both being alkyl groups.
An ester’s name consists of two parts, one taken from the alcohol and one from the carboxylic acid it is made from. First write the name of the alkyl group attached to the oxygen atom; then take the acid name, drop the ending ‘ic acid’ and replace it with ‘ate’.
For example, ethanol and ethanoic acid give ethyl ethanoate — ‘ethyl’ from the alcohol and ‘ethanoate’ from the acid. Ethanol with methanoic acid gives ethyl methanoate instead. Table 12.9 gives the first few.
| \(\mathrm{HCOOCH_3}\) |
methyl methanoate |
methyl formate |
| \(\mathrm{CH_3COOCH_3}\) |
methyl ethanoate |
methyl acetate |
| \(\mathrm{CH_3COOC_2H_5}\) |
ethyl ethanoate |
ethyl acetate |
| \(\mathrm{C_2H_5COOCH_3}\) |
methyl propanoate |
methyl propionate |
| \(\mathrm{C_2H_5COOC_2H_5}\) |
ethyl propanoate |
ethyl propionate |
Table 12.9 : Names of some common esters
12.10.5 Worked Examples Of IUPAC Names
Naming A Branched Alkane
1. Give the IUPAC name of \(\mathrm{CH_3-CH_2-CH(CH_3)-CH_2-CH_3}\).
Trace the longest continuous chain. It runs through five carbon atoms, so the word root is ‘pent’.
All the bonds between carbon atoms are single, so the primary suffix is ‘ane’. There is no functional group, so there is no secondary suffix.
The methyl substituent sits on the middle carbon. Numbering from either end gives it position 3, so no choice needs to be made.
The name is 3-methylpentane.
Naming A Compound With A Double Bond
1. Give the IUPAC name of \(\mathrm{CH_3-CH=CH-CH_3}\).
The chain has four carbon atoms, so the word root is ‘but’.
There is a double bond between two carbon atoms, so the primary suffix is ‘ene’.
The double bond must be given the smallest possible number. It lies between carbon 2 and carbon 3, and is numbered by the lower of the two, that is 2. Counting from the other end gives the same answer.
The name is but-2-ene.
Naming An Alcohol With A Branch
1. Give the IUPAC name of \(\mathrm{CH_3-CH(CH_3)-CH_2-CH_2-OH}\).
The longest chain carrying the \(\mathrm{-OH}\) group has four carbon atoms, so the word root is ‘but’ and the primary suffix is ‘ane’.
Number the chain so that the functional group gets the smallest number. Counting from the \(\mathrm{-OH}\) end puts it on carbon 1, giving the secondary suffix ‘1-ol’.
With that numbering, the methyl substituent falls on carbon 3, so the prefix is ‘3-methyl’.
The name is 3-methylbutan-1-ol.
Naming A Carboxylic Acid
1. Give the IUPAC name of \(\mathrm{CH_3-CH_2-CH_2-COOH}\).
Count every carbon atom, including the one in the \(\mathrm{-COOH}\) group, which is part of the chain. There are four, so the word root is ‘but’ and the primary suffix is ‘ane’.
The functional group is carboxyl, so the secondary suffix is ‘-oic acid’. The carboxyl carbon is always carbon 1, so no position number is needed.
The name is butanoic acid.
Naming A Ketone
1. Give the IUPAC name of \(\mathrm{CH_3-CO-CH_2-CH_2-CH_3}\).
The chain has five carbon atoms, so the word root is ‘pent’ and the primary suffix is ‘ane’.
The keto group \(\mathrm{>C=O}\) gives the secondary suffix ‘-one’. Numbering from the left places it on carbon 2; numbering from the right would place it on carbon 4, which is larger.
The name is pentan-2-one.
Give the IUPAC name of each of the following: (i) \(\mathrm{CH_3-CH(CH_3)-CH_3}\), (ii) \(\mathrm{CH_3-C\equiv C-CH_2-CH_3}\), (iii) \(\mathrm{CH_3-CH_2-CHO}\), (iv) \(\mathrm{CH_3-CH(Cl)-CH_2-CH_3}\).
12.12 Isomerism
Two chemists were once handed samples with the same molecular formula, \(\mathrm{C_4H_{10}}\), and found that one liquefied at \(0\ ^\circ\text{C}\) and the other only at \(-12\ ^\circ\text{C}\). The atoms were the same in number and kind, so the difference had to lie in how they were joined.
Isomers And Isomerism Compounds having the same molecular formula but different structural formulae are known as isomers, and the phenomenon is known as isomerism. The word comes from the Greek iso, meaning same, and meros, meaning parts.
Isomers differ in physical properties, or in chemical properties, or in both. The two samples above are butane and isobutane, drawn in Fig. 12.12-1. In butane the four carbon atoms lie in one continuous chain; in isobutane, three lie in a chain and the fourth branches off the middle one.
Q1. The same four carbon atoms and ten hydrogen atoms can be built into two different substances. How can two compounds be made of exactly the same atoms and still not be the same substance?
A. Because a molecular formula counts atoms but does not fix how they are linked. Four carbon atoms can be joined in one straight chain, or as a three-carbon chain with the fourth branching from the middle. The two arrangements are different molecules — butane and isobutane — with different boiling points.
The main cause of structural isomerism is a difference in the mode of linking of the atoms. The formula \(\mathrm{C_4H_{10}O}\), for instance, allows the oxygen atom to sit at three different places along the chain, and so stands for three different ethers: diethyl ether \(\mathrm{C_2H_5-O-C_2H_5}\), methyl propyl ether \(\mathrm{CH_3-O-C_3H_7}\), and methyl isopropyl ether \(\mathrm{CH_3-O-CH(CH_3)_2}\).
Isomerism is itself of more than one kind, as FC 12.2 shows. The two kinds studied here are chain isomerism and position isomerism; functional isomerism and metamerism are further varieties of structural isomerism, and stereo isomerism, or space isomerism, is a separate branch altogether.
12.12.1 Chain Isomerism
When two or more compounds have the same molecular formula but differ in the arrangement of the carbon atoms in straight or branched chains, the compounds are referred to as chain isomers, and the phenomenon is termed chain isomerism.
Butane and isobutane are chain isomers. Pentane, \(\mathrm{C_5H_{12}}\), goes one better and has three, shown in Fig. 12.12-2. The five carbon atoms may run in a single chain, giving n-pentane; or four may form a chain with one carbon branching off, giving iso-pentane, whose IUPAC name is 2-methylbutane; or three may form a chain with two carbons branching from the middle one, giving neo-pentane, that is 2,2-dimethylpropane.
Iso And Neo These older prefixes describe a shape at a glance. ‘Iso’ means at least one carbon atom is attached to three others; ‘neo’ means one carbon atom is attached to four. Both are common names, and the IUPAC name always says the same thing more precisely.
12.12.2 Position Isomerism
When two or more compounds with the same molecular formula differ in the position of a substituent atom or of a functional group on the carbon chain, they are called position isomers, and the phenomenon is termed position isomerism. Here the chain itself is unchanged; only the place of attachment moves.
Butyne, \(\mathrm{C_4H_6}\), has two position isomers. In but-1-yne, \(\mathrm{CH_3-CH_2-C\equiv CH}\), the triple bond lies at the end of the chain; in but-2-yne, \(\mathrm{CH_3-C\equiv C-CH_3}\), it lies in the middle. Both are shown in Fig. 12.12-3.
A functional group moves in exactly the same way. In pentan-3-one, \(\mathrm{CH_3CH_2-CO-CH_2CH_3}\), the keto group sits on the middle carbon of a five-carbon chain, while in pentan-2-one, \(\mathrm{CH_3-CO-CH_2CH_2CH_3}\), it sits on the second. The two are position isomers.
How Many Isomers Are There?
The number of isomers increases sharply with the number of carbon atoms, because a longer chain offers more places to branch. Butane, with four carbon atoms, has 2 isomers; hexane has 5. An organic compound of formula \(\mathrm{C_{21}H_{44}}\) has about a million isomers, and the count keeps climbing after that.
Butane And Isobutane In A Fuel Canister Camping stoves and lighters run on liquefied butane, which is easy to store as a liquid under mild pressure. But butane will not vaporise readily in cold weather, so canisters meant for winter use are blended with its isomer isobutane, which boils at a lower temperature and keeps the flame going. Two compounds, one formula — and the branched one is the one that works in the cold.
Draw the structural formulae of the two chain isomers of butane, \(\mathrm{C_4H_{10}}\), and give the IUPAC name of each. Then draw the two position isomers of butene, \(\mathrm{C_4H_8}\), that have an unbranched chain.
12.13 Alkanes
Alkanes are hydrocarbons in which all the linkages between the carbon atoms are single covalent bonds. They are known as saturated hydrocarbons, since all four valencies of every carbon in the formula \(\mathrm{C_nH_{2n+2}}\) are fully satisfied by single bonds — carbon to carbon, and carbon to hydrogen.
The simple alkanes share many properties. All of them enter into combustion reactions with oxygen to produce carbon dioxide and water vapour, which is another way of saying that many alkanes are flammable. This makes them good fuels: methane is the principal component of natural gas, and butane is the common lighter fluid.
methane + oxygen → carbon dioxide + water
\[\mathrm{CH_4}\text{(g)} + 2\,\mathrm{O_2}\text{(g)} \rightarrow \mathrm{CO_2}\text{(g)} + 2\,\mathrm{H_2O}\text{(l)}\]
Apart from burning, these hydrocarbons are relatively unreactive under ordinary conditions. Because of this they are also known as paraffins, from the Latin parum, meaning little, and affinis, meaning affinity — substances with little affinity for anything else.
The first six members of the series are given in Table 12.10, and the structural formulae of the first five are drawn out in Fig. 12.13-1.
| Methane |
\(\mathrm{CH_4}\) |
\(\mathrm{CH_4}\) |
| Ethane |
\(\mathrm{C_2H_6}\) |
\(\mathrm{CH_3CH_3}\) |
| Propane |
\(\mathrm{C_3H_8}\) |
\(\mathrm{CH_3CH_2CH_3}\) |
| Normal butane |
\(\mathrm{C_4H_{10}}\) |
\(\mathrm{CH_3CH_2CH_2CH_3}\) |
| Pentane |
\(\mathrm{C_5H_{12}}\) |
\(\mathrm{CH_3CH_2CH_2CH_2CH_3}\) |
| Hexane |
\(\mathrm{C_6H_{14}}\) |
\(\mathrm{CH_3CH_2CH_2CH_2CH_2CH_3}\) |
Table 12.10 : Homologous series of alkanes, general formula \(\mathrm{C_nH_{2n+2}}\)
Because several compounds may share one molecular formula, it is appropriate to represent alkanes by the structural formula, or by the condensed formula, rather than by the molecular formula alone.
12.13.1 Sources Of Alkanes
The principal sources of alkanes are natural gas and petroleum. Natural gas contains mainly methane, with smaller amounts of ethane, propane and butane.
Why A Gas Leak Can Be Smelled Methane and the other alkanes in natural gas are odourless, so a leak would give no warning at all. Gas suppliers therefore add a trace of a strong-smelling sulphur compound before the gas is distributed. The smell is not the fuel; it is a deliberate safety measure, and it is why a gas leak must be reported and the supply turned off at once rather than investigated with a flame.
12.13.2 Isomerism In Alkanes
Alkanes with more than three carbon atoms form isomers. Methane, ethane and propane have only one possible arrangement each, but from four carbon atoms onward there is a choice of framework.
The various isomers of an alkane differ in the framework of the carbon chains, and not in any functional group, since alkanes have none. They therefore show chain isomerism. Butane has the two isomers drawn in Fig. 12.12-1, and pentane the three drawn in Fig. 12.12-2 — in each case one unbranched chain and the rest branched.
Hexane, \(\mathrm{C_6H_{14}}\), has five chain isomers. Draw any three of them and give the IUPAC name of each.
12.14 Methane And Ethane
12.14.1 Occurrence Of Methane And Ethane
Methane is the simplest alkane, and its older name, marsh gas, records where it was first noticed. It arises in five main ways.
- Marsh gas is formed at the bottom of marshes, due to the fermentation of cellulose by a special type of bacteria.
- It is also present in the air exhaled by animals whose food contains cellulose.
- It is contained in the intestinal gases and in the blood of animals and human beings.
- Cavities in coal contain 90% methane, where it is called fire-damp.
- Methane is produced also by the dry distillation of wood and of peat.
Ethane keeps methane company. It occurs to the extent of 10–20 percent alongside methane.
Methane As A Greenhouse Gas Methane is a primary constituent of natural gas. It absorbs outgoing heat radiation from the earth and so contributes to the greenhouse effect, which makes it a greenhouse gas. It stays in the atmosphere for about 10 years, and while it is there it is some twenty times more effective at trapping heat than carbon dioxide.
It is emitted from a variety of natural and human-influenced sources: landfills, natural gas and petroleum systems, agricultural activities, coal mining, stationary and mobile combustion, waste water treatment and certain industrial processes. Since methane is also a valuable energy source, capturing it instead of letting it escape brings both economic and environmental benefit — a landfill that pipes its gas to a generator is doing both at once.
12.14.2 Structure Of Methane
The methane molecule is not flat. Carbon has four outer electrons, and electrons repel one another, so the four move as far apart from each other as they possibly can.
They are furthest apart when the four carbon–hydrogen bonds point towards the corners of a tetrahedron — a pyramid-shaped, three-dimensional structure, the name coming from the Greek tetra, meaning four. The angle between any two bonds is then \(109.5^\circ\).
12.14.3 Laboratory Preparation Of Methane
Reactants : sodium ethanoate (sodium acetate) and soda lime.
The reagent that actually attacks the sodium acetate is sodium hydroxide. Pure sodium hydroxide, however, is deliquescent — it draws in moisture from the air and turns into a sticky liquid, which is useless in a dry heating tube. Soda lime is used instead. It is a mixture of sodium hydroxide and calcium oxide, and comes as a white porous mass or as granules that stay dry.
Procedure : A mixture of sodium ethanoate and soda lime is taken in a hard glass test tube, as shown in Fig. 12.14-3, and heated over a bunsen burner.
Reaction : sodium ethanoate + sodium hydroxide → sodium carbonate + methane
\[\mathrm{CH_3COONa} + \mathrm{NaOH} \xrightarrow{\ \mathrm{CaO},\ 300^\circ\text{C}\ } \mathrm{Na_2CO_3} + \mathrm{CH_4}\uparrow\]
This reaction is known as decarboxylation, because a carbon dioxide molecule is eliminated from a carboxylic acid — here it leaves not as the gas but locked up as the carbonate. The chain is one carbon atom shorter than the salt it came from, which is why a two-carbon salt yields a one-carbon gas.
Collection : The gas evolved is collected by the downward displacement of water, since methane is only slightly soluble in water and is lighter than air.
Take care at the end of the experiment. The delivery tube must be lifted out of the water before the burner is removed, or cold water will be sucked back into the hot tube and crack it.
Soda Lime A dry mixture of sodium hydroxide and calcium oxide → supplies the alkali for decarboxylation without the stickiness of sodium hydroxide alone → heat it with the sodium salt of a carboxylic acid and the alkane with one carbon atom less is given off.
12.14.4 Laboratory Preparation Of Ethane
Reactants : sodium propanoate (sodium propionate) and soda lime.
Procedure : A mixture of sodium propionate and soda lime is taken in a boiling tube and heated over a bunsen burner, in apparatus similar to that of Fig. 12.14-3.
Reaction : sodium propanoate + sodium hydroxide → sodium carbonate + ethane
\[\mathrm{C_2H_5COONa} + \mathrm{NaOH} \xrightarrow{\ \mathrm{CaO},\ 300^\circ\text{C}\ } \mathrm{Na_2CO_3} + \mathrm{C_2H_6}\uparrow\]
Collection : The gas evolved is collected by the downward displacement of water.
Compare the two preparations and the pattern is the same throughout. To obtain an alkane by this route, start from the sodium salt of the carboxylic acid with one carbon atom more than the alkane wanted.
12.14.5 Other Methods Of Preparation
I. From iodomethane and bromoethane. Iodomethane (methyl iodide) and bromoethane (ethyl bromide) are reduced by nascent hydrogen at ordinary room temperature.
\[\mathrm{CH_3I} + 2[\mathrm{H}] \rightarrow \mathrm{CH_4} + \mathrm{HI}\]
\[\mathrm{C_2H_5Br} + 2[\mathrm{H}] \rightarrow \mathrm{C_2H_6} + \mathrm{HBr}\]
The symbol \([\mathrm{H}]\) stands for nascent hydrogen — hydrogen at the moment of its formation, which is far more reactive than hydrogen from a cylinder. It is produced here by the action of zinc powder with dilute hydrochloric acid, or by a zinc–copper couple in alcohol.
II. From aluminium carbide. When water is added to aluminium carbide at room temperature, methane is prepared.
\[\mathrm{Al_4C_3} + 12\,\mathrm{H_2O} \rightarrow 3\,\mathrm{CH_4}\uparrow + 4\,\mathrm{Al(OH)_3}\downarrow\]
III. Ethane from alkyl halides. When methyl iodide or methyl bromide is warmed with sodium metal in the presence of dry ether, ethane is produced.
\[2\,\mathrm{CH_3I} + 2\,\mathrm{Na} \xrightarrow{\ \text{dry ether}\ } \mathrm{H_3C-CH_3} + 2\,\mathrm{NaI}\]
This reaction is referred to as the Wurtz reaction. Two one-carbon fragments are joined end to end, so the product has twice as many carbon atoms as the halide. Methane therefore cannot be prepared by this method, and the method is not suitable for preparing alkanes with an odd number of carbon atoms.
Decarboxylation And The Wurtz Reaction Decarboxylation takes one carbon atom away: a three-carbon salt gives ethane. The Wurtz reaction doubles the carbon atoms: two one-carbon halide molecules give ethane. One shortens the chain, the other builds it.
Name the sodium salt you would heat with soda lime to obtain propane, and write the balanced equation for its decarboxylation. Then state why propane cannot be obtained by the Wurtz reaction.
12.15 Properties And Uses Of Methane And Ethane
12.15.1 Physical Properties
Methane
- It is a colourless and odourless gas with vapour density 8.
- Its melting point is \(-183\ ^\circ\text{C}\) and its boiling point is \(-162\ ^\circ\text{C}\).
- It is negligibly soluble in water, and soluble in organic solvents.
Ethane
- It is a colourless, odourless, tasteless and non-poisonous gas with vapour density 15.
- Its melting point is \(-172\ ^\circ\text{C}\) and its boiling point is \(-89\ ^\circ\text{C}\).
- It is sparingly soluble in water, but wholly soluble in organic solvents like alcohols, acetone and ether.
Both boiling points lie far below room temperature, which is why the two are gases at ordinary temperatures. Notice also that ethane, the heavier molecule, boils higher than methane — the gradation met in Table 12.6 at work again.
Vapour Density Vapour density compares a gas with hydrogen, and it is half the molecular mass: \[\text{vapour density} = \frac{\text{molecular mass}}{2}\] Methane has molecular mass 16, so its vapour density is 8. It is a ratio, and so carries no unit.
Chemically, methane and ethane are saturated hydrocarbons and are hard to attack. They do not react with bases such as sodium hydroxide, nor with oxidising agents such as potassium permanganate, nor with reducing agents such as sodium metal. Two reactions they do undergo are combustion and substitution.
12.15.2 Combustion
Complete Combustion
Methane and ethane burn in air with a bluish, non-sooty flame to form carbon dioxide and water vapour. A large amount of heat is given out at the same time, which is what makes alkanes good gaseous fuels.
methane + oxygen → carbon dioxide + water
\[\mathrm{CH_4} + 2\,\mathrm{O_2} \rightarrow \mathrm{CO_2} + 2\,\mathrm{H_2O}\]
ethane + oxygen → carbon dioxide + water
\[2\,\mathrm{C_2H_6} + 7\,\mathrm{O_2} \rightarrow 4\,\mathrm{CO_2} + 6\,\mathrm{H_2O}\]
The flame is worth looking at. Because methane and ethane hold little carbon for the hydrogen they carry, all their carbon is fully oxidised to carbon dioxide, and no unburnt carbon is left glowing in the flame. That is why they burn with a clean, non-sooty flame.
Combustion In Insufficient Supply Of Air
When alkanes burn in an insufficient supply of air, there is not enough oxygen to take every carbon atom all the way to carbon dioxide. They form carbon monoxide and water instead.
\[2\,\mathrm{CH_4} + 3\,\mathrm{O_2} \rightarrow 2\,\mathrm{CO} + 4\,\mathrm{H_2O}\]
\[2\,\mathrm{C_2H_6} + 5\,\mathrm{O_2} \rightarrow 4\,\mathrm{CO} + 6\,\mathrm{H_2O}\]
With still less oxygen the carbon is not oxidised at all, and is deposited as soot.
\[\mathrm{CH_4} + \mathrm{O_2} \rightarrow \mathrm{C} + 2\,\mathrm{H_2O}\]
Soot is not merely a nuisance; it is collected and used in the manufacture of printing inks and tyres.
A Gas Flame That Turns Yellow Compare the three equations above and notice how the oxygen supply alone changes the product. A gas burner starved of air produces carbon monoxide, a colourless and odourless gas that is poisonous even in small amounts, and the flame turns yellow and sooty as unburnt carbon glows in it. A yellow flame on a burner meant to burn blue is a warning that the air holes are blocked. It is also why a gas heater must never be run in a closed room.
Complete And Incomplete Combustion Complete combustion has plenty of oxygen and gives carbon dioxide with a clean blue flame. Incomplete combustion is short of oxygen and gives carbon monoxide, or soot, with a yellow smoky flame. The fuel is the same; only the air supply differs.
Catalytic Oxidation
Under carefully chosen conditions, alkanes can be oxidised part of the way rather than burnt, and the product then depends on the catalyst and the conditions used.
When a mixture of methane and oxygen in the ratio 9 : 1 by volume is compressed to about 120 atm and passed over copper tubes at 475 K, methyl alcohol is formed.
\[2\,\mathrm{CH_4} + \mathrm{O_2} \xrightarrow{\ \mathrm{Cu}\ \text{tubes},\ 475\ \text{K},\ 120\ \text{atm}\ } 2\,\mathrm{CH_3OH}\]
When a mixture of methane and oxygen is passed through heated molybdenum oxide, the mixture is oxidised to methanal (formaldehyde).
\[\mathrm{CH_4} + \mathrm{O_2} \xrightarrow{\ \mathrm{MoO}\ } \mathrm{HCHO} + \mathrm{H_2O}\]
When a manganese-based catalyst is used at \(100\ ^\circ\text{C}\), methane can be oxidised to methanoic acid (formic acid).
\[2\,\mathrm{CH_4} + 3\,\mathrm{O_2} \xrightarrow{\ \mathrm{Mn}\ \text{compound},\ 100^\circ\text{C}\ } 2\,\mathrm{HCOOH} + 2\,\mathrm{H_2O}\]
Ethane behaves in the same way, oxidising to ethyl alcohol, to ethanal (acetaldehyde), or on to ethanoic acid (acetic acid), according to the conditions.
\[2\,\mathrm{C_2H_6} + \mathrm{O_2} \xrightarrow{\ \mathrm{Cu}\ \text{tubes},\ 475\ \text{K},\ 120\ \text{atm}\ } 2\,\mathrm{C_2H_5OH}\]
\[\mathrm{C_2H_6} + \mathrm{O_2} \xrightarrow{\ \mathrm{MoO}\ } \mathrm{CH_3CHO} + \mathrm{H_2O}\]
\[\mathrm{C_2H_5OH} + \mathrm{O_2} \xrightarrow{\ \mathrm{Pt},\ 300^\circ\text{C}\ } \mathrm{CH_3COOH} + \mathrm{H_2O}\]
12.15.3 Substitution With Halogens
Alkanes react with chlorine, bromine or iodine in the presence of sunlight or ultraviolet light, giving halogen substituted products that are known as alkyl halides. An alkyl halide contains one or more halogen atoms in place of hydrogen.
Methane reacts with chlorine in diffused sunlight, or when heated to 600 K, to give chloromethane.
methane + chlorine → chloromethane + hydrogen chloride
\[\mathrm{CH_4} + \mathrm{Cl_2} \xrightarrow{\ \text{diffused sunlight or }600\ \text{K}\ } \mathrm{CH_3Cl} + \mathrm{HCl}\]
Here \(\mathrm{CH_3Cl}\) is formed by the substitution of a hydrogen atom of the methane molecule by a chlorine atom. This type of reaction is known as a substitution reaction, and the product formed is known as the substitution product.
The reaction does not stop there. The chloromethane formed reacts with further chlorine, substituting one more hydrogen atom and producing dichloromethane; this in turn produces trichloromethane, and finally tetrachloromethane.
\[\mathrm{CH_3Cl} + \mathrm{Cl_2} \rightarrow \mathrm{CH_2Cl_2} + \mathrm{HCl}\]
\[\mathrm{CH_2Cl_2} + \mathrm{Cl_2} \rightarrow \mathrm{CHCl_3} + \mathrm{HCl}\]
\[\mathrm{CHCl_3} + \mathrm{Cl_2} \rightarrow \mathrm{CCl_4} + \mathrm{HCl}\]
Trichloromethane is the compound better known as chloroform, and tetrachloromethane as carbon tetrachloride. A mixture of all four products is obtained, and they can be separated from one another by fractional distillation, because of the appreciable differences in their respective boiling points.
Ethane reacts with chlorine in the same way, producing successively chloroethane, dichloroethane, trichloroethane, tetrachloroethane, pentachloroethane and hexachloroethane. The six steps are set out in Table 12.11.
| \(\mathrm{C_2H_6}\) (ethane) |
\(\mathrm{Cl_2}\) |
\(\mathrm{C_2H_5Cl}\) (monochloroethane) |
\(\mathrm{HCl}\) |
| \(\mathrm{C_2H_5Cl}\) (chloroethane) |
\(\mathrm{Cl_2}\) |
\(\mathrm{C_2H_4Cl_2}\) (dichloroethane) |
\(\mathrm{HCl}\) |
| \(\mathrm{C_2H_4Cl_2}\) (dichloroethane) |
\(\mathrm{Cl_2}\) |
\(\mathrm{C_2H_3Cl_3}\) (trichloroethane) |
\(\mathrm{HCl}\) |
| \(\mathrm{C_2H_3Cl_3}\) (trichloroethane) |
\(\mathrm{Cl_2}\) |
\(\mathrm{C_2H_2Cl_4}\) (tetrachloroethane) |
\(\mathrm{HCl}\) |
| \(\mathrm{C_2H_2Cl_4}\) (tetrachloroethane) |
\(\mathrm{Cl_2}\) |
\(\mathrm{C_2HCl_5}\) (pentachloroethane) |
\(\mathrm{HCl}\) |
| \(\mathrm{C_2HCl_5}\) (pentachloroethane) |
\(\mathrm{Cl_2}\) |
\(\mathrm{C_2Cl_6}\) (hexachloroethane) |
\(\mathrm{HCl}\) |
Table 12.11 : Successive chlorination of ethane
All alkanes react with chlorine, bromine and iodine in a similar manner, producing the corresponding substituted products.
12.15.4 Uses Of Methane And Ethane
Methane
- It is a source of carbon monoxide and hydrogen.
- It is used in the preparation of useful compounds such as ethyne (acetylene), methanal (formaldehyde), methanol, chloromethane and tetrachloromethane (carbon tetrachloride).
- It is employed as a domestic fuel.
Trichloromethane, or chloroform, was once used to produce anaesthesia. Because a higher dose can kill a patient, it has been replaced; halothane, \(\mathrm{CF_3CHBrCl}\), is now used for anaesthetic purposes.
Ethane
- It is used in the preparation of ethene, ethanol, ethanal (acetaldehyde) and ethanoic acid (acetic acid).
- It forms ethyl chloride, which is used to make tetraethyllead; and 1,1,1-trichloroethane is a solvent much used in dry cleaning.
- Ethane is also a good fuel.
Both gases are flammable and are the primary components of natural gas, which ignites easily and is used as a fuel. Both also form explosive mixtures with air, which is why a suspected leak is treated as an emergency.
Write the balanced equation for the burning of ethane in an insufficient supply of air, and name the poisonous product. Then write the equation for the second step in the chlorination of methane, and name the product formed.
12.16 Alkenes
Alkenes are compounds with at least one carbon–carbon double bond. They form a homologous series having the general formula \(\mathrm{C_nH_{2n}}\), and they are unsaturated hydrocarbons, since the valencies of the carbon atoms are not all satisfied by hydrogen and a double bond is present between two of them.
Alkenes are also called olefins, a name meaning oil-forming, because the lower members of the family form oily products when they are treated with chlorine or bromine.
Since alkenes are reactive, they seldom occur free in nature. The lower alkenes occur in minute quantities in coal gas, and they are produced on a large scale by the cracking of petroleum — a process in which the long-chain hydrocarbons of petroleum are heated until they break into shorter molecules, some of which carry a double bond.
12.16.1 The Homologous Series Of Alkenes
The first four members are given in Table 12.12, and their structural formulae are drawn in Fig. 12.16-1.
| \(\mathrm{C_2H_4}\) |
\(\mathrm{CH_2=CH_2}\) |
Ethylene |
Ethene |
| \(\mathrm{C_3H_6}\) |
\(\mathrm{CH_3-CH=CH_2}\) |
Propylene |
Propene |
| \(\mathrm{C_4H_8}\) |
\(\mathrm{CH_3-CH_2-CH=CH_2}\) |
Butylene |
Butene |
| \(\mathrm{C_5H_{10}}\) |
\(\mathrm{CH_3-CH_2-CH_2-CH=CH_2}\) |
Pentylene |
Pentene |
Table 12.12 : Homologous series of alkenes, general formula \(\mathrm{C_nH_{2n}}\)
Count the hydrogen atoms in any of these and the general formula holds: pentene has five carbon atoms and ten hydrogen atoms, exactly \(\mathrm{C_nH_{2n}}\) with \(n = 5\). An alkene always carries two hydrogen atoms fewer than the alkane with the same number of carbon atoms, and those two missing hydrogens are the space taken up by the double bond.
Cracking Petroleum is rich in long-chain alkanes but poor in the small reactive molecules industry wants. Cracking heats those long chains, often over a catalyst, until they snap into shorter pieces — and the pieces include alkenes such as ethene.
12.16.2 Isomerism In Alkenes
Alkenes with four or more carbon atoms can form isomers, and they show two kinds.
Position isomerism arises from the position of the double bond in the chain. In but-1-ene, \(\mathrm{CH_3CH_2CH=CH_2}\), the double bond is at the end; in but-2-ene, \(\mathrm{CH_3CH=CHCH_3}\), it lies in the middle. The chain is the same in both.
Chain isomerism arises from the nature of the parent chain, or from branches on it. But-1-ene has an unbranched chain of four carbon atoms, while 2-methylpropene, \(\mathrm{CH_3-C(CH_3)=CH_2}\), has a chain of three with a methyl group on the middle carbon.
Butene, \(\mathrm{C_4H_8}\), therefore has three isomers in all: but-1-ene, but-2-ene and 2-methylpropene. They are drawn together in Fig. 12.16-2.
Pentene has the molecular formula \(\mathrm{C_5H_{10}}\). Draw two of its position isomers with an unbranched chain and name each one. State the molecular formula of the alkene that is the next higher homologue of pentene.
12.17 Ethene
Ethene, \(\mathrm{C_2H_4}\), is the first member of the alkene series. It is present in natural gas, and in small amounts it occurs as a plant hormone, where it is responsible for the ripening of fruits.
12.17.1 Structure Of Ethene
Each carbon atom of ethene is attached to two hydrogen atoms by single covalent bonds, and to the other carbon atom by a double covalent bond. There are four carbon–hydrogen single covalent bonds and one carbon–carbon double covalent bond in the molecule.
Unlike methane, ethene is a planar molecule — the whole molecule is flat, and all six atoms lie in one plane. All the bond angles, both \(\mathrm{H-C-H}\) and \(\mathrm{H-C=C}\), are of \(120^\circ\), as shown in Fig. 12.17-1.
12.17.2 Preparation By Dehydration Of Ethanol
Reactants : ethanol and concentrated sulphuric acid.
Procedure : Arrange the apparatus as shown in Fig. 12.17-2. Take one part of ethyl alcohol in a flask and add two parts of concentrated sulphuric acid, pouring the acid in slowly and with cooling, since the mixing gives out a great deal of heat. Heat the mixture gradually to about \(170\ ^\circ\text{C}\), and ethene gas is evolved.
The gas is passed through sodium hydroxide solution, which removes the carbon dioxide and sulphur dioxide formed as impurities when some of the alcohol is oxidised by the acid. Ethene is then collected by the downward displacement of water; a little aluminium sulphate is added to the water in order to avoid frothing.
Reaction : The change takes place in two steps. First the acid combines with the alcohol to give ethyl hydrogen sulphate:
\[\mathrm{CH_3CH_2OH} + \mathrm{H_2SO_4} \rightarrow \mathrm{CH_3CH_2HSO_4} + \mathrm{H_2O}\]
On further heating in excess acid, the ethyl hydrogen sulphate breaks down, giving ethene and returning the sulphuric acid unchanged:
\[\mathrm{CH_3CH_2HSO_4} \xrightarrow{\ \text{excess } \mathrm{H_2SO_4},\ 170^\circ\text{C}\ } \mathrm{H_2C=CH_2} + \mathrm{H_2SO_4}\]
Since the acid is recovered at the end, the two steps are usually written as one:
ethanol → ethene + water
\[\mathrm{C_2H_5OH} \xrightarrow{\ \text{conc. } \mathrm{H_2SO_4},\ 170^\circ\text{C}\ } \mathrm{C_2H_4} + \mathrm{H_2O}\]
The removal of a molecule of water from a compound in this way is called dehydration, and the sulphuric acid acts as the dehydrating agent.
Collection : The gas is collected by the downward displacement of water, because it is an inflammable gas and because it is insoluble in water.
Ethene is also prepared by passing ethanol vapours through a tube containing alumina, \(\mathrm{Al_2O_3}\), at \(350\ ^\circ\text{C}\). Here a hot solid does the same work as the acid.
\[\mathrm{C_2H_5OH} \xrightarrow{\ \mathrm{Al_2O_3},\ 350^\circ\text{C}\ } \mathrm{C_2H_4} + \mathrm{H_2O}\]
12.17.3 Preparation By Dehydrohalogenation
Ethene is also prepared by the dehydrohalogenation of an ethyl halide, that is a haloethane, using hot concentrated alcoholic potassium hydroxide. The name says what happens: a hydrogen atom and a halogen atom are removed from adjacent carbon atoms, and the double bond closes the gap.
\[\mathrm{C_2H_5Cl} + \mathrm{KOH}\ (\text{alc., hot and conc.}) \rightarrow \mathrm{C_2H_4} + \mathrm{KCl} + \mathrm{H_2O}\]
\[\mathrm{C_2H_5Br} + \mathrm{KOH}\ (\text{alc., hot and conc.}) \rightarrow \mathrm{C_2H_4} + \mathrm{KBr} + \mathrm{H_2O}\]
\[\mathrm{C_2H_5I} + \mathrm{KOH}\ (\text{alc., hot and conc.}) \rightarrow \mathrm{C_2H_4} + \mathrm{KI} + \mathrm{H_2O}\]
The word ‘alcoholic’ is doing real work in those equations. Potassium hydroxide dissolved in alcohol removes hydrogen halide and gives an alkene, whereas the same alkali dissolved in water replaces the halogen and gives an alcohol instead.
Dehydration And Dehydrohalogenation Dehydration takes out a molecule of water — ethanol with conc. sulphuric acid gives ethene. Dehydrohalogenation takes out a molecule of hydrogen halide — chloroethane with alcoholic KOH gives ethene. Different starting material, different reagent, same product.
12.17.4 Physical Properties Of Ethene
- Ethene is a colourless and inflammable gas with a peculiar odour, a faint sweetish smell, and vapour density 14.
- Its melting point is \(-169\ ^\circ\text{C}\) and its boiling point is \(-102\ ^\circ\text{C}\).
- It is sparingly soluble in water, but highly soluble in organic solvents like alcohol, ether and chloroform.
- It produces an anaesthetic effect upon inhalation.
12.17.5 Addition Reactions Of Ethene
The double bond is where the chemistry of ethene happens. One of the two bonds between the carbon atoms opens up, and the two freed valencies pick up an atom each — so the characteristic reaction of an alkene is addition, and the product is saturated.
Addition of hydrogen (hydrogenation) : When ethene and hydrogen are passed over finely divided catalysts such as platinum or palladium at ordinary temperature, or nickel at \(200\ ^\circ\text{C}\), the two atoms of the hydrogen molecule are added to the unsaturated molecule, which thus becomes a saturated one.
ethene + hydrogen → ethane
\[\mathrm{C_2H_4} + \mathrm{H_2} \xrightarrow{\ \mathrm{Ni},\ 200^\circ\text{C}\ } \mathrm{C_2H_6}\]
Addition of halogens (halogenation) : Reaction with fluorine is explosive, and the compound formed is not stable. Chlorine, bromine and iodine, however, are added to the double bond of ethene at room temperature to form saturated dichloro, dibromo and diiodo ethane.
\[\mathrm{CH_2=CH_2} + \mathrm{Cl_2} \rightarrow \mathrm{CH_2Cl-CH_2Cl}\ \ (1,2\text{-dichloroethane})\]
\[\mathrm{CH_2=CH_2} + \mathrm{Br_2} \rightarrow \mathrm{CH_2Br-CH_2Br}\ \ (1,2\text{-dibromoethane})\]
\[\mathrm{C_2H_4} + \mathrm{I_2} \rightarrow \mathrm{C_2H_4I_2}\ \ (1,2\text{-di-iodoethane})\]
Reaction with iodine is very slow, and the di-iodoethane formed is unstable; it eliminates the iodine readily and regenerates the ethene.
The bromine reaction is also a test. A solution of bromine in carbon tetrachloride has a reddish brown colour. When it is added dropwise to ethene, the reddish brown colour of the bromine disappears, because the colourless dibromoethane is formed.
Bromine In Carbon Tetrachloride A reddish brown solution → add it dropwise to a hydrocarbon → if the colour disappears, the hydrocarbon has a double or triple bond that has taken the bromine up by addition. If the colour stays, the hydrocarbon is saturated.
Addition of water (hydration) : A water molecule gets added to alkenes in the presence of acids, such as sulphuric acid, to form alcohols.
\[\mathrm{CH_2=CH_2} + \mathrm{H_2O} \xrightarrow{\ \mathrm{H^{+}}\ } \mathrm{C_2H_5OH}\]
Addition of hydrogen chloride : When ethene gas is treated with hydrochloric acid, chloroethane is formed.
\[\mathrm{CH_2=CH_2} + \mathrm{HCl}\text{(aq)} \rightarrow \mathrm{CH_3-CH_2Cl}\]
If that chloroethane is then treated with aqueous potassium hydroxide, ethanol is formed.
\[\mathrm{CH_3CH_2Cl} + \mathrm{KOH}\text{(aq)} \rightarrow \mathrm{CH_3CH_2OH} + \mathrm{KCl}\]
Polymerization : When two or more molecules of the same compound join together to form a bigger molecule, the reaction is called polymerisation. Ethene polymerises to produce polythene.
\[n\,\mathrm{H_2C=CH_2} \xrightarrow{\ \text{high temperature, high pressure, catalyst}\ } \mathrm{[H_2C-CH_2]}_n\]
Oxidation : Ethene is oxidised by cold alkaline potassium permanganate solution at room temperature, the permanganate supplying oxygen as \([\mathrm{O}]\).
\[\mathrm{H_2C=CH_2} + \mathrm{H_2O} + [\mathrm{O}] \xrightarrow{\ \text{cold alk. } \mathrm{KMnO_4}\ } \mathrm{HOCH_2-CH_2OH}\]
The product is 1,2-ethanediol, better known as ethylene glycol. The purple colour of the potassium permanganate is decolourised in the process, and this too serves as a test for unsaturation.
Predicting The Product Of An Addition
1. Ethene is treated with chlorine at room temperature. Name and write the formula of the product, and state what type of compound it is.
Ethene is unsaturated, so it reacts by addition. The double bond opens, leaving one free valency on each carbon atom.
The chlorine molecule supplies one chlorine atom to each of those free valencies, so one chlorine goes to carbon 1 and one to carbon 2:
\[\mathrm{CH_2=CH_2} + \mathrm{Cl_2} \rightarrow \mathrm{CH_2Cl-CH_2Cl}\]
- The product is 1,2-dichloroethane, \(\mathrm{C_2H_4Cl_2}\). Every carbon now has four single bonds, so it is a saturated compound — an alkyl halide.
12.17.6 Combustion Of Ethene
Ethene burns in air with a sooty flame, producing a large amount of heat. The soot appears because ethene holds proportionally more carbon than an alkane does, and in a limited air supply not all of it is oxidised.
If the supply of air is in excess, it burns instead with a pale blue flame to produce carbon dioxide, water and a large amount of heat.
\[\mathrm{C_2H_4} + 3\,\mathrm{O_2} \rightarrow 2\,\mathrm{CO_2} + 2\,\mathrm{H_2O} + \text{heat}\]
12.17.7 Uses Of Ethene
- Ethene is used in making polythene.
- It is converted into ethanol, which is used as a starting material for other products, mainly cosmetics and toiletry preparations.
- It is used for the ripening of fruits.
- It is used in making epoxyethane, used in the manufacture of detergents.
- It is used for producing the oxy-ethylene flame, which is used for cutting and welding metals.
- It is used in the manufacture of synthetic chemicals.
The Gas That Ripens Fruit Ethene is not only an industrial gas; a ripening fruit makes it and gives it off, and the gas then sets off ripening in fruit nearby. This is the chemistry behind the old advice to keep a ripe banana with unripe ones, and behind the practice of shipping bananas and mangoes unripe and treating them with a controlled amount of ethene on arrival. The same effect explains why one over-ripe fruit spoils a whole basket.
Give balanced equations for the conversion of (i) ethanol into ethene, (ii) ethene into ethane, (iii) ethene into 1,2-dibromoethane. State in each case whether the reaction is a dehydration, an addition, or both.
12.18 Alkynes
The aliphatic hydrocarbons that contain a triple bond, \(\mathrm{-C\equiv C-}\), between two carbon atoms are known as alkynes. The triple bond is also called the acetylenic bond, after the first member of the family.
Alkynes are unsaturated hydrocarbons, because of that triple bond between the two carbon atoms. They form a homologous series with the general formula \(\mathrm{C_nH_{2n-2}}\) — four hydrogen atoms fewer than the alkane with the same number of carbon atoms, since the triple bond uses up two valencies on each of two carbons.
The sources of alkynes are natural gas and petroleum.
12.18.1 The Homologous Series Of Alkynes
The first four members are listed in Table 12.13, and their structural formulae are drawn in Fig. 12.18-1.
| \(\mathrm{C_2H_2}\) |
\(\mathrm{CH\equiv CH}\) |
Acetylene |
Ethyne |
| \(\mathrm{C_3H_4}\) |
\(\mathrm{CH_3-C\equiv CH}\) |
Allylene, or methyl acetylene |
Propyne |
| \(\mathrm{C_4H_6}\) |
\(\mathrm{CH_3-CH_2-C\equiv CH}\) |
Crotonylene, or ethyl acetylene |
Butyne |
| \(\mathrm{C_5H_8}\) |
\(\mathrm{CH_3-CH_2-CH_2-C\equiv CH}\) |
Propyl acetylene |
Pentyne |
Table 12.13 : Homologous series of alkynes, general formula \(\mathrm{C_nH_{2n-2}}\)
Names Ending In Acetylene The older names in Table 12.13 all treat ethyne as the parent: methyl acetylene is ethyne with a methyl group in place of one hydrogen, ethyl acetylene with an ethyl group. The IUPAC names say the same thing by counting carbon atoms instead.
12.18.2 Isomerism In Alkynes
Alkynes with four or more carbon atoms can form isomers, and they show position isomerism as well as chain isomerism.
Butyne shows position isomerism only. Its two isomers, but-1-yne and but-2-yne, were drawn in Fig. 12.12-3; both have the same unbranched four-carbon chain, and only the position of the triple bond differs.
Pentyne, \(\mathrm{C_5H_8}\), shows both kinds at once and has three isomers:
- \(\mathrm{CH_3-CH_2-CH_2-C\equiv CH}\), pent-1-yne
- \(\mathrm{CH_3-CH_2-C\equiv C-CH_3}\), pent-2-yne
- \(\mathrm{CH_3-CH(CH_3)-C\equiv CH}\), 3-methylbut-1-yne
The first two have the same straight chain of five carbon atoms and differ only in where the triple bond sits, so they are position isomers. The third has a chain of four carbon atoms with a methyl branch, so it differs from each of the other two in the framework itself, and is a chain isomer of both. The three are drawn in Fig. 12.18-2.
Write the molecular formula of the alkyne with six carbon atoms, using the general formula. Then draw two isomers of butyne and state which type of isomerism they show.
12.19 Ethyne
Ethyne, \(\mathrm{C_2H_2}\), whose common name is acetylene, is the first member of the alkyne series. Being an unsaturated hydrocarbon, it is too reactive to survive in the free state and is not found as such.
Traces of ethyne are present in coal gas, and in the gases obtained by the decomposition of certain complex organic compounds. It is obtained on a working scale by the cracking of alkanes from various fractions of petroleum.
12.19.1 Structure Of Ethyne
Each carbon atom in ethyne is attached to one hydrogen atom by a single covalent bond, and to the other carbon atom by a triple covalent bond. Each carbon therefore uses three of its four valencies on its neighbour and one on hydrogen.
With only one other atom attached to each carbon, there is nothing to bend the molecule. The shape of the ethyne molecule is linear — all four atoms lie in one straight line.
12.19.2 Laboratory Preparation From Calcium Carbide
Reactants : water and calcium carbide.
Procedure : Arrange the apparatus as shown in Fig. 12.19-2. Take a few pieces of calcium carbide in a conical flask and add a few drops of water through the thistle funnel. Calcium carbide reacts with water to produce a colourless gas with a sweet, ether-like odour. The reaction is exothermic, so the water is added a few drops at a time through a tap or pinch cock, and never poured in.
Reaction : calcium carbide + water → calcium hydroxide + ethyne
\[\mathrm{CaC_2}\text{(s)} + 2\,\mathrm{H_2O}\text{(l)} \rightarrow \mathrm{Ca(OH)_2}\text{(aq)} + \mathrm{C_2H_2}\text{(g)}\uparrow\]
Collection : The gas is collected by the downward displacement of water, since it is insoluble in water.
Purification : Impurities such as phosphine, hydrogen sulphide, ammonia and arsine are formed along with the ethyne, because commercial calcium carbide is contaminated with calcium sulphide, calcium phosphide and similar compounds. On passing the gas through water, all these impurities except phosphine are absorbed. The phosphine is then absorbed by passing the gas through acidified copper sulphate or acidified potassium dichromate solution.
The flask should not contain air, as ethyne forms an explosive mixture with it. The air in the flask is therefore displaced with oil or with some inert gas before the reaction is started.
12.19.3 Preparation From 1,2-Dibromoethane
When 1,2-dibromoethane, also called ethylene dibromide, is boiled with alcoholic potassium hydroxide, ethyne is formed.
1,2-dibromoethane + potassium hydroxide → ethyne + potassium bromide + water
\[\mathrm{CH_2Br\text{-}CH_2Br} + 2\,\mathrm{KOH}\ (\text{alcoholic}) \xrightarrow{\ \text{boiling},\ 200^\circ\text{C}\ } \mathrm{CH\equiv CH} + 2\,\mathrm{KBr} + 2\,\mathrm{H_2O}\]
Two molecules of hydrogen bromide are removed here, one from each carbon atom, and each removal creates one more bond between the carbons. That is why the product carries a triple bond and not a double one.
Which Halide Gives Which Gas Bromoethane, \(\mathrm{C_2H_5Br}\), has one bromine atom; with alcoholic KOH it loses one HBr and gives ethene. 1,2-dibromoethane has two, on adjacent carbons; it loses two HBr and gives ethyne. One halogen atom, a double bond; two, a triple bond.
12.19.4 Physical Properties Of Ethyne
- Ethyne is a colourless gas with an ether-like sweet odour when it is pure. The gas prepared from calcium carbide has a garlic odour, due to the traces of phosphine and hydrogen sulphide in it.
- It is negligibly soluble in water, but highly soluble in organic solvents like acetone and alcohol.
- It is lighter than air, with vapour density 13.
- It liquefies at \(-84\ ^\circ\text{C}\).
12.19.5 Combustion Of Ethyne
Ethyne is a highly reactive compound, because of the triple bond between its two carbon atoms. Its first reaction is with oxygen.
Ethyne contains a greater proportion of carbon than either the alkanes such as ethane or the alkenes such as ethene. Since all the carbon particles do not get oxidised completely in an ordinary air supply, it burns with a sooty flame.
In excess air the story changes. Ethyne then burns with a brilliant white flame to produce carbon dioxide, water vapour and a large amount of heat.
\[2\,\mathrm{CH\equiv CH} + 5\,\mathrm{O_2} \rightarrow 4\,\mathrm{CO_2} + 2\,\mathrm{H_2O} + \text{heat}\]
12.19.6 Addition Reactions Of Ethyne
Alkynes are unsaturated compounds, so they are associated with addition reactions, since triple bonds break up easily. The breaking up of a triple bond gives a product that is still unsaturated, carrying a double bond; further breaking of that double bond gives a saturated product. Addition to an alkyne therefore takes place in two stages.
Addition of hydrogen (catalytic hydrogenation) : In the presence of nickel, platinum or palladium, ethyne first takes up two atoms of hydrogen to change to ethene, which further takes up two atoms of hydrogen to give ethane.
\[\mathrm{C_2H_2} + \mathrm{H_2} \xrightarrow{\ \mathrm{Ni}\ } \mathrm{C_2H_4}\]
\[\mathrm{C_2H_4} + \mathrm{H_2} \xrightarrow{\ \mathrm{Ni}\ } \mathrm{C_2H_6}\]
Addition of halogens : Ethyne, in an inert solvent like carbon tetrachloride, adds chlorine to change first into an ethene derivative with a carbon–carbon double bond, and then into an ethane derivative with a carbon–carbon single bond.
\[\mathrm{CH\equiv CH} \xrightarrow{\ \mathrm{Cl_2}\ } \mathrm{CHCl=CHCl} \xrightarrow{\ \mathrm{Cl_2}\ } \mathrm{CHCl_2\text{-}CHCl_2}\]
The products are 1,2-dichloroethene and then 1,1,2,2-tetrachloroethane. Away from an inert solvent the reaction is far more violent: acetylene reacts vigorously with chlorine gas in the presence of sunlight, giving out flames and depositing carbon.
\[\mathrm{C_2H_2} + \mathrm{Cl_2} \rightarrow 2\,\mathrm{C} + 2\,\mathrm{HCl}\]
When bromine in carbon tetrachloride is added to ethyne, its reddish brown colour disappears, because addition products are formed. The reaction is similar to the addition of chlorine, giving 1,2-dibromoethene and then 1,1,2,2-tetrabromoethane.
\[\mathrm{C_2H_2} \xrightarrow{\ \mathrm{Br_2}\ } \mathrm{C_2H_2Br_2} \xrightarrow{\ \mathrm{Br_2}\ } \mathrm{C_2H_2Br_4}\]
Iodine reacts slowly with ethyne in the presence of alcohol to form a di-iodo derivative, and stops at the first stage.
\[\mathrm{CH\equiv CH} + \mathrm{I_2} \rightarrow \mathrm{ICH=CHI}\ \ (1,2\text{-di-iodoethene})\]
Reaction with hydrogen chloride : Ethyne reacts with hydrochloric acid to form first chloroethene and finally 1,1-dichloroethane.
\[\mathrm{CH\equiv CH} \xrightarrow{\ \mathrm{HCl}\ } \mathrm{CH_2=CHCl} \xrightarrow{\ \mathrm{HCl}\ } \mathrm{CH_3\text{-}CHCl_2}\]
Two Stages Of An Addition
1. Excess bromine dissolved in carbon tetrachloride is added to ethyne. Write the formula of the final product, name it, and state what is observed.
Ethyne has a triple bond, so addition takes place in two stages. In the first stage one bromine molecule opens the triple bond down to a double bond, giving \(\mathrm{CHBr=CHBr}\).
Bromine is in excess, so a second molecule adds across that double bond, giving \(\mathrm{CHBr_2-CHBr_2}\).
The final product is 1,1,2,2-tetrabromoethane, \(\mathrm{C_2H_2Br_4}\), which is saturated. The observation is that the reddish brown colour of the bromine solution disappears, since the product is colourless.
12.19.7 Uses Of Ethyne
Ethyne is used
- for oxy-acetylene welding at very high temperatures, which are obtained when ethyne burns in oxygen;
- as an illuminant in the oxy-acetylene lamp;
- for the artificial ripening and preservation of fruits;
- for the manufacture of synthetic products like polymers, artificial rubber and oxalic acid;
- for the manufacture of important organic compounds like acetaldehyde, acetic acid, plastic and rubber.
Why An Acetylene Cylinder Is Filled With Acetone Burning ethyne in pure oxygen rather than air gives a flame hot enough to melt steel, which is what makes oxy-acetylene welding possible. Storing the gas is the difficulty: compressed ethyne is unstable and forms an explosive mixture with air. A welding cylinder is therefore packed with a porous solid soaked in acetone, in which ethyne is highly soluble, so the gas is stored dissolved rather than merely compressed. It is also why such a cylinder must be kept upright and away from heat.
Write balanced equations for the preparation of ethyne from calcium carbide and for its complete combustion in excess air. Then name the two products formed, in order, when ethyne reacts with excess chlorine in carbon tetrachloride.
12.20 Chemical Tests To Distinguish Alkanes, Alkenes And Alkynes
Three colourless gases in three unlabelled jars cannot be told apart by looking at them. They can be told apart by what they do to a reagent, and the tests follow directly from the bonding: a saturated hydrocarbon has nothing to add to, while an unsaturated one takes up the reagent and destroys its colour.
Four tests are used, and they are collected in Table 12.14. Observe first, and conclude afterwards.
Table 12.14 — Tests For Alkanes, Alkenes And Alkynes
| Add a few drops of bromine solution in carbon tetrachloride to the hydrocarbon |
No change is observed. |
The reddish brown colour of the bromine solution is decolourised. |
The reddish brown colour is decolourised. |
| Add a few drops of alkaline potassium permanganate, which is purple, to the hydrocarbon |
No change is observed. |
The purple colour fades. |
The purple colour fades. |
| Add ammoniacal cuprous chloride |
No change is observed. |
No change is observed. |
A red precipitate of copper acetylide is formed. |
| Add ammoniacal silver nitrate |
No change. |
No change. |
A white precipitate of silver acetylide is formed. |
The first two tests separate the saturated from the unsaturated. In both, the reagent is taken up by the double or triple bond, and since the product is colourless, the colour of the reagent disappears. An alkane, having no such bond, leaves the reagent untouched.
The second test is called Baeyer’s test. Alkaline potassium permanganate supplies oxygen, shown as \([\mathrm{O}]\), and in the case of ethyne the oxidation goes as far as oxalic acid:
\[\mathrm{CH\equiv CH} + 4[\mathrm{O}] \xrightarrow{\ \text{alkaline } \mathrm{KMnO_4}\ } \mathrm{COOH\text{-}COOH}\]
Distinguishing An Alkyne From An Alkene
The first two tests are passed by both an alkene and an alkyne, so a third is needed to separate them. That is the work of the last two rows of the table.
Ethyne carries a hydrogen atom on the triply bonded carbon which can be replaced by a metal, and neither ethene nor ethane can do this. With ammoniacal cuprous chloride, ethyne therefore gives a red precipitate of copper acetylide; with ammoniacal silver nitrate, it gives a white precipitate of silver acetylide. Neither reagent gives any change with an alkene or an alkane.
These precipitates are washed away with dilute acid as soon as the test is read, and are never collected or allowed to dry, since dry metal acetylides are unstable.
Ammoniacal Silver Nitrate Silver nitrate solution with ammonia added → gives a white precipitate of silver acetylide with ethyne → no change with ethene or ethane. Reach for it only after a decolourisation test has already shown the gas to be unsaturated.
Identifying An Unknown Gas
1. A colourless gas X decolourises bromine solution in carbon tetrachloride and fades the purple colour of alkaline potassium permanganate. It gives no precipitate with ammoniacal silver nitrate. Identify X and give a reason for each conclusion.
The bromine solution is decolourised, so X takes up bromine by addition. It must therefore contain a double or a triple bond, and cannot be an alkane.
Alkaline potassium permanganate is also decolourised, which confirms the same conclusion — X is unsaturated.
Ammoniacal silver nitrate gives no precipitate, so X has no hydrogen atom on a triply bonded carbon. This rules out ethyne.
X is unsaturated but is not an alkyne, so it is an alkene. The gas is ethene, \(\mathrm{C_2H_4}\).
You are given three jars containing ethane, ethene and ethyne. Describe, with the observation in each case, how you would identify all three using only two reagents.
12.21 Alcohols
Alcohols are the hydroxyl derivatives of alkanes. They are formed by replacing one or more hydrogen atoms of the alkane with an \(\mathrm{-OH}\) group, and it is that group which gives the whole family its chemistry.
12.21.1 Homologous Series Of Alcohols
Replacing one hydrogen atom of \(\mathrm{C_nH_{2n+2}}\) by \(\mathrm{-OH}\) gives the general formula \(\mathrm{C_nH_{2n+1}OH}\). The first four members are given in Table 12.15, and their structural formulae are drawn in Fig. 12.21-1.
| Methyl alcohol |
\(\mathrm{CH_3OH}\) |
\(\mathrm{CH_3-OH}\) |
Methanol |
| Ethyl alcohol |
\(\mathrm{C_2H_5OH}\) |
\(\mathrm{CH_3-CH_2-OH}\) |
Ethanol |
| Propyl alcohol |
\(\mathrm{C_3H_7OH}\) |
\(\mathrm{CH_3-CH_2-CH_2-OH}\) |
Propanol |
| Butyl alcohol |
\(\mathrm{C_4H_9OH}\) |
\(\mathrm{CH_3-CH_2-CH_2-CH_2-OH}\) |
Butanol |
Table 12.15 : Homologous series of alcohols, general formula \(\mathrm{C_nH_{2n+1}OH}\)
Notice how the IUPAC names are built. The chain gives the root, the single bonds give the primary suffix -ane, and the \(\mathrm{-OH}\) group gives the secondary suffix -ol, so methane becomes methanol and ethane becomes ethanol.
The \(\mathrm{-OH}\) Group And The \(\mathrm{OH^{-}}\) Ion Sodium hydroxide contains the hydroxide ion \(\mathrm{OH^{-}}\) and is an alkali. An alcohol holds its \(\mathrm{-OH}\) by a covalent bond and does not release it, so ethanol is neutral to litmus and does not behave as an alkali at all.
12.21.2 Monohydric, Dihydric And Trihydric Alcohols
An alkane has several hydrogen atoms, and more than one of them may be replaced. Alcohols are therefore classified by how many \(\mathrm{-OH}\) groups they carry.
Alcohols can be monohydric, that is, with one \(\mathrm{-OH}\) group attached to the carbon atom. Methyl alcohol, \(\mathrm{CH_3OH}\), and ethyl alcohol, \(\mathrm{C_2H_5OH}\), are examples.
They can be dihydric, with two \(\mathrm{-OH}\) groups attached to the carbon atoms. Glycol, \(\mathrm{OH-H_2C-CH_2-OH}\), is the example, and its IUPAC name is ethane-1,2-diol. This is the compound met earlier as the product of oxidising ethene with cold alkaline potassium permanganate.
They can also be trihydric, with three \(\mathrm{-OH}\) groups attached to the carbon atoms. Glycerol is the familiar one:
\[\mathrm{CH_2(OH)-CH(OH)-CH_2(OH)}\]
Its IUPAC name is propane-1,2,3-triol. The name is read straight off the structure — three carbon atoms give ‘propane’, and hydroxyl groups on carbons 1, 2 and 3 give ‘1,2,3-triol’.
Write the molecular formula of the alcohol with five carbon atoms and one \(\mathrm{-OH}\) group, and give its IUPAC name. Then state how many \(\mathrm{-OH}\) groups are present in a dihydric alcohol and name one such alcohol.
12.22 Ethanol
12.22.1 Occurrence Of Ethanol
Since alcohols are not found naturally in the earth’s atmosphere, they are obtained by artificial synthesis. Methanol, called wood spirit, is obtained from the destructive distillation of wood, while ethanol is obtained by the fermentation of sugar.
glucose → ethanol + carbon dioxide
\[\mathrm{C_6H_{12}O_6}\text{(aq)} \xrightarrow{\ \text{enzymes},\ 37^\circ\text{C}\ } 2\,\mathrm{C_2H_5OH} + 2\,\mathrm{CO_2}\uparrow\]
Fermentation is an anaerobic process: it takes place under conditions where there is no air or oxygen available. The temperature of \(37\ ^\circ\text{C}\) is the temperature at which the enzymes of yeast work best.
Ethyl alcohol is an essential constituent of all wines and is called spirit of wine. It is also obtained from starchy grain, and for that reason is known as grain alcohol. A second route begins with petroleum: cracking gives ethane, and ethane can be converted into ethanol.
12.22.2 Isomerism In Alcohols
Methanol and ethanol do not show isomerism. There is only one place an \(\mathrm{-OH}\) group can go on a one-carbon or a two-carbon chain, so no alternative arrangement exists.
Alcohols with three or more carbon atoms do form isomers. Propanol has two, differing in the position of the \(\mathrm{-OH}\) group:
- \(\mathrm{H_3C-CH_2-CH_2OH}\), propan-1-ol
- \(\mathrm{H_3C-CH(OH)-CH_3}\), propan-2-ol
Butanol has four, since the chain may now be branched as well:
- \(\mathrm{H_3C-CH_2-CH_2-CH_2OH}\), butan-1-ol
- \(\mathrm{H_3C-CH_2-CH(OH)-CH_3}\), butan-2-ol
- \(\mathrm{H_3C-CH(CH_3)-CH_2OH}\), 2-methylpropan-1-ol
- \(\mathrm{(CH_3)_3C-OH}\), 2-methylpropan-2-ol
The first two of the butanols are position isomers of each other, and the last two have a branched chain, so they are chain isomers of the first two.
12.22.3 Laboratory Preparation By Hydrolysis Of Alkyl Halides
Alcohol can be prepared by the hydrolysis of alkyl halides, that is haloalkanes, on reaction with a hot dilute alkali, or when an alkyl halide is boiled with aqueous alkalis. The halogen atom is replaced by an \(\mathrm{-OH}\) group, so this is a substitution reaction.
| \(\mathrm{RX} + \mathrm{KOH}\text{(aq)}\) |
boil |
\(\mathrm{ROH} + \mathrm{KX}\) |
| \(\mathrm{CH_3Br} + \mathrm{KOH}\text{(aq)}\) |
boil |
\(\mathrm{CH_3OH} + \mathrm{KBr}\) |
| \(\mathrm{C_2H_5Cl} + \mathrm{KOH}\text{(aq)}\) |
boil |
\(\mathrm{C_2H_5OH} + \mathrm{KCl}\) |
| \(\mathrm{C_2H_5Br} + \mathrm{NaOH}\text{(aq)}\) |
boil |
\(\mathrm{C_2H_5OH} + \mathrm{NaBr}\) |
Table 12.16 : Preparation of alcohols by the hydrolysis of alkyl halides
Here R stands for an alkyl group and X for a halogen. Potassium hydroxide reacts differently in aqueous and in alcoholic solution, and the difference decides the product.
Aqueous potassium hydroxide is mild in its action and shows substitution reactions:
\[\mathrm{C_2H_5Br} + \mathrm{KOH}\text{(aq)} \xrightarrow{\ \Delta\ } \mathrm{C_2H_5OH} + \mathrm{KBr}\]
Alcoholic potassium hydroxide is strong in its action and undergoes elimination reactions:
\[\mathrm{C_2H_5Br} + \mathrm{KOH}\ (\text{alc.}) \rightarrow \mathrm{C_2H_4} + \mathrm{KBr} + \mathrm{H_2O}\]
Aqueous KOH And Alcoholic KOH The same alkali, two different products. Dissolved in water it substitutes, replacing the halogen with \(\mathrm{-OH}\) to give an alcohol. Dissolved in alcohol, hot and concentrated, it eliminates \(\mathrm{HBr}\) to give an alkene. Always read which solvent is stated.
12.22.4 Industrial Preparation By Hydration Of Ethene
On the industrial scale ethanol is made not from a halide but from ethene, which is obtained from the cracking of petroleum.
When concentrated sulphuric acid is added to ethene at a temperature of \(80\ ^\circ\text{C}\) and a pressure of 30 atm, ethyl hydrogen sulphate is produced.
\[\mathrm{C_2H_4} + \mathrm{H_2SO_4} \xrightarrow{\ 80^\circ\text{C},\ 30\ \text{atm}\ } \mathrm{C_2H_5HSO_4}\]
The ethyl hydrogen sulphate, on hydrolysis with boiling water, gives ethanol and returns the acid.
\[\mathrm{C_2H_5HSO_4} + \mathrm{H_2O} \xrightarrow{\ \text{boiling}\ } \mathrm{C_2H_5OH} + \mathrm{H_2SO_4}\]
Alternatively, ethanol is produced when ethene is heated with water at \(300\ ^\circ\text{C}\) and 60 atmospheric pressure, in the presence of phosphoric acid as catalyst.
\[\mathrm{C_2H_4} + \mathrm{H_2O} \xrightarrow{\ \mathrm{H_3PO_4},\ 300^\circ\text{C},\ 60\ \text{atm}\ } \mathrm{C_2H_5OH}\]
Compare this with the dehydration in section 12.17.2 and the pair is a matched set: taking water out of ethanol gives ethene, and adding water to ethene gives ethanol back.
12.22.5 Physical Properties Of Ethanol
- Nature : Alcohols are inflammable volatile liquids.
- Boiling point : Their boiling point increases with an increase in molecular weight. Methanol boils at \(64.5\ ^\circ\text{C}\) and ethanol at \(78.3\ ^\circ\text{C}\).
- Solubility : They are soluble in water as well as in organic solvents.
- Density : Ethanol is lighter than water; its specific gravity is 0.79 at 293 K.
- They are colourless, have a faint odour and a burning taste.
- Some are toxic. Methyl alcohol, if consumed, causes blindness and even death.
Ethyl alcohol affects the part of the brain that controls muscular movement. It gives temporary relief from tiredness, but in excess quantities it damages the liver and the kidney.
12.22.6 Chemical Properties Of Ethanol
Alcohols are a reactive class of compounds, and all their characteristic reactions involve the \(\mathrm{-OH}\) group.
Combustion : Alcohol burns readily in air to produce carbon dioxide and water vapour, and a lot of heat is produced during the combustion. Ethanol burns with a pale blue flame.
\[\mathrm{C_2H_5OH} + 3\,\mathrm{O_2} \rightarrow 2\,\mathrm{CO_2} + 3\,\mathrm{H_2O} + \Delta\]
Oxidation : Alcohols react with oxidising agents like acidified potassium dichromate and acidified potassium permanganate. These oxidising agents supply nascent oxygen, \([\mathrm{O}]\), so that alcohols first get converted into aldehydes and later change into their respective acids.
alcohol \(\xrightarrow{[\mathrm{O}]}\) aldehyde \(\xrightarrow{[\mathrm{O}]}\) acid
\[\mathrm{CH_3OH} \xrightarrow{\ [\mathrm{O}],\ \mathrm{K_2Cr_2O_7}\ } \mathrm{HCHO} + \mathrm{H_2O} \xrightarrow{\ [\mathrm{O}]\ } \mathrm{HCOOH}\]
\[\mathrm{C_2H_5OH} \xrightarrow{\ [\mathrm{O}],\ \mathrm{K_2Cr_2O_7}\ } \mathrm{CH_3CHO} + \mathrm{H_2O} \xrightarrow{\ [\mathrm{O}]\ } \mathrm{CH_3COOH}\]
Methanol gives methanal and then methanoic acid; ethanol gives ethanal and then ethanoic acid. The chain length never changes — only the group at its end.
Acidified Potassium Dichromate An orange solution that supplies nascent oxygen \([\mathrm{O}]\) → oxidises an alcohol first to an aldehyde and then to a carboxylic acid → the orange colour changes to green as the dichromate is reduced.
Q5. Wine left standing open begins to taste sour, like vinegar. What could the air be doing to the alcohol in it?
A. Oxygen from the air oxidises the ethanol in the wine, in the same way that a laboratory oxidising agent does, converting it to ethanoic acid. Ethanoic acid is the acid in vinegar, so an opened bottle left standing for a few days really does begin to taste of vinegar.
Action with sodium : When sodium reacts with methyl alcohol or ethyl alcohol at room temperature, hydrogen is evolved, with the formation of sodium methoxide and sodium ethoxide respectively.
\[2\,\mathrm{CH_3OH} + 2\,\mathrm{Na} \xrightarrow{\ \Delta\ } 2\,\mathrm{CH_3ONa} + \mathrm{H_2}\uparrow\]
\[2\,\mathrm{C_2H_5OH} + 2\,\mathrm{Na} \xrightarrow{\ \Delta\ } 2\,\mathrm{C_2H_5ONa} + \mathrm{H_2}\uparrow\]
The effervescence of hydrogen serves as a test for alcohols. Only the hydrogen of the \(\mathrm{-OH}\) group is replaced; the hydrogen atoms attached to carbon are untouched.
Reaction with acetic acid : When alcohols react with ethanoic (acetic) acid in the presence of concentrated sulphuric acid at high temperature, the respective esters are produced, and the process is known as esterification.
\[\mathrm{CH_3OH} + \mathrm{CH_3COOH} \xrightarrow{\ \text{conc. } \mathrm{H_2SO_4}\ } \mathrm{CH_3COOCH_3} + \mathrm{H_2O}\]
\[\mathrm{C_2H_5OH} + \mathrm{CH_3COOH} \xrightarrow{\ \text{conc. } \mathrm{H_2SO_4}\ } \mathrm{CH_3COOC_2H_5} + \mathrm{H_2O}\]
Esters can be detected by their fruity smell, and the two products above are methyl acetate and ethyl acetate.
Dehydration with concentrated sulphuric acid : When concentrated sulphuric acid is added to ethyl alcohol and the mixture is heated up to \(170\ ^\circ\text{C}\), dehydration gives ethene, as described in section 12.17.2. Ethanol can also be dehydrated by aluminium oxide at \(350\ ^\circ\text{C}\).
At the lower temperature of \(140\ ^\circ\text{C}\), however, concentrated sulphuric acid dehydrates alcohol to give the respective ether instead. Two alcohol molecules lose one molecule of water between them.
\[2\,\mathrm{CH_3OH} \xrightarrow{\ \text{conc. } \mathrm{H_2SO_4},\ 140^\circ\text{C}\ } \mathrm{CH_3-O-CH_3} + \mathrm{H_2O}\]
\[2\,\mathrm{C_2H_5OH} \xrightarrow{\ \text{conc. } \mathrm{H_2SO_4},\ 140^\circ\text{C}\ } \mathrm{C_2H_5-O-C_2H_5} + \mathrm{H_2O}\]
The temperature, and nothing else, decides which product is obtained — \(140\ ^\circ\text{C}\) for the ether, \(170\ ^\circ\text{C}\) for the alkene.
Identifying A Compound From Its Reactions
1. A colourless liquid A reacts with sodium metal at room temperature, giving off a gas that burns with a pop sound. On warming with acidified potassium dichromate, A gives a compound B, which on further oxidation gives a compound C with a vinegar-like smell. A contains two carbon atoms. Identify A, B and C.
The gas given off with sodium burns with a pop, so it is hydrogen. Of the compounds studied, an alcohol releases hydrogen with sodium at room temperature, so A carries an \(\mathrm{-OH}\) group.
A has two carbon atoms, so A is ethanol, \(\mathrm{C_2H_5OH}\).
Oxidation of an alcohol gives first the aldehyde, so B is ethanal, \(\mathrm{CH_3CHO}\); further oxidation gives the acid, so C is ethanoic acid, \(\mathrm{CH_3COOH}\). The vinegar smell confirms C.
12.22.8 Uses Of Ethanol
- It is a good solvent for gums and resins.
- It is used in thermometers and as a preservative for biological specimens, due to its low freezing point.
- It is used in the manufacture of chemicals such as chloroform, iodoform, ether and acetic acid, and of synthetic products like dyes, esters used as perfumes, antiseptics and preservatives.
- It is used in alcoholic drinks. Beers contain about 4% ethanol, wines about 11%, and spirits such as rum, brandy and whisky 40% or more.
- It is used as a fuel in powdered form, and as an antifreeze for automobile radiators.
Write balanced equations for the reaction of ethanol with (i) sodium, (ii) ethanoic acid in the presence of concentrated sulphuric acid. Name the product in each case, and state one observation you would make in each.
12.23 Carboxylic Acids
An organic compound containing the carboxyl group, \(\mathrm{-COOH}\), is known as a carboxylic acid. These compounds possess acidic properties, and the carboxyl group is the reason for it.
Carboxylic Acid A carboxylic acid is an organic compound containing the carboxyl group \(\mathrm{-COOH}\), in which a carbon atom is joined to one oxygen atom by a double bond and to a hydroxyl group by a single bond. Its general formula is \(\mathrm{C_nH_{2n+1}COOH}\), written for short as \(\mathrm{RCOOH}\).
The name of the group records how it is built. It holds a carbonyl part, \(\mathrm{>C=O}\), and a hydroxyl part, \(\mathrm{-OH}\), on the same carbon atom — ‘carb’ from one and ‘oxyl’ from the other. That neighbouring double-bonded oxygen is what loosens the hydrogen of the \(\mathrm{-OH}\) group and lets the molecule behave as an acid, which a plain alcohol cannot do.
12.23.1 Monocarboxylic And Dicarboxylic Acids
An organic compound may contain one or more \(\mathrm{-COOH}\) groups. Those which contain one \(\mathrm{-COOH}\) group are called monocarboxylic acids. Methanoic acid, \(\mathrm{HCOOH}\), known commonly as formic acid, and ethanoic acid, \(\mathrm{CH_3COOH}\), known commonly as acetic acid, are the two met most often.
Those which contain two \(\mathrm{-COOH}\) groups are called dicarboxylic acids. Oxalic acid is the example, written \(\mathrm{COOH-COOH}\) or \(\mathrm{(COOH)_2}\); its IUPAC name is ethanedioic acid. It is the same acid obtained when ethyne is oxidised by alkaline potassium permanganate.
12.23.2 Homologous Series Of Monocarboxylic Acids
The IUPAC name of a carboxylic acid is given by replacing the final ‘e’ of the corresponding alkane by ‘oic acid’. Ethane becomes ethanoic acid, propane becomes propanoic acid. This is done because the acids are derived from alkanes by replacing one hydrogen atom with the carboxyl group, and for that reason they are also known as alkanoic acids.
The first four members are given in Table 12.17.
| \(\mathrm{HCOOH}\) |
Formic acid |
Methanoic acid |
| \(\mathrm{CH_3COOH}\) |
Acetic acid |
Ethanoic acid |
| \(\mathrm{CH_3CH_2COOH}\) |
Propionic acid |
Propanoic acid |
| \(\mathrm{CH_3CH_2CH_2COOH}\) |
Butyric acid |
Butanoic acid |
Table 12.17 : Homologous series of monocarboxylic acids
Count the carbon atoms in the table and one point needs care: the carbon of the \(\mathrm{-COOH}\) group is counted as part of the chain. Ethanoic acid has two carbon atoms in all, one in the methyl group and one in the carboxyl group, which is why its root word is ‘eth’ and not ‘meth’.
Butter And Butanoic Acid Butyric acid takes its common name from the Latin butyrum, meaning butter. It is present in butter, and it is the compound largely responsible for the sharp smell of butter that has gone rancid.
Write the formula and the IUPAC name of the monocarboxylic acid containing five carbon atoms. State how many \(\mathrm{-COOH}\) groups are present in a dicarboxylic acid, and name one such acid.
12.24 Ethanoic Acid
12.24.1 Structure And Occurrence Of Ethanoic Acid
Ethanoic acid, \(\mathrm{CH_3COOH}\), is the second member of the carboxylic acid series and by far the most familiar. Its structure is a methyl group joined to a carboxyl group, as drawn in Fig. 12.24-1.
Ethanoic acid occurs in the free state in many fruits, and as esters in a number of essential oils.
A dilute solution of ethanoic acid, of about 4 to 5 percent, is what is called vinegar. The presence of a colouring matter gives vinegar its greyish, reddish or brownish colour, while the presence of other organic compounds gives it the usual taste and flavour. Vinegar is used for flavouring and for preserving foods.
12.24.2 Physical Properties Of Ethanoic Acid
- Physical state : Pure ethanoic acid is a colourless liquid.
- Odour : It has a characteristic pungent smell — the smell of vinegar.
- Melting point : The anhydrous acid, on cooling, forms a crystalline mass resembling ice. Its melting point is \(17\ ^\circ\text{C}\), and for this reason it is called glacial acetic acid.
- Boiling point : It boils at \(118\ ^\circ\text{C}\).
- Nature : It is a hygroscopic liquid, and its specific gravity at \(0\ ^\circ\text{C}\) is 1.08.
- Solubility : It is miscible with water, alcohol and ether in all proportions.
A melting point of \(17\ ^\circ\text{C}\) is worth pausing on. On a cold day the temperature of a laboratory may fall below it, and a bottle of the pure acid will then set into ice-like crystals — which is exactly what the word ‘glacial’ records.
A Weak Acid And A Dilute Acid ‘Weak’ describes the acid itself: ethanoic acid only partly ionises in water, however much is present. ‘Dilute’ describes the solution: plenty of water to little acid. Vinegar is both weak and dilute; glacial acetic acid is weak but not dilute at all.
12.24.3 Chemical Properties Of Ethanoic Acid
1. It is a weak acid, and behaves like one in four ways.
It turns moist blue litmus red.
It reacts with active metals such as zinc and magnesium, evolving hydrogen.
ethanoic acid + zinc → zinc acetate + hydrogen
\[2\,\mathrm{CH_3COOH} + \mathrm{Zn} \rightarrow \mathrm{(CH_3COO)_2Zn} + \mathrm{H_2}\uparrow\]
\[2\,\mathrm{CH_3COOH} + \mathrm{Mg} \rightarrow \mathrm{(CH_3COO)_2Mg} + \mathrm{H_2}\uparrow\]
It reacts with alkalis to form a salt and water.
\[\mathrm{CH_3COOH} + \mathrm{NaOH} \rightarrow \mathrm{CH_3COONa} + \mathrm{H_2O}\]
\[\mathrm{CH_3COOH} + \mathrm{NH_4OH} \rightarrow \mathrm{CH_3COONH_4} + \mathrm{H_2O}\]
It liberates brisk effervescence of carbon dioxide with carbonates and with hydrogen carbonates.
\[2\,\mathrm{CH_3COOH} + \mathrm{Na_2CO_3} \rightarrow 2\,\mathrm{CH_3COONa} + \mathrm{H_2O} + \mathrm{CO_2}\uparrow\]
\[\mathrm{CH_3COOH} + \mathrm{NaHCO_3} \rightarrow \mathrm{CH_3COONa} + \mathrm{H_2O} + \mathrm{CO_2}\uparrow\]
The gas given off is identified in the usual way: it turns lime water milky, and is therefore carbon dioxide.
2. Ester formation : Ethanoic acid forms an ester, a pleasant fruity smelling compound, on reacting with an alcohol in the presence of a dehydrating agent like concentrated sulphuric acid. The reaction is known as esterification.
\[\mathrm{CH_3COOH} + \mathrm{HOC_2H_5} \xrightleftharpoons[\ \ ]{\ \text{conc. } \mathrm{H_2SO_4}\ } \mathrm{CH_3COOC_2H_5} + \mathrm{H_2O}\]
The product is ethyl acetate, or ethyl ethanoate, whose structure is drawn in Fig. 12.24-2. Notice the double arrow: the reaction is reversible, and the ester and water can react back to give the acid and the alcohol. This is prevented by using concentrated sulphuric acid, which removes the water as it forms and so drives the change forward.
3. Formation of acetic anhydride : It forms acetic anhydride on heating with phosphorus pentoxide, as it loses a water molecule.
\[2\,\mathrm{CH_3COOH} \xrightarrow{\ \mathrm{P_2O_5},\ \Delta\ } \mathrm{(CH_3CO)_2O} + \mathrm{H_2O}\]
4. Reduction : A strong reducing agent such as lithium aluminium hydride, \(\mathrm{LiAlH_4}\), will reduce ethanoic acid to ethanol.
\[\mathrm{CH_3COOH} + 4[\mathrm{H}] \rightarrow \mathrm{C_2H_5OH} + \mathrm{H_2O}\]
This is the reverse of the oxidation met in Section 12.22.6, where ethanol was carried up to ethanoic acid. Oxidation climbs from alcohol to acid; reduction climbs back down.
12.24.4 Tests For Ethanoic Acid
- On adding ethanoic acid to carbonates and bicarbonates, carbon dioxide is evolved, seen as brisk effervescence.
- When warmed with ethyl alcohol and concentrated sulphuric acid, a pleasant fruity smell of ethyl acetate is produced.
- On adding neutral iron(III) chloride, a wine red colour is produced.
Neutral Iron(III) Chloride A pale yellow solution, neutralised before use → add it to a solution suspected of containing ethanoic acid or its salt → a wine red colour appears. Used to confirm the acid once effervescence with a carbonate has shown that an acid is present.
Identifying A Liquid From Its Reactions
1. A colourless liquid with a pungent smell turns blue litmus red, gives brisk effervescence with sodium hydrogen carbonate, and gives a sweet fruity smell when warmed with ethanol and a little concentrated sulphuric acid. Identify the liquid and name the gas and the fruity compound.
Turning blue litmus red shows an acid. Effervescence with a hydrogen carbonate shows that the gas released is carbon dioxide, which would turn lime water milky; so the substance is an acid that attacks carbonates.
The fruity smell on warming with an alcohol and concentrated sulphuric acid is the test for esterification, which is the reaction of a carboxylic acid.
The liquid is ethanoic acid, \(\mathrm{CH_3COOH}\). The gas is carbon dioxide, and the fruity compound is ethyl ethanoate, \(\mathrm{CH_3COOC_2H_5}\).
12.24.5 Uses Of Ethanoic Acid
Ethanoic acid is used
- as a solvent for phosphorus, sulphur, iodine, resins and cellulose;
- as a laboratory reagent;
- as vinegar;
- in dyes, perfumes and medicines, and in the production of esters;
- for coagulating rubber from latex;
- for the manufacture of cellulose acetate, which is used for making packaging materials, varnishes and rayon; of amyl acetate, used as a flavouring agent for chewing gums; and of acetic anhydride, which is used in making aspirin.
Vinegar In The Kitchen, Glacial Acetic Acid In The Laboratory The same compound behaves very differently at different concentrations. At 4 to 5 percent it is vinegar: it is safe to eat, and it preserves pickles because few micro-organisms will grow in an acid medium. Pure glacial acetic acid is corrosive, burns the skin, and gives off choking vapour, so it is handled in a fume cupboard and diluted by adding the acid to water, never water to the acid. Concentration, not the formula, decides how a substance must be treated.
Write balanced equations for the reaction of ethanoic acid with (i) magnesium, (ii) sodium hydroxide, (iii) sodium carbonate. Name the gas evolved in the reactions where one is produced, and give the test for it.
12.25 Summary
- The word ‘organic’ means pertaining to life. Substances obtained from plants and animals were called organic compounds, and those from minerals were called inorganic compounds.
- The vital force theory held that organic compounds could be made only under the influence of a vital force present in living things.
- The theory was discarded when Wöhler, in 1828, obtained urea by heating ammonium cyanate: \(\mathrm{NH_4CNO} \xrightarrow{\ \Delta\ } \mathrm{CO(NH_2)_2}\). Kolbe prepared acetic acid in 1845, and Berthelot synthesized methane in 1856.
- Organic chemistry is the study of carbon compounds, excluding the oxides of carbon, metallic carbonates, metal cyanides and metal carbides.
- Organic compounds are obtained from plants, animals, coal, petroleum, fermentation, wood and synthetic methods.
- Organic compounds are covalent: they have low melting and boiling points, are generally insoluble in water but soluble in organic solvents, burn easily, are non-electrolytes, and show isomerism.
- Carbon has atomic number 6 and electron arrangement 2, 4. It shares its four outer electrons and forms four covalent bonds, which is its tetravalency.
- Catenation is the self-linking of atoms of an element through covalent bonds to form straight, branched and cyclic chains. Carbon shows it best, because the carbon–carbon bond is strong and carbon is tetracovalent.
- Two carbon atoms may share one, two or three pairs of electrons, giving single, double and triple bonds. Carbon also forms \(\mathrm{>C=O}\) with oxygen and \(\mathrm{-C\equiv N}\) with nitrogen.
- Tetravalency, catenation and isomerism together account for the more than 10 million organic compounds known.
- Hydrocarbons are compounds made up only of carbon and hydrogen. They are aliphatic (open chain) or cyclic (closed chain).
- Aliphatic hydrocarbons are saturated (alkanes, \(\mathrm{C_nH_{2n+2}}\)) or unsaturated (alkenes, \(\mathrm{C_nH_{2n}}\), and alkynes, \(\mathrm{C_nH_{2n-2}}\)).
- Saturated hydrocarbons are less reactive and undergo substitution reactions; unsaturated hydrocarbons are more reactive and undergo addition reactions.
- In substitution, an atom of a molecule is replaced by another atom or group. In addition, atoms are added across a double or triple bond to give a saturated product.
- Cyclic hydrocarbons are alicyclic, or aromatic. An aromatic compound contains at least one benzene ring — six carbon atoms with alternate single and double bonds.
- A structural formula shows how the atoms are linked; a condensed formula shortens it; a carbon skeleton shows only the carbon linkages.
- Carbon atoms are primary, secondary, tertiary or quaternary according to whether they are joined to one, two, three or four other carbon atoms.
- An alkyl group is obtained by removing one hydrogen atom from an alkane, and has the general formula \(\mathrm{-C_nH_{2n+1}}\). It is named by changing -ane to -yl.
- A functional group is an atom or group of atoms responsible for the characteristic chemical properties of a compound. The alkyl part mainly decides physical properties.
- A homologous series is a group of compounds with the same general formula and similar chemical properties, in which successive members differ by a \(\mathrm{CH_2}\) group and by 14 a.m.u.
- Members of a homologous series show gradation in physical properties: melting point, boiling point and density increase with increasing molecular mass.
- An IUPAC name is built as: prefix + word root + primary suffix + secondary suffix. The root counts the carbon atoms of the longest chain, the primary suffix records the bond, and the secondary suffix names the functional group.
- The chain is numbered so that the functional group, and then the substituents, receive the smallest possible numbers; when two or more substituents are present the lowest sum rule applies and they are named alphabetically.
- Isomers have the same molecular formula but different structural formulae. Chain isomers differ in the carbon framework; position isomers differ in the position of a bond or a group.
- Alkanes are saturated hydrocarbons, relatively unreactive, and are also called paraffins. Their sources are natural gas and petroleum.
- Methane is prepared in the laboratory by heating sodium ethanoate with soda lime, a reaction called decarboxylation: \[\mathrm{CH_3COONa} + \mathrm{NaOH} \xrightarrow{\ \mathrm{CaO},\ 300^\circ\text{C}\ } \mathrm{Na_2CO_3} + \mathrm{CH_4}\uparrow\]
- Ethane is prepared in the same way from sodium propanoate, and also by the Wurtz reaction: \(2\,\mathrm{CH_3I} + 2\,\mathrm{Na} \xrightarrow{\ \text{dry ether}\ } \mathrm{C_2H_6} + 2\,\mathrm{NaI}\).
- The methane molecule is tetrahedral, with bond angles of \(109.5^\circ\); ethene is planar, with bond angles of \(120^\circ\); ethyne is linear.
- Alkanes burn completely in plenty of air to carbon dioxide and water, and incompletely in insufficient air to carbon monoxide, or to soot.
- Alkanes react with chlorine in sunlight by substitution, methane giving chloromethane, dichloromethane, trichloromethane and tetrachloromethane in turn.
- Ethene is prepared by dehydration of ethanol with concentrated sulphuric acid at \(170\ ^\circ\text{C}\), and by dehydrohalogenation of a haloethane with hot concentrated alcoholic potassium hydroxide.
- Ethene undergoes addition with hydrogen (giving ethane), with halogens (giving 1,2-dihaloethanes), with water (giving ethanol) and with hydrogen chloride; it also polymerises to polythene.
- Ethyne is prepared from calcium carbide and water: \(\mathrm{CaC_2} + 2\,\mathrm{H_2O} \rightarrow \mathrm{Ca(OH)_2} + \mathrm{C_2H_2}\uparrow\), and from 1,2-dibromoethane with alcoholic potassium hydroxide.
- Addition to ethyne takes place in two stages, giving first an ethene derivative and then an ethane derivative.
- Unsaturation is tested with bromine in carbon tetrachloride, whose reddish brown colour is decolourised, and with alkaline potassium permanganate, whose purple colour fades. Ethyne alone gives a red precipitate with ammoniacal cuprous chloride and a white precipitate with ammoniacal silver nitrate.
- Alcohols are hydroxyl derivatives of alkanes, general formula \(\mathrm{C_nH_{2n+1}OH}\). They are monohydric, dihydric or trihydric according to the number of \(\mathrm{-OH}\) groups.
- Ethanol is obtained by fermentation of sugar, by hydrolysis of an alkyl halide with aqueous alkali, and industrially by the hydration of ethene.
- Aqueous potassium hydroxide substitutes and gives an alcohol; alcoholic potassium hydroxide eliminates and gives an alkene.
- Ethanol burns with a pale blue flame, gives hydrogen with sodium, is oxidised by acidified potassium dichromate to ethanal and then to ethanoic acid, forms esters with acids, and is dehydrated by concentrated sulphuric acid to ethene at \(170\ ^\circ\text{C}\) and to ether at \(140\ ^\circ\text{C}\).
- Denatured alcohol, or methylated spirit, is ethanol made undrinkable with methyl alcohol, a dye and pyridine. Spurious alcohol contains methanol and is fatal if consumed.
- Carboxylic acids contain the group \(\mathrm{-COOH}\) and have the general formula \(\mathrm{RCOOH}\). Their IUPAC names end in -oic acid.
- Pure ethanoic acid melts at \(17\ ^\circ\text{C}\) to an ice-like solid and is called glacial acetic acid; a 4 to 5 percent solution is vinegar.
- Ethanoic acid is a weak acid: it turns blue litmus red, gives hydrogen with active metals, gives a salt and water with alkalis, and gives carbon dioxide with carbonates and hydrogen carbonates.
- Ethanoic acid reacts with an alcohol in the presence of concentrated sulphuric acid to form an ester, a reversible reaction called esterification: \[\mathrm{CH_3COOH} + \mathrm{C_2H_5OH} \xrightleftharpoons[\ \ ]{\ \text{conc. } \mathrm{H_2SO_4}\ } \mathrm{CH_3COOC_2H_5} + \mathrm{H_2O}\]
- Ethanoic acid is identified by effervescence with a carbonate, by the fruity smell of an ester on warming with ethanol and concentrated sulphuric acid, and by the wine red colour with neutral iron(III) chloride.
12.26 Key Terms
- Abbreviated formula — a shortened structural formula in which the hydrogen atoms on each carbon are written after it, as in \(\mathrm{CH_3CH_2CH_3}\).
- Acetylenic bond — the triple bond between two carbon atoms, \(\mathrm{-C\equiv C-}\), characteristic of alkynes.
- Addition reaction — a reaction in which atoms or molecules add across a double or triple bond to give a saturated product.
- Alcohols — hydroxyl derivatives of alkanes, of general formula \(\mathrm{C_nH_{2n+1}OH}\).
- Alicyclic compounds — ringed carbon compounds containing three or more carbon atoms arranged in a closed structure.
- Aliphatic compounds — open chain compounds, in which the carbon chain has two ends.
- Alkanes — saturated hydrocarbons in which all the linkages between carbon atoms are single covalent bonds; general formula \(\mathrm{C_nH_{2n+2}}\).
- Alkanoic acids — another name for the carboxylic acids, since they are derived from alkanes by replacing one hydrogen atom with a carboxyl group.
- Alkenes — unsaturated hydrocarbons containing a carbon–carbon double bond; general formula \(\mathrm{C_nH_{2n}}\).
- Alkyl group — the fragment left when one hydrogen atom is removed from an alkane; general formula \(\mathrm{-C_nH_{2n+1}}\).
- Alkyl halide — a compound in which one or more hydrogen atoms of an alkane have been replaced by halogen atoms.
- Alkynes — unsaturated hydrocarbons containing a carbon–carbon triple bond; general formula \(\mathrm{C_nH_{2n-2}}\).
- Aromatic compounds — compounds whose molecules contain at least one benzene ring.
- Benzene ring — a ring of six carbon atoms with carbon–carbon single and double bonds in alternate positions.
- Carbocyclic compounds — cyclic compounds whose ring is built entirely of carbon atoms.
- Carbon skeleton — a structure showing only the linking of the carbon atoms in a molecule.
- Carboxyl group — the group \(\mathrm{-COOH}\), in which one carbon carries both a double-bonded oxygen and a hydroxyl group.
- Carboxylic acid — an organic compound containing the carboxyl group; general formula \(\mathrm{RCOOH}\).
- Catenation — the self-linking of atoms of an element through covalent bonds to form straight, branched and cyclic chains.
- Chain isomerism — isomerism in which compounds of the same molecular formula differ in the arrangement of the carbon chain.
- Common names — names given to compounds from their source or their history, rather than by rule; also called trivial names.
- Cyclic compounds — closed chain compounds, in which the carbon chain has joined up into a ring.
- Decarboxylation — the removal of carbon dioxide from a carboxylic acid or its salt, as when sodium ethanoate and soda lime give methane.
- Dehydration — the removal of a molecule of water from a compound, as when ethanol gives ethene.
- Dicarboxylic acids — carboxylic acids containing two \(\mathrm{-COOH}\) groups, such as oxalic acid.
- Dihydric alcohol — an alcohol carrying two hydroxyl groups, such as glycol.
- Ester — a compound formed when a carboxylic acid reacts with an alcohol, with the elimination of water.
- Esterification — the reaction of a carboxylic acid with an alcohol, in the presence of concentrated sulphuric acid, to form an ester.
- Fermentation — the anaerobic breaking down of sugar by enzymes to give ethanol and carbon dioxide.
- Fire-damp — the name given to the methane found in cavities in coal.
- Functional group — an atom or group of atoms responsible for the characteristic chemical properties of an organic compound.
- Glacial acetic acid — pure ethanoic acid, so called because it freezes at \(17\ ^\circ\text{C}\) to an ice-like crystalline mass.
- Homologous series — a group of compounds with the same general formula and similar chemical properties, whose successive members differ by a \(\mathrm{CH_2}\) group.
- Hydrocarbons — compounds made up only of carbon and hydrogen atoms.
- Hydrolysis — the replacement of a halogen atom of an alkyl halide by a hydroxyl group on boiling with aqueous alkali.
- Inorganic chemistry — the branch of chemistry dealing with compounds obtained from minerals and other non-living sources.
- Inorganic compounds — compounds obtained from minerals and non-living sources; the carbon compounds excluded from organic chemistry.
- Isomerism — the phenomenon in which two or more compounds have the same molecular formula but different structural formulae.
- Isomers — the compounds that show isomerism.
- Linear molecule — a molecule in which all the atoms lie in one straight line, as in ethyne.
- Monocarboxylic acids — carboxylic acids containing one \(\mathrm{-COOH}\) group.
- Monohydric alcohol — an alcohol carrying one hydroxyl group, such as ethanol.
- Nomenclature — the system of assigning names to organic compounds.
- Olefins — another name for the alkenes, meaning oil-forming.
- Organic chemistry — the study of carbon compounds, excluding the oxides of carbon, metallic carbonates, metal cyanides and metal carbides.
- Organic compounds — the compounds of carbon.
- Paraffins — another name for the alkanes, from Latin words meaning little affinity, since they are relatively unreactive.
- Planar molecule — a flat molecule, all of whose atoms lie in one plane, as in ethene.
- Polythene — the polymer formed when many molecules of ethene join together.
- Position isomerism — isomerism in which compounds of the same molecular formula differ in the position of a substituent or a functional group.
- Prefix — the part of an IUPAC name placed before the word root; the primary prefix shows whether the chain is open or closed, and the secondary prefix names the substituents.
- Primary carbon — a carbon atom linked to only one other carbon atom.
- Quaternary carbon — a carbon atom linked to four other carbon atoms.
- Root word — the part of an IUPAC name that records the number of carbon atoms in the longest chain.
- Secondary carbon — a carbon atom linked to two other carbon atoms.
- Structural formula — the formula that shows how atoms of different elements are linked together in a molecule.
- Substitution product — the compound formed when one atom of a molecule is replaced by another atom or group.
- Substitution reaction — a reaction in which one atom of a molecule is replaced by another atom or group of atoms.
- Suffix — the part of an IUPAC name placed after the word root; the primary suffix records the type of carbon–carbon bond and the secondary suffix names the functional group.
- Tertiary carbon — a carbon atom linked to three other carbon atoms.
- Tetrahedron — the pyramid-shaped, three-dimensional arrangement of the four bonds of carbon, as in methane.
- Tetravalency — the characteristic of the carbon atom by virtue of which it forms four covalent bonds.
- Trihydric alcohol — an alcohol carrying three hydroxyl groups, such as glycerol.
- Trivial names — see common names.
- Vital force theory — the discarded belief that organic compounds could be produced only under the influence of a vital force present in living organisms.
- Wurtz reaction — the reaction in which an alkyl halide is warmed with sodium in dry ether, joining two alkyl groups to give a larger alkane.
12.27 Points To Ponder
Wöhler’s single experiment was enough to unsettle the vital force theory, yet chemists went on to prepare acetic acid and methane before the old idea was fully abandoned. What does it take for one result to overturn a theory, and why might a single success not be enough?
Silicon sits directly below carbon and is also tetravalent, yet the chemistry of the earth’s crust is built on silicon while the chemistry of living things is built on carbon. What properties of the two elements might explain why life uses one and rock uses the other?
Homologous series make it possible to predict the properties of a compound nobody has yet prepared. How far should such a prediction be trusted, and what kind of property would you expect the prediction to get right, and what kind might it get wrong?
Both bromine solution and alkaline potassium permanganate show that a hydrocarbon is unsaturated, but neither tells a double bond from a triple bond. What does this suggest about the difference between a test that detects a class of compounds and a test that identifies one compound?
Ethanol and ethanoic acid are separated by nothing more than an oxidation, and the same conversion happens slowly in an open bottle of wine. Where else in this chapter does an industrial process do deliberately what nature does slowly, and what does the comparison suggest about what a chemist actually controls?
Concentrated sulphuric acid turns ethanol into an ether at \(140\ ^\circ\text{C}\) and into ethene at \(170\ ^\circ\text{C}\). If the reagents are identical and only the temperature differs, what must temperature be deciding about the reaction?
12.28 Objective Questions
Multiple Choice Questions
Choose the correct option.
- The general formula \(\mathrm{C_nH_{2n-2}}\) belongs to: [1]
- alkanes
- alkenes
- alkynes
- alcohols
- The compound that will decolourise bromine solution in carbon tetrachloride is: [1]
- \(\mathrm{CH_4}\)
- \(\mathrm{C_2H_6}\)
- \(\mathrm{C_2H_4}\)
- \(\mathrm{C_3H_8}\)
- Two adjacent members of a homologous series differ by a molecular mass of: [1]
- 12 a.m.u.
- 14 a.m.u.
- 16 a.m.u.
- 28 a.m.u.
- The IUPAC name of \(\mathrm{CH_3CH(OH)CH_3}\) is: [1]
- propan-1-ol
- propan-2-ol
- propanal
- propanoic acid
- The reagent used with sodium ethanoate for the laboratory preparation of methane is: [1]
- quicklime
- soda lime
- slaked lime
- lime water
- Concentrated sulphuric acid acts on ethanol at \(170\ ^\circ\text{C}\) to give: [1]
- ethane
- ethoxyethane
- ethene
- ethanoic acid
- The number of isomers of pentane is: [1]
- two
- three
- four
- five
- A carbon atom linked to four other carbon atoms is described as: [1]
- primary
- secondary
- tertiary
- quaternary
- Ethyne is obtained when water is added to: [1]
- calcium carbonate
- calcium carbide
- aluminium carbide
- calcium hydroxide
- Which of these is not excluded from organic chemistry? [1]
- carbon dioxide
- calcium carbonate
- methanol
- potassium cyanide
Assertion–Reason
For each of the following, choose from: (a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true but R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true.
Assertion (A) : Alkanes undergo substitution reactions. Reason (R) : In an alkane all four valencies of every carbon atom are satisfied by single covalent bonds. [1]
Assertion (A) : Ethene is a planar molecule. Reason (R) : All the bond angles in ethene are of \(109.5^\circ\). [1]
Assertion (A) : Ethanol is neutral to litmus although it contains an \(\mathrm{-OH}\) group. Reason (R) : The hydroxyl group in an alcohol is covalently bonded and is not released as the hydroxide ion. [1]
Assertion (A) : Bromoethane gives ethene when heated with alcoholic potassium hydroxide. Reason (R) : Alcoholic potassium hydroxide brings about elimination. [1]
Match The Following
- Match each reagent or condition in Column I with the product it gives from the starting material named. [5]
| a. Ethene with hydrogen over nickel at \(200\ ^\circ\text{C}\) |
i. Ethyl ethanoate |
| b. Ethanol with acidified potassium dichromate |
ii. 1,2-dibromoethane |
| c. Ethanoic acid with ethanol and conc. sulphuric acid |
iii. Ethane |
| d. Ethene with bromine |
iv. Ethyne |
| e. Calcium carbide with water |
v. Ethanoic acid |
Fill In The Blanks
The self-linking of carbon atoms to form long chains is called ……………. . [1]
An alkyl group is obtained by removing one ……………. atom from an alkane. [1]
The compounds ethanol and ethoxyethane have the same molecular formula and are therefore ……………. . [1]
Pure ethanoic acid is called ……………. acetic acid because it freezes to an ice-like solid. [1]
True Or False
Alkynes contain two carbon atoms fewer than the corresponding alkanes. [1]
Fermentation is an anaerobic process. [1]
Aromatic compounds are so called because every one of them has a pleasant smell. [1]
Name The Following
The hydrocarbon that is the principal component of natural gas. [1]
The gas evolved when sodium is dropped into ethanol. [1]
The alcohol whose consumption causes blindness. [1]
The reaction in which an alkyl halide is warmed with sodium in dry ether. [1]
Give One Word Or Phrase For
A compound containing carbon and hydrogen only. [1]
An atom or group of atoms responsible for the chemical properties of an organic compound. [1]
Compounds having the same molecular formula but different structural formulae. [1]
Odd One Out, With A Reason
\(\mathrm{C_2H_6}\), \(\mathrm{C_3H_8}\), \(\mathrm{C_4H_8}\), \(\mathrm{C_5H_{12}}\) [1]
Methanol, ethanol, propan-2-ol, ethanoic acid [1]
Bromine water in carbon tetrachloride, alkaline potassium permanganate, ammoniacal silver nitrate, lime water [1]
12.29 Check Your Concepts
Define catenation. [2]
State two characteristics of a homologous series. [2]
Give a reason why alkanes are described as saturated hydrocarbons. [2]
Why is soda lime, and not sodium hydroxide alone, used in the laboratory preparation of methane? [2]
Name the products formed when ethanol is oxidised in stages by acidified potassium dichromate. [2]
Give a reason why ethyne burns with a sooty flame while ethane burns with a clean blue flame. [3]
Differentiate between chain isomerism and position isomerism, stating the basis of difference and giving one example of each. [3]
Differentiate between a substitution reaction and an addition reaction, stating the basis of difference. [3]
Explain, with an example, why the compounds of a homologous series have similar chemical properties but different physical properties. [3]
Why does a molecule of methane take up a tetrahedral shape rather than a flat one? [3]
What would happen if a gas burner were used in a room with the air holes blocked? Give the chemistry behind your answer. [3]
Differentiate between glacial acetic acid and vinegar, stating the basis of difference, and explain why the terms ‘weak’ and ‘dilute’ are not interchangeable. [4]
Ethene, ethane and ethyne are supplied in three unlabelled jars. Describe how you would identify each, naming the reagents and stating the observations. [4]
Explain why potassium hydroxide gives an alcohol from bromoethane in one solvent and an alkene in another. Give both equations. [4]
Account for the fact that carbon forms a far greater number of compounds than any other element, giving three separate reasons and explaining each. [5]
Describe the laboratory preparation of ethene from ethanol, stating the reactants, the conditions, why the gas is passed through sodium hydroxide solution, and how it is collected. [5]
12.30 Equations, Conversions & Identification
Write the balanced equation for the complete combustion of ethane in air. [2]
Write the balanced equation for the laboratory preparation of ethane from sodium propanoate, stating the condition. [2]
Complete and balance: \(\mathrm{CaC_2} + \mathrm{H_2O} \rightarrow\) ……………. [2]
Complete and balance: \(\mathrm{CH_3COOH} + \mathrm{NaHCO_3} \rightarrow\) ……………. [2]
Complete and balance, stating the condition: \(\mathrm{C_2H_5Br} + \mathrm{KOH}\ (\text{alcoholic}) \rightarrow\) ……………. [2]
Name the gas evolved when ethanoic acid is added to sodium carbonate, and give the test by which you would identify it. [2]
State what you would see when bromine dissolved in carbon tetrachloride is added dropwise to ethene, and explain the observation. [2]
State what you would see when ethyne is passed through ammoniacal silver nitrate solution, and name the product formed. [2]
How would you obtain ethanol from ethene? Give the equation and the conditions. [2]
How would you obtain ethyne from ethanol? Give the equations for each step in order. [3]
How would you obtain ethyl ethanoate starting from bromoethane? Give the equations for both steps. [3]
A colourless gas burns with a clean blue flame, gives no change with alkaline potassium permanganate, and reacts with chlorine in sunlight to give a compound of formula \(\mathrm{CH_3Cl}\). Identify the gas and give a reason for each conclusion. [3]
A liquid X turns blue litmus red, gives brisk effervescence with a carbonate, and gives a wine red colour with neutral iron(III) chloride. Identify X and write the equation for its reaction with sodium hydroxide. [3]
Identify the labelled parts of the apparatus in Fig. 12.14-3 and state the purpose of each. Name the gas being collected and state why it is collected in that way. [4]
Identify the labelled parts of the apparatus in Fig. 12.19-2, state the purpose of the pinch cock, and give the balanced equation for the reaction taking place in the flask. [4]