Chapter 12: Aldehydes, Ketones and Carboxylic Acids
Before the middle of the nineteenth century, making a good mirror was dangerous work. Glass was backed with a shining film of tin–mercury amalgam, and the workers who spread it slowly poisoned themselves breathing mercury vapour.
Around the 1830s the German chemist Justus von Liebig found a completely different route. He took a solution of a silver salt, made it alkaline, added ammonia, and then poured in a sugar solution. Within minutes the inside of the glass vessel had turned into a smooth, brilliant mirror of metallic silver.
Nothing had been painted on. The silver had been pulled out of the solution by the sugar itself. Glucose carries a –CHO group, and that group gives up electrons so readily that it hands them to silver ions, reducing them to a film of silver metal, while the –CHO group is itself oxidized. Replace that –CHO with the C=O of a ketone, tucked between two carbon chains, and nothing happens at all — no mirror, no silver.
That single structural difference, a hydrogen atom sitting on the carbon of the C=O group, is the thread running through this chapter. Aldehydes, ketones and carboxylic acids are all built around the same carbon–oxygen double bond, as shown in Fig. 12.0, yet they smell different, boil at different temperatures, and behave differently with the same reagent.
Diagram
Fig. 12.0 – A round-bottomed flask with a mirror-bright silvered inner surface, and beside it the three general structures R–CHO, R–CO–R′ and R–COOH labelled formyl group, ketonic carbonyl group and carboxyl group
The chapter ahead will give you everything you need to reason out each of these questions for yourself.
12.1 Introduction
Moving from a Single Bond to a Double Bond
In the previous chapter you studied organic compounds that contain a carbon–oxygen single bond. In this chapter we study organic compounds that contain a carbon–oxygen double bond, \(>\mathrm{C=O}\), which is called the carbonyl group. It is one of the most important functional groups in organic chemistry.
A functional group is the small cluster of atoms that decides how a molecule behaves chemically. This is why the carbonyl group repays close study: once you understand how one \(>\mathrm{C=O}\) reacts, you can predict the behaviour of a very large number of compounds built around it.
The two atoms of this group have names of their own. The carbon of the double bond is called the carbonyl carbon and the oxygen is called the carbonyl oxygen, as marked in Fig. 12.1a.
Diagram
Fig. 12.1a – General carbonyl compound R–C(=O)–R′ with an arrow to the oxygen labelled “carbonyl oxygen” and an arrow to the central carbon labelled “carbonyl carbon”
Three Families Built on One Group
Both aldehydes and ketones contain a carbon–oxygen double bond as their functional group. For this reason they are together called carbonyl compounds.
In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen atom apart from an alkyl or aryl group. The functional group of aldehydes is therefore \(\mathrm{-CHO}\), which is called the formyl group or the aldehydic carbonyl group. Here an alkyl group means a group derived from an alkane, such as \(\mathrm{CH_3-}\) or \(\mathrm{CH_3CH_2-}\), while an aryl group means a group derived from an aromatic ring, such as \(\mathrm{C_6H_5-}\).
In a ketone, on the other hand, the carbonyl carbon is bonded to two alkyl or aryl groups. These two groups may be identical (\(\mathrm{R = R}\)) or different (\(\mathrm{R \neq R'}\)). This group is called the ketonic carbonyl group. Notice the single structural difference from an aldehyde: the hydrogen on the carbonyl carbon has been replaced by a second carbon-containing group.
The functional group of carboxylic acids is \(\mathrm{-COOH}\), called the carboxyl group. Because of the \(\mathrm{-OH}\) group bonded to the \(>\mathrm{C=O}\) group, carboxylic acids are distinct from aldehydes and ketones. In other words, a carboxyl group is a carbonyl group and a hydroxyl group sitting on the same carbon atom, and this combination behaves so differently from either group alone that carboxylic acids are treated as a separate family.
12.2 Classification of Aldehydes, Ketones and Carboxylic Acids
Aldehydes, ketones and carboxylic acids are classified according to the nature of the carbon skeleton bonded to the \(>\mathrm{C=O}\) group. In other words, we do not look at the carbonyl group itself — that is the same in every case — but at what kind of carbon framework it is attached to. If that framework is an open chain of carbon atoms, the compound is called aliphatic; if it is a benzene ring or another aromatic ring, the compound is called aromatic. The complete classification scheme is summarised in FC 12.2a.
Flowchart
FC 12.2a – Classification of carbonyl compounds: Carbonyl-containing compounds → (branch 1) Aldehydes, R–CHO → aliphatic (acetaldehyde) / aromatic (benzaldehyde); (branch 2) Ketones, R–CO–R′ → aliphatic → simple (acetone) / mixed (ethyl methyl ketone), and aromatic (acetophenone, benzophenone); (branch 3) Carboxylic acids, R–COOH → aliphatic → mono (acetic acid) / di (oxalic acid) / tri (citric acid), and aromatic (benzoic acid)
12.2.1 Classification of Aldehydes
Aldehydes are classified as aliphatic and aromatic aldehydes.
Aliphatic Aldehydes
The compounds in which the \(\mathrm{-CHO}\) group (formyl group) is attached directly to an \(sp^3\) hybridized carbon atom, that is, to a saturated carbon atom, are called aliphatic aldehydes. An \(sp^3\) hybridized carbon is one that forms four single bonds and no multiple bonds, so this condition simply says that the carbon holding the \(\mathrm{-CHO}\) group is a plain, saturated chain carbon.
There is one exception. Formaldehyde, \(\mathrm{H\!-\!CHO}\), is also classified as an aliphatic aldehyde even though its \(\mathrm{-CHO}\) group is not attached to any carbon at all. For example:
\[\mathrm{CH_3\!-\!CHO} \qquad \mathrm{CH_3\!-\!CH_2\!-\!CHO}\]
\[\text{(Acetaldehyde)} \qquad \text{(Propionaldehyde)}\]
The general formula of an aliphatic aldehyde is drawn in Fig. 12.2a, where R may be a hydrogen atom or an alkyl group.
Diagram
Fig. 12.2a – General formula of an aliphatic aldehyde: R–C(=O)–H with the –CHO portion shaded and labelled “general formula (R = H or alkyl group)”
Aromatic Aldehydes
The compounds in which the \(\mathrm{-CHO}\) group is attached directly to an aromatic ring are called aromatic aldehydes. The word “directly” is important here: the carbon of the \(\mathrm{-CHO}\) group must be bonded to a ring carbon itself, with no chain carbon in between. Three examples are shown in Fig. 12.2b.
Diagram
Fig. 12.2b – Three aromatic aldehydes: benzaldehyde (C₆H₅–CHO); salicylaldehyde (benzene ring with –CHO and an –OH on the adjacent carbon); p-nitrobenzaldehyde (benzene ring with –CHO and –NO₂ at the para position); each labelled with its name
12.2.2 Classification of Ketones
Ketones are classified as aliphatic and aromatic ketones.
Aliphatic Ketones
The compounds in which the \(>\mathrm{C=O}\) group is attached to two alkyl groups are called aliphatic ketones. In a ketone, the carbonyl group is also called the oxo group or the ketonic carbonyl group; its general formula is shown in Fig. 12.2c, where R and \(\mathrm{R'}\) are alkyl groups that may be identical or different.
Diagram
Fig. 12.2c – General formula of an aliphatic ketone: R–C(=O)–R′ with an arrow to the C=O labelled “oxo or ketonic carbonyl group” and the caption “R, R′ = alkyl group, identical or different”
Simple and Mixed Ketones
On the basis of the types of alkyl groups bonded to the carbonyl carbon, aliphatic ketones are further classified as simple and mixed ketones.
Simple or symmetrical ketones are the ketones in which both the alkyl groups bonded to the carbonyl carbon are identical. Because the two halves of such a molecule are the same, the molecule is symmetrical about the carbonyl carbon. For example:
\[\mathrm{H_3C\!-\!CO\!-\!CH_3} \qquad \mathrm{H_5C_2\!-\!CO\!-\!C_2H_5}\]
\[\text{(Dimethyl ketone) (Acetone)} \qquad \text{(Diethyl ketone)}\]
Mixed or unsymmetrical ketones are the ketones in which the two alkyl groups bonded to the carbonyl carbon are different. For example:
\[\mathrm{H_5C_2\!-\!CO\!-\!CH_3} \qquad \mathrm{H_5C_2\!-\!CO\!-\!CH_2\!-\!CH_2\!-\!CH_3}\]
\[\text{(Ethyl methyl ketone)} \qquad \text{(Ethyl n-propyl ketone)}\]
Aromatic Ketones
The compounds in which a \(>\mathrm{C=O}\) group is attached to either two aryl groups, or to one aryl and one alkyl group, are called aromatic ketones. Two common examples are drawn in Fig. 12.2d.
Diagram
Fig. 12.2d – Two aromatic ketones: benzophenone (two benzene rings joined through a C=O) and acetophenone (a benzene ring joined through a C=O to a CH₃ group); label their trivial names diphenyl ketone and methyl phenyl ketone
12.2.3 Classification of Carboxylic Acids
Carboxylic acids are classified as aliphatic and aromatic carboxylic acids.
Aliphatic Carboxylic Acids
The organic compounds in which the carboxyl (\(\mathrm{-COOH}\)) group is bonded to an alkyl group are called aliphatic carboxylic acids, or fatty acids. The second name is a reminder of where they were first obtained: the higher members of this series were isolated from natural fats and oils.
As with aldehydes, there is one exception to the definition. Formic acid, \(\mathrm{H\!-\!COOH}\), is also classified as an aliphatic carboxylic acid, even though its \(\mathrm{-COOH}\) group is not attached to any carbon. For example:
\[\mathrm{H_3C\!-\!COOH} \qquad \mathrm{H_3C\!-\!CH_2\!-\!COOH}\]
\[\text{(Acetic acid)} \qquad \text{(Propionic acid)}\]
The general formula of a carboxylic acid, with the carboxyl functional group marked, is given in Fig. 12.2e, where R is a hydrogen atom or an alkyl group.
Diagram
Fig. 12.2e – General formula of a carboxylic acid: R–C(=O)–OH with the –COOH portion boxed and an arrow labelled “carboxyl functional group”; caption “R = H or alkyl group”
Classification by Number of Carboxyl Groups
A molecule need not be limited to one carboxyl group. Depending on the number of \(\mathrm{-COOH}\) groups present, carboxylic acids are classified as mono, di, tri carboxylic acids and so on. For example, propionic acid, \(\mathrm{H_3C\!-\!CH_2\!-\!COOH}\), is a monocarboxylic acid; oxalic acid, \(\mathrm{HOOC\!-\!COOH}\), is a dicarboxylic acid; and citric acid, whose structure is drawn in Fig. 12.2f, is a tricarboxylic acid.
Diagram
Fig. 12.2f – Structure of citric acid: a central carbon bearing –OH and –COOH, with a –CH₂–COOH group above and a –CH₂–COOH group below; label “citric acid (a tricarboxylic acid)”
Aromatic Carboxylic Acids
These are the compounds in which one or more carboxyl groups (\(\mathrm{-COOH}\)) are attached directly to the aromatic ring. Three examples are shown in Fig. 12.2g.
Diagram
Fig. 12.2g – Three aromatic carboxylic acids: benzoic acid (ring + COOH); phthalic acid (ring with two COOH groups on adjacent carbons); acetyl salicylic acid or aspirin (ring with COOH and an OCOCH₃ group on the adjacent carbon); label each with its name
Once again, the word “directly” decides the classification, so a compound whose ring and carboxyl group are separated by a chain carbon needs a name of its own.
Carboxylic Acids in Nature
Carboxylic acids are widely distributed in nature; they are found in both plants and animals. L-lactic acid is present in curd and citric acid is found in citrus fruits such as lemons. Acetic acid is the key ingredient of vinegar.
12.3 Nomenclature of Aldehydes, Ketones and Carboxylic Acids
12.3.1 Nomenclature of Aldehydes and Carboxylic Acid
The names of aldehydes and carboxylic acids are related to each other. There are two systems of naming aldehydes and carboxylic acids: trivial and IUPAC.
Trivial Names of Aldehydes and Carboxylic Acids
Trivial names of aliphatic aldehydes are derived from the corresponding trivial names of carboxylic acids. Here the ending ‘ic acid’ of the carboxylic acid is replaced by the ending ‘aldehyde’. Acetic acid therefore gives acetaldehyde, and propionic acid gives propionaldehyde.
In the case of substituted aliphatic carboxylic acids and aldehydes, the position of the substituent is indicated by labelling the carbon serially as \(\alpha\), \(\beta\), \(\gamma\) and so on. The carbon atom adjacent to the carbonyl carbon is labelled as \(\alpha\), the next one as \(\beta\), and so on, as shown in Fig. 12.3a. Note that the carbonyl carbon itself gets no Greek letter — the counting starts from its neighbour. Some trivial names formed in this way are listed in Table 12.1.
Diagram
Fig. 12.3a – A four-carbon chain ending in –COOH, with the carbon next to the –COOH carbon labelled α, the next labelled β and the last labelled γ; alongside, the same labelling on a chain ending in –CHO
IUPAC Names of Aldehydes and Carboxylic Acids
According to the IUPAC system, the name of an aliphatic aldehyde is derived from the name of the corresponding alkane by replacing the ending ‘e’ of the alkane with ‘al’. An aldehyde is therefore named as an alkanal, as shown in Table 12.1. The IUPAC name of an aliphatic carboxylic acid is derived from the name of the corresponding alkane by replacing the ending ‘e’ of the alkane with ‘oic acid’.
\[\text{Alkane} \longrightarrow \text{Alkanal}\]
\[\text{Alkane} \longrightarrow \text{Alkanoic acid}\]
The longest chain including the \(\mathrm{-CHO}\) or \(\mathrm{-COOH}\) group is identified as the parent chain. Numbering of the chain is done by giving number 1 to the \(\mathrm{-CHO}\) or \(\mathrm{-COOH}\) carbon. The name of the substituent is included along with its locant, that is, the number of the carbon on which it sits.
\[^4\mathrm{CH_3}\!-\!^3\mathrm{CH_2}\!-\!^2\mathrm{CH_2}\!-\!^1\mathrm{CHO} \ ;\qquad ^4\mathrm{CH_3}\!-\!^3\mathrm{CH_2}\!-\!^2\mathrm{CH_2}\!-\!^1\mathrm{COOH}\]
The aldehyde (\(\mathrm{-CHO}\)) group and the carboxyl (\(\mathrm{-COOH}\)) group are always present at the end of the parent straight chain. The complete naming procedure is set out in FC 12.3a.
Flowchart
FC 12.3a – Writing the IUPAC name of an aliphatic aldehyde or carboxylic acid: identify the longest carbon chain that includes the –CHO or –COOH carbon → number that chain giving locant 1 to the –CHO or –COOH carbon → name the parent alkane and replace the final ‘e’ with ‘al’ (aldehyde) or ‘oic acid’ (acid) → add each substituent as a prefix with its locant, in alphabetical order → assemble the full name
Chains Carrying Two Groups
When two \(\mathrm{-CHO}\) groups are present at the two ends of the chain, the ending ‘e’ of the alkane is retained and the suffix ‘dial’ is added to the name of the parent aldehyde. In the case of dicarboxylic acids, ‘dioic acid’ is added to the name of the parent alkane. Retaining the ‘e’ here keeps the name easy to pronounce and signals at once that two groups are present.
Naming Ring Compounds: Carbaldehyde and Carboxylic Acid
In IUPAC nomenclature, an alicyclic compound in which the \(\mathrm{-CHO}\) group is attached directly to the ring is named as a carbaldehyde. The suffix ‘carbaldehyde’ is added after the full name of the parent cycloalkane structure. Similarly, an alicyclic compound having a carboxyl group directly attached to the alicyclic ring is named as a cycloalkane carboxylic acid. An alicyclic compound is a ring compound that is not aromatic, such as cyclohexane.
Substituted Aromatic Aldehydes and Carboxylic Acids
When two or more different functional groups are attached to a ring, the higher priority group (Std. XI Chemistry Textbook, Chapter 14, sec. 14.4.7) is given the lower number. When the \(\mathrm{-CHO}\) group appears as a substituent, the prefix ‘formyl’ is used in the IUPAC name. Three worked examples of such names are drawn in Fig. 12.3b.
Diagram
Fig. 12.3b – Three substituted aromatic compounds with names: a benzene ring with CHO at C1, CH₃ at C3 and OH at C4, labelled 4-hydroxy-3-methylbenzaldehyde; a ring with COOH at C1, OH at C3 and CH₃ at C4, labelled 3-hydroxy-4-methylbenzoic acid; a ring with COOH at C1 and CHO at C2, labelled 2-formylbenzoic acid
Trivial and IUPAC names of some aldehydes and carboxylic acids are given in Table 12.1.
Table 12.1 Trivial and IUPAC names of carboxylic acids and aldehydes
| Structure (acid) | Trivial name | IUPAC name | Structure (aldehyde) | Trivial name | IUPAC name |
|---|---|---|---|---|---|
| \(\mathrm{H\!-\!COOH}\) | Formic acid | Methanoic acid | \(\mathrm{H\!-\!CHO}\) | Formaldehyde | Methanal |
| \(\mathrm{CH_3\!-\!COOH}\) | Acetic acid | Ethanoic acid | \(\mathrm{CH_3\!-\!CHO}\) | Acetaldehyde | Ethanal |
| \(\mathrm{CH_3\!-\!CH_2\!-\!COOH}\) | Propionic acid | Propanoic acid | \(\mathrm{CH_3\!-\!CH_2\!-\!CHO}\) | Propionaldehyde | Propanal |
| \(\mathrm{CH_3\!-\!CH_2\!-\!CH_2\!-\!COOH}\) | n-Butyric acid | Butanoic acid | \(\mathrm{CH_3\!-\!CH_2\!-\!CH_2\!-\!CHO}\) | Butyraldehyde | Butanal |
| \(\mathrm{(CH_3)_2CH\!-\!COOH}\) | Isobutyric acid (\(\alpha\)-methylpropionic acid) | 2-Methylpropanoic acid | \(\mathrm{(CH_3)_2CH\!-\!CHO}\) | Isobutyraldehyde (\(\alpha\)-methylpropionaldehyde) | 2-Methylpropanal |
| \(\mathrm{CH_2{=}CH\!-\!COOH}\) | Acrylic acid | Prop-2-enoic acid | \(\mathrm{CH_2{=}CH\!-\!CHO}\) | Acrolein | Prop-2-enal |
| \(\mathrm{HOOC\!-\!COOH}\) | Oxalic acid | Ethanedioic acid | \(\mathrm{CHO\!-\!CHO}\) | Oxaldehyde (Glyoxal) | Ethanedial |
| \(\mathrm{C_6H_5\!-\!COOH}\) | Benzoic acid (Benzenecarboxylic acid) | Benzoic acid | \(\mathrm{C_6H_5\!-\!CHO}\) | Benzaldehyde | Benzaldehyde (Benzenecarbaldehyde) |
| \(\mathrm{2\text{-}CH_3\!-\!C_6H_4\!-\!COOH}\) | o-Toluic acid | 2-Methylbenzoic acid | \(\mathrm{2\text{-}CH_3\!-\!C_6H_4\!-\!CHO}\) | o-Tolualdehyde | 2-Methylbenzaldehyde |
| \(\mathrm{2\text{-}OH\!-\!C_6H_4\!-\!COOH}\) | Salicylic acid | 2-Hydroxybenzoic acid | \(\mathrm{2\text{-}OH\!-\!C_6H_4\!-\!CHO}\) | Salicylaldehyde | 2-Hydroxybenzaldehyde |
| \(\mathrm{1,2\text{-}C_6H_4(COOH)_2}\) | Phthalic acid | Benzene-1,2-dicarboxylic acid | \(\mathrm{1,2\text{-}C_6H_4(CHO)_2}\) | Phthalaldehyde | Benzene-1,2-dicarbaldehyde |
| \(\mathrm{C_6H_{11}\!-\!COOH}\) | Cyclohexylcarboxylic acid | Cyclohexanecarboxylic acid | \(\mathrm{C_6H_{11}\!-\!CHO}\) | Cyclohexane aldehyde | Cyclohexanecarbaldehyde |
12.3.2 Trivial and IUPAC Names of Ketones
Trivial Names of Ketones
The trivial names of aliphatic ketones are based on the names of the alkyl groups or aryl groups attached to the carbonyl carbon. The names of the alkyl or aryl groups are written in alphabetical order, followed by the word ketone. Thus a carbonyl carbon carrying an ethyl group and a methyl group gives ethyl methyl ketone.
In the case of substituted aliphatic ketones, the position of the substituent is indicated by labelling the carbon serially as \(\alpha\), \(\beta\), \(\gamma\) and so on. The carbon atom adjacent to the carbonyl carbon is labelled as \(\alpha\) and the next one is \(\beta\), and so on — the same convention you met for aldehydes and acids. Names of aromatic ketones are based on a phenone, as seen in Table 12.2.
IUPAC Names of Ketones
The IUPAC names of aliphatic ketones are derived from the names of the corresponding alkanes by replacing the ending ‘e’ of the alkane with ‘one’. They are named as alkanone. The longest chain of carbon atoms containing the ketonic carbonyl group is numbered from the end closer to the carbonyl carbon.
\[\text{Alkane} \longrightarrow \text{Alkanone}\]
When two \(>\mathrm{C=O}\) groups are present, the ending ‘e’ of the alkane is retained and the suffix ‘dione’ is added to the name of the parent ketone, indicating the locants of the ketonic carbonyl groups. In the case of polyfunctional ketones, the higher priority group is given the lower number. When the ketonic carbonyl is a lower priority group, it is named as ‘oxo’, preceded by the locant. In alicyclic ketones, the carbonyl carbon is numbered as 1. These conventions can all be traced in Table 12.2.
Table 12.2 Trivial and IUPAC names of some ketones
| Sr. No. | Compound | Trivial name | IUPAC name |
|---|---|---|---|
| 1 | \(\mathrm{CH_3\!-\!CO\!-\!CH_3}\) | Dimethyl ketone (Acetone) | Propanone |
| 2 | \(\mathrm{CH_3\!-\!CO\!-\!CH_2\!-\!CH_3}\) | Ethyl methyl ketone | Butanone |
| 3 | \(\mathrm{CH_3\!-\!CO\!-\!CH_2\!-\!CH_2\!-\!CH_3}\) | Methyl n-propyl ketone | Pentan-2-one |
| 4 | \(\mathrm{CH_3\!-\!CH_2\!-\!CO\!-\!CH_2\!-\!CH_3}\) | Diethyl ketone | Pentan-3-one |
| 5 | \(\mathrm{CH_3\!-\!CHBr\!-\!CO\!-\!CH_2\!-\!CH_2\!-\!CH_3}\) | \(\alpha\)-Bromoethyl n-propyl ketone | 2-Bromohexan-3-one |
| 6 | \(\mathrm{(CH_3)_2C{=}CH\!-\!CO\!-\!CH_3}\) | Mesityl oxide | 4-Methylpent-3-en-2-one |
| 7 | \(\mathrm{CH_3\!-\!CO\!-\!CH_2\!-\!CO\!-\!CH_2\!-\!CH_3}\) | — | Hexane-2,4-dione |
| 8 | \(\mathrm{CH_3\!-\!CO\!-\!CH_2\!-\!CHO}\) | — | 3-Oxobutanal |
| 9 | \(\mathrm{C_6H_5\!-\!CO\!-\!C_6H_5}\) | Diphenyl ketone (Benzophenone) | Benzophenone |
| 10 | \(\mathrm{C_6H_5\!-\!CO\!-\!CH_3}\) | Methyl phenyl ketone (Acetophenone) | Acetophenone |
| 11 | Cyclohexanone ring bearing \(\mathrm{-Cl}\) at C2 and \(\mathrm{-CH_3}\) at C4 | — | 2-Chloro-4-methylcyclohexanone |
Entry 8 in Table 12.2 is worth a second look. The molecule contains both a \(>\mathrm{C=O}\) and a \(\mathrm{-CHO}\) group; the aldehyde group has the higher priority, so it takes the suffix and locant 1, and the ketonic carbonyl is demoted to the prefix ‘oxo’ at C-3, giving 3-oxobutanal.
Diagram
Fig. 12.3c – Five structures to be named: (i) a branched chain bearing two Br atoms and a terminal –CHO; (ii) a branched chain containing two ketonic C=O groups; (iii) an unbranched chain with a terminal –CHO; (iv) a benzene ring carrying –CHO and a –CH₃ substituent; (v) a cyclohexanone ring carrying a –CH₃ substituent
12.4 Preparation of Aldehydes and Ketones
12.4.1 General Methods of Preparation of Aldehydes and Ketones
By Oxidation of Alcohols
Aldehydes and ketones are prepared by the oxidation of primary and secondary alcohols respectively (see Chapter 11). The pairing is worth fixing in your mind: a primary alcohol has two hydrogen atoms on its carbon-bearing \(\mathrm{-OH}\) group, and removing two of the hydrogens from that carbon and its \(\mathrm{-OH}\) leaves a \(\mathrm{-CHO}\) group. A secondary alcohol has only one such hydrogen, so oxidation can go no further than a \(>\mathrm{C=O}\) flanked by two carbon groups, which is a ketone.
By Dehydrogenation of Alcohols
This method has industrial application. Aldehydes and ketones are prepared by passing the vapours of primary and secondary alcohols respectively over hot copper powder (see Chapter 11). Here no oxidising agent is consumed at all; hydrogen is simply pulled off the alcohol at the hot metal surface, which is why the method suits large-scale continuous production.
From Hydrocarbons
By ozonolysis: An alkene reacts with ozone to give an ozonide, which on decomposition with zinc dust and water gives aldehydes and/or ketones (see Std. XI Chemistry Textbook, Chapter 15). Whether the product is an aldehyde or a ketone depends on what the doubly bonded carbons were carrying: a carbon holding a hydrogen becomes a \(\mathrm{-CHO}\) group, while a carbon holding two alkyl groups becomes a ketonic carbonyl.
By hydration of alkynes: Alkynes react with water in the presence of 40% sulfuric acid and 1% mercuric sulfate to give aldehydes or ketones (see Std. XI Chemistry Textbook, Chapter 15).
12.4.2 Other Methods of Preparation of Aldehydes and Ketones
Some methods of preparation of aldehydes and ketones involve common starting functional groups but different types. In other words, the same starting compound can be steered towards an aldehyde or towards a ketone, and what decides the outcome is the reagent chosen. The routes covered in this subsection are collected in FC 12.4a.
Flowchart
FC 12.4a – Routes to aldehydes and ketones from a common starting material: acyl chloride R–COCl → (H₂, Pd–BaSO₄) → aldehyde R–CHO, or → (R₂Cd) → ketone R′–CO–R; nitrile R–C≡N → (SnCl₂/HCl, then H₃O⁺) or (DIBAl-H, then H₃O⁺) → aldehyde R–CHO, or → (R′MgX, dry ether, then H₃O⁺) → ketone R–CO–R′; arene → (Etard / CrO₃ / side-chain chlorination / Gatterman–Koch) → aromatic aldehyde, or → (Friedel–Crafts acylation) → aromatic ketone; ester R–CO–O–R′ → (DIBAl-H at 195 K) → aldehyde
From Acyl Chlorides: Preparation of Aldehydes
Aldehydes and ketones both can be obtained from acyl chloride, but the reactions involved are different.
An acyl chloride is reduced to the corresponding aldehyde by hydrogen using a palladium catalyst poisoned with barium sulfate. This reaction is known as Rosenmund reduction.
\[\mathrm{R\!-\!COCl} \xrightarrow{\ \mathrm{H_2},\ \mathrm{Pd\text{-}BaSO_4}\ } \mathrm{R\!-\!CHO} + \mathrm{HCl}\]
\[\text{(Acyl chloride)} \qquad\qquad \text{(Aldehyde)}\]
From Acyl Chlorides: Preparation of Ketones
Preparation of aliphatic ketones from acyl chloride. Ketones are obtained from acyl chloride by reaction with dialkyl cadmium, which is prepared by the treatment of cadmium chloride with a Grignard reagent.
\[2\,\mathrm{R\!-\!MgX} + \mathrm{CdCl_2} \longrightarrow \mathrm{R_2Cd} + 2\,\mathrm{Mg(X)Cl}\]
\[2\,\mathrm{R'\!-\!COCl} + \mathrm{R_2Cd} \longrightarrow 2\,\mathrm{R'\!-\!CO\!-\!R} + \mathrm{CdCl_2}\]
\[\text{(Acyl chloride)} \qquad\qquad \text{(Ketone)}\]
For example,
\[2\,\mathrm{CH_3\!-\!COCl} + \mathrm{(CH_3)_2Cd} \longrightarrow 2\,\mathrm{CH_3\!-\!CO\!-\!CH_3} + \mathrm{CdCl_2}\]
\[\text{(Ethanoyl chloride)} \quad \text{(Dimethyl cadmium)} \quad \text{(Propanone, acetone)}\]
\[2\,\mathrm{C_6H_5\!-\!COCl} + \mathrm{(CH_3)_2Cd} \longrightarrow \mathrm{C_6H_5\!-\!CO\!-\!CH_3} + \mathrm{CdCl_2}\]
\[\text{(Benzoyl chloride)} \quad \text{(Dimethyl cadmium)} \quad \text{(Acetophenone)}\]
Preparation of aromatic ketones from acyl chloride. Aromatic ketones are prepared by Friedel–Crafts acylation reaction (Std. XI Chemistry Textbook, Chapter 15, sec. 15.4.6).
From Nitriles: Preparation of Aldehydes
Aldehydes and ketones both can be obtained from nitriles, but by different reactions.
Nitriles are reduced to imine hydrochloride by stannous chloride in the presence of hydrochloric acid, which on acid hydrolysis gives the corresponding aldehyde. This reaction is called the Stephen reaction.
\[\mathrm{R\!-\!C{\equiv}N} + 2[\mathrm{H}] \xrightarrow{\ \mathrm{SnCl_2},\ \mathrm{HCl}\ } \mathrm{R\!-\!HC{=}NH\cdot HCl}\]
\[\text{(Alkane nitrile)} \qquad\qquad \text{(imine hydrochloride)}\]
\[\mathrm{R\!-\!HC{=}NH\cdot HCl} \xrightarrow{\ \mathrm{H_3O^{+}}\ } \mathrm{R\!-\!CHO} + \mathrm{NH_4Cl}\]
\[\text{(Aldehyde)}\]
Here \(2[\mathrm{H}]\) stands for the two hydrogen atoms supplied by the reducing system; the triple bond is reduced only as far as a \(\mathrm{C{=}N}\) double bond, and water then replaces the nitrogen in the hydrolysis step. For example,
\[\mathrm{H_3C\!-\!C{\equiv}N} + 2[\mathrm{H}] \xrightarrow[\text{(reduction)}]{\ \mathrm{SnCl_2},\ \mathrm{HCl}\ } \mathrm{CH_3\!-\!HC{=}NH\cdot HCl}\]
\[\text{(Ethanenitrile)} \qquad\qquad \text{(ethanimine hydrochloride)}\]
\[\mathrm{CH_3\!-\!HC{=}NH\cdot HCl} \xrightarrow{\ \mathrm{H_3O^{+}}\ } \mathrm{CH_3\!-\!CHO} + \mathrm{NH_4Cl}\]
\[\text{(Ethanal)}\]
\[\mathrm{C_6H_5\!-\!C{\equiv}N} + 2[\mathrm{H}] \xrightarrow{\ \mathrm{SnCl_2},\ \mathrm{HCl}\ } \mathrm{C_6H_5\!-\!HC{=}NH\cdot HCl}\]
\[\text{(Benzonitrile)} \qquad\qquad \text{(Benzanimine hydrochloride)}\]
\[\mathrm{C_6H_5\!-\!HC{=}NH\cdot HCl} \xrightarrow{\ \mathrm{H_3O^{+}}\ } \mathrm{C_6H_5\!-\!CHO} + \mathrm{NH_4Cl}\]
\[\text{(Benzaldehyde)}\]
Alternatively, nitriles are also reduced by diisobutylaluminium hydride (DIBAl-H), or \(\mathrm{AlH(i\text{-}Bu)_2}\), to imines, followed by acid hydrolysis to aldehydes. An advantage of this method is that a double or triple bond present in the same molecule is not reduced.
\[\mathrm{H_3C\!-\!CH{=}CH\!-\!CH_2\!-\!C{\equiv}N} \xrightarrow[\ \mathrm{H_3O^{+}}\ ]{\ \mathrm{AlH(i\text{-}Bu)_2}\ } \mathrm{H_3C\!-\!CH{=}CH\!-\!CH_2\!-\!CHO}\]
\[\text{(Pent-3-enenitrile)} \qquad\qquad \text{(Pent-3-enal)}\]
Notice that the \(\mathrm{C{=}C}\) double bond at position 3 survives untouched — that is exactly the selectivity being claimed.
From Nitriles: Preparation of Ketones
Ketones are prepared by reacting nitriles with a Grignard reagent in dry ether as solvent, followed by acid hydrolysis.
\[\mathrm{H_3C\!-\!C{\equiv}N} + \mathrm{H_3CMgCl} \xrightarrow{\ \text{dry ether}\ } \mathrm{CH_3\!-\!C(CH_3){=}NMgCl}\]
\[\text{(Ethanenitrile)}\]
\[\mathrm{CH_3\!-\!C(CH_3){=}NMgCl} \xrightarrow{\ \mathrm{H_3O^{+}}\ } \mathrm{CH_3\!-\!CO\!-\!CH_3} + \mathrm{NH_3} + \mathrm{Mg(Cl)OH}\]
\[\text{(Acetone)}\]
\[\mathrm{C_6H_5\!-\!C{\equiv}N} + \mathrm{C_6H_5\!-\!MgBr} \xrightarrow{\ \text{dry ether}\ } \mathrm{C_6H_5\!-\!C(C_6H_5){=}NMgBr}\]
\[\text{(Benzonitrile)}\]
\[\mathrm{C_6H_5\!-\!C(C_6H_5){=}NMgBr} \xrightarrow{\ \mathrm{H_3O^{+}}\ } \mathrm{C_6H_5\!-\!CO\!-\!C_6H_5} + \mathrm{NH_3} + \mathrm{Mg(Br)OH}\]
\[\text{(Benzophenone)}\]
The alkyl or aryl group of the Grignard reagent becomes the second group on the carbonyl carbon, so the ketone you obtain can be designed by choosing the Grignard reagent.
From Aromatic Hydrocarbons
Aromatic aldehydes and ketones are both prepared from aromatic hydrocarbons, but by different methods.
For the aldehydes there is a difficulty to overcome. Strong oxidizing agents transform a \(\mathrm{-CH_3}\) group bonded to an aromatic ring straight into a carboxyl group (\(\mathrm{-COOH}\)), overshooting the aldehyde stage. For obtaining an aromatic aldehyde from a methyl arene, the following special methods are therefore used.
Etard Reaction
The methyl group in methyl benzene (or a methyl arene) is oxidized by the oxidizing agent chromyl chloride in carbon disulfide as solvent, to form a chromium complex, from which the corresponding benzaldehyde is obtained on acid hydrolysis. This reaction is known as the Etard reaction. The two stages are drawn in Fig. 12.4a.
\[\mathrm{C_6H_5\!-\!CH_3} + \mathrm{CrO_2Cl_2} \xrightarrow{\ \mathrm{CS_2}\ } \text{chromium complex} \xrightarrow{\ \mathrm{H_3O^{+}}\ } \mathrm{C_6H_5\!-\!CHO}\]
Diagram
Fig. 12.4a – Etard reaction: toluene ring with a CH₃ group plus CrO₂Cl₂ in CS₂ giving the chromium complex, a benzene ring bearing a CH with two OCr(OH)Cl₂ groups, which on H₃O⁺ gives benzaldehyde; label toluene, chromyl chloride, chromium complex, benzaldehyde
The key idea is that the chromium is held back at an intermediate stage as a stable complex; only when that complex is hydrolysed is the carbon released, and it is released as \(\mathrm{-CHO}\), not \(\mathrm{-COOH}\).
Oxidation of Methyl Arene Using CrO₃
Methylarene is converted into a benzylidene diacetate on treatment with chromium oxide in acetic anhydride at 273–278 K. The diacetate derivative on acid hydrolysis gives the corresponding aldehyde. The sequence is shown in Fig. 12.4b.
\[\mathrm{C_6H_5\!-\!CH_3} + \mathrm{CrO_3} + \mathrm{(CH_3CO)_2O} \xrightarrow{\ 273\text{–}278\ \text{K}\ } \mathrm{C_6H_5\!-\!CH(OCOCH_3)_2}\]
\[\text{(Methylbenzene)} \qquad \text{(Acetic anhydride)} \qquad \text{(benzylidene diacetate)}\]
\[\mathrm{C_6H_5\!-\!CH(OCOCH_3)_2} \xrightarrow[\ \mathrm{H_3O^{+}}\ ]{\ \Delta\ } \mathrm{C_6H_5\!-\!CHO}\]
\[\text{(Benzaldehyde)}\]
Diagram
Fig. 12.4b – Oxidation of methylbenzene with CrO₃ in acetic anhydride at 273–278 K to give benzylidene diacetate (ring with CH carrying two OCOCH₃ groups), which on Δ/H₃O⁺ gives benzaldehyde
Here the low temperature and the acetic anhydride together do the protecting: as soon as the carbon is oxidized it is capped as a diacetate, so it cannot be attacked further.
Side Chain Chlorination of Toluene
Side chain chlorination of toluene gives benzal chloride, which on acid hydrolysis at 373 K gives benzaldehyde. Benzaldehyde is manufactured commercially by this method.
\[\mathrm{C_6H_5\!-\!CH_3} \xrightarrow{\ \mathrm{Cl\!-\!Cl},\ h\nu\ } \mathrm{C_6H_5\!-\!CHCl_2} \xrightarrow[\ \Delta\ 373\ \text{K}\ ]{\ \mathrm{H_3O^{+}}\ } \mathrm{C_6H_5\!-\!CHO}\]
\[\text{(Toluene)} \qquad \text{(Benzal chloride)} \qquad \text{(Benzaldehyde)}\]
The light, written \(h\nu\) over the first arrow, is what sends the chlorination to the side chain rather than to the ring. Two chlorine atoms on the same carbon are unstable towards water, so hydrolysis converts that carbon directly into \(\mathrm{-CHO}\).
Gatterman–Koch Formylation of Arene
Benzene or substituted benzene is treated under high pressure with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride to give benzaldehyde or substituted benzaldehyde.
\[\mathrm{C_6H_6} \xrightarrow[\ \text{high pressure}\ ]{\ \mathrm{CO},\ \mathrm{HCl},\ \text{Anhyd.}\ \mathrm{AlCl_3}\ } \mathrm{C_6H_5\!-\!CHO}\]
\[\text{(Benzene)} \qquad\qquad \text{(Benzaldehyde)}\]
This is the one method in the group that does not need a methyl group to start with: the \(\mathrm{-CHO}\) group is installed directly on the ring.
Preparation of Aromatic Ketones from Hydrocarbons
Aromatic ketones are prepared by Friedel–Crafts acylation of arene (refer to sec. 12.4.2 (a) ii and Std. XI Chemistry Textbook, Chapter 15, sec. 15.4.6).
12.4.3 Preparation of Aldehydes Only from Esters
Aliphatic or aromatic esters are reduced to aldehydes by using diisobutylaluminium hydride, DIBAl-H, or \(\mathrm{AlH(i\text{-}Bu)_2}\). The reaction is usually carried out at 195 K to prevent further reduction of the aldehyde produced.
\[\mathrm{R\!-\!CO\!-\!O\!-\!R'} \xrightarrow[\ \mathrm{H_3O^{+}}\ ]{\ \mathrm{AlH(i\text{-}Bu)_2}\ } \mathrm{R\!-\!CO\!-\!H}\]
\[\text{(Ester)} \qquad\qquad \text{(Aldehyde)}\]
The low temperature is not a detail to be skimmed over. An aldehyde is itself easily reduced, so if the mixture is allowed to warm, the reaction continues past the aldehyde to the alcohol. Cooling to 195 K slows that second step enough for the aldehyde to be isolated.
12.5 Preparation of Carboxylic Acids
A carboxylic acid can be reached from several different starting materials. Broadly, either a group that already contains a carbon–oxygen or carbon–nitrogen multiple bond is hydrolysed, or a carbon skeleton is oxidized until the carbon in question carries the \(\mathrm{-COOH}\) group. The six routes of this section are gathered in FC 12.5a.
Flowchart
FC 12.5a – Routes to carboxylic acids: nitrile R–C≡N → (dil. acid) → amide R–CONH₂ → (dil. acid) → R–COOH; acyl chloride R–COCl → (H₂O) → R–COOH; anhydride → (H₂O) → 2 R–COOH; ester → (dil. H₂SO₄) → R–COOH, or → (dil. NaOH, then conc. HCl) → R–COOH; alkyl benzene → (alk./acidic KMnO₄ or dil. HNO₃ or chromic acid) → aromatic acid; alkene → (KMnO₄ in dil. H₂SO₄) → R–COOH; Grignard reagent R–MgX → (dry ice in dry ether) → R–COOMgX → (dil. HCl) → R–COOH
12.5.1 From Nitriles and Amides
Alkyl nitriles or aryl nitriles on acid hydrolysis give amides. Amides on further acid hydrolysis give the corresponding carboxylic acids. Hydrolysis is carried out by using dilute mineral acids like dilute sulfuric acid or dilute hydrochloric acid.
The first molecule of water adds across the \(\mathrm{C{\equiv}N}\) bond to give an unstable intermediate, which rearranges to the amide:
\[\mathrm{R\!-\!C{\equiv}N} + \mathrm{H_2O} \longrightarrow [\,\mathrm{R\!-\!C(OH){=}NH}\,] \longrightarrow \mathrm{R\!-\!CO\!-\!NH_2}\]
\[\text{(Nitrile)} \qquad\qquad \text{(Amide)}\]
\[\mathrm{R\!-\!CO\!-\!NH_2} \xrightarrow[\ \text{dil. HCl}\ ]{\ \mathrm{H_2O}\ } \mathrm{R\!-\!COOH} + \mathrm{NH_4Cl}\]
\[\text{(carboxylic acid)}\]
Written as a single overall change, the two stages become
\[\mathrm{R\!-\!C{\equiv}N} + 2\,\mathrm{H_2O} + \text{dil. HCl} \xrightarrow{\ \Delta\ } \mathrm{R\!-\!COOH} + \mathrm{NH_4Cl}\]
Two molecules of water are needed because two separate additions take place — one to reach the amide and one to replace the nitrogen. The nitrogen leaves as ammonium chloride because the medium is acidic.
12.5.2 From Acyl Chloride and Anhydrides
Acyl chlorides on hydrolysis with water give carboxylic acids. This method is useful for the preparation of aliphatic as well as aromatic acid.
\[\mathrm{R\!-\!COCl} + \mathrm{H_2O} \longrightarrow \mathrm{R\!-\!COOH} + \mathrm{H\!-\!Cl}\]
\[\text{(Acyl chloride)} \qquad \text{(Carboxylic acid)}\]
Anhydrides on hydrolysis with water give carboxylic acids. Since an anhydride is, in effect, two acid molecules joined with the loss of one water molecule, putting that water back releases both acid molecules.
\[\mathrm{R\!-\!CO\!-\!O\!-\!CO\!-\!R} + \mathrm{H_2O} \longrightarrow 2\,\mathrm{R\!-\!CO\!-\!O\!-\!H}\]
\[\text{(Anhydride)} \qquad\qquad \text{(Carboxylic acid)}\]
12.5.3 From Esters
Carboxylic acids can be obtained from esters either by acid hydrolysis or by alkaline hydrolysis.
Acid Hydrolysis of Ester
Esters on hydrolysis with a dilute mineral acid like dilute HCl or dilute \(\mathrm{H_2SO_4}\) give the corresponding carboxylic acid.
\[\mathrm{R\!-\!CO\!-\!O\!-\!R'} + \mathrm{H_2O} \xrightarrow[\ \Delta\ ]{\ \text{dil.}\ \mathrm{H_2SO_4}\ } \mathrm{R\!-\!CO\!-\!O\!-\!H} + \mathrm{R'\!-\!OH}\]
\[\text{(Ester)} \qquad\qquad \text{(Carboxylic acid)} \qquad \text{(Alcohol)}\]
Note that the acid here is a catalyst, not a reactant: it is not consumed, and the reaction is reversible, which is why an excess of water is used to drive it forward.
Alkaline Hydrolysis of Ester
Alkaline hydrolysis of ester using dilute alkali like dilute NaOH or dilute KOH forms a solution of the water soluble sodium or potassium salt of the acid (carboxylate). On acidification with concentrated HCl, the free acid is formed.
\[\mathrm{H_5C_2\!-\!CO\!-\!O\!-\!CH_3} + \text{dil. NaOH} \xrightarrow{\ \Delta\ } \mathrm{H_5C_2\!-\!CO\!-\!O^{-}Na^{+}} + \mathrm{CH_3\!-\!OH}\]
\[\text{(Methyl propanoate)} \qquad\qquad \text{(Sodium propanoate)}\]
\[\mathrm{H_5C_2\!-\!CO\!-\!O^{-}Na^{+}} + \mathrm{H_2O} \xrightarrow[\ \text{Conc. HCl}\ ]{\ \mathrm{H^{+}}\ } \mathrm{H_5C_2\!-\!CO\!-\!O\!-\!H} + \mathrm{NaOH}\]
\[\text{(Sodium propanoate)} \qquad\qquad \text{(Propanoic acid)}\]
The sodium or potassium salts of higher fatty acids are known as soaps. Hence alkaline hydrolysis of esters is called saponification (Std. XI Chemistry Textbook, Chapter 16).
12.5.4 From Alkyl Benzene
Aromatic carboxylic acids can be prepared by oxidation of alkyl benzene with dilute \(\mathrm{HNO_3}\) or alkaline/acidic \(\mathrm{KMnO_4}\) or chromic acid. The entire alkyl chain, regardless of its length, is oxidized to a carboxyl group. A tertiary alkyl substituent on benzene, however, is not oxidized.
For example, both toluene and cumene give the same product, benzoic acid, as shown in Fig. 12.5a.
\[\mathrm{C_6H_5\!-\!CH_3} \xrightarrow[\ \Delta\ ]{\ \mathrm{KMnO_4}\text{, }\mathrm{KOH}\ } \mathrm{C_6H_5\!-\!COO^{-}K^{+}} \xrightarrow{\ \mathrm{H_3O^{+}}\ } \mathrm{C_6H_5\!-\!COOH}\]
\[\text{(Toluene)} \qquad \text{(Potassium benzoate)} \qquad \text{(Benzoic acid)}\]
\[\mathrm{C_6H_5\!-\!CH(CH_3)_2} \xrightarrow[\ \Delta\ ]{\ \mathrm{KMnO_4}\text{, }\mathrm{KOH}\ } \mathrm{C_6H_5\!-\!COO^{-}K^{+}} \xrightarrow{\ \mathrm{H_3O^{+}}\ } \mathrm{C_6H_5\!-\!COOH}\]
\[\text{(Cumene)} \qquad \text{(Potassium benzoate)} \qquad \text{(Benzoic acid)}\]
Diagram
Fig. 12.5a – Two parallel schemes: toluene (ring + CH₃) and cumene (ring + CH(CH₃)₂), each with KMnO₄/KOH and Δ giving potassium benzoate (ring + COO⁻K⁺), which with H₃O⁺ gives benzoic acid; label all four compounds
12.5.5 From Alkenes
Carboxylic acids can also be prepared by the oxidation of alkenes by \(\mathrm{KMnO_4}\) in dilute \(\mathrm{H_2SO_4}\). The double bond is cleaved and each doubly bonded carbon that carries hydrogen ends up as a carboxyl carbon.
\[\mathrm{H_5C_6\!-\!CH{=}CH_2} \xrightarrow[\ \Delta\ ]{\ \mathrm{KMnO_4},\ \text{dil.}\ \mathrm{H_2SO_4}\ } \mathrm{H_5C_6\!-\!COOH}\]
\[\text{(Phenyl ethene)} \qquad\qquad \text{(Benzoic acid)}\]
When the double bond lies inside a ring, cleaving it does not give two separate molecules; the ring simply opens out into a chain carrying a \(\mathrm{-COOH}\) group at each end, as drawn in Fig. 12.5b.
\[\text{Cyclohexene} \xrightarrow[\ \Delta\ ]{\ \mathrm{KMnO_4},\ \text{dil.}\ \mathrm{H_2SO_4}\ } \mathrm{HOOC\!-\!(CH_2)_4\!-\!COOH}\]
\[\text{(Adipic acid)}\]
Diagram
Fig. 12.5b – Cyclohexene ring with KMnO₄/dil. H₂SO₄ and Δ opening to an open chain bearing COOH at both ends, labelled adipic acid
12.5.6 From Grignard Reagent
A Grignard reagent in dry ether solvent is added to solid carbon dioxide (dry ice) to give a complex, which on acid hydrolysis gives the corresponding carboxylic acid.
\[\mathrm{R\!-\!Mg\!-\!X} + \mathrm{O{=}C{=}O} \xrightarrow{\ \text{dry ether}\ } \mathrm{R\!-\!COOMgX}\]
\[\text{(Alkyl magnesium halide)} \quad \text{(dry ice)} \quad \text{(complex)}\]
\[\mathrm{R\!-\!COOMgX} \xrightarrow[\ \text{dil. HCl}\ ]{\ \mathrm{HOH}\ } \mathrm{R\!-\!COOH} + \mathrm{Mg(X)OH}\]
\[\text{(carboxylic acid)}\]
This route is valuable because it adds a carbon atom: the carbon of the carbon dioxide becomes the carboxyl carbon, so the acid formed always has one carbon more than the alkyl halide you started from.
12.6 Physical Properties
12.6.1 Nature of Intermolecular Forces
The carbonyl bond (\(>\mathrm{C=O}\)) in aldehydes and ketones is a polar covalent bond. Oxygen is much more electronegative than carbon, so the shared electrons of the double bond are pulled towards oxygen, leaving the oxygen slightly negative (\(\delta-\)) and the carbon slightly positive (\(\delta+\)). Every molecule therefore behaves as a small permanent dipole.
As a result, these compounds contain dipole–dipole forces of attraction, illustrated in Fig. 12.1. The molecules orient in such a way as to have oppositely polarized atoms facing each other, so that the positive end of one molecule sits next to the negative end of the next.
Diagram
Fig. 12.1 – Dipole–dipole attraction in carbonyl compounds: two >C=O groups drawn head to tail, with δ⁻ marked on each oxygen and δ⁺ on each carbon, and dotted lines showing the attraction between the δ⁻ oxygen of one molecule and the δ⁺ carbon of the other
The carboxyl group of a carboxylic acid contains an O–H bond which is responsible for the formation of hydrogen bonding. Thus, carboxylic acids have the strongest intermolecular forces of attraction (Fig. 12.3 in section 12.6.4).
12.6.2 Physical State and Boiling Points of Aldehydes and Ketones
Formaldehyde is a gas at room temperature and has an irritating odour. Acetaldehyde is an extremely volatile, colourless liquid. Higher aldehydes have a pleasant odour. Acetone is a liquid at room temperature and has a pleasant odour, but most of the higher ketones have bland odours.
Increasing boiling points in the homologous series of aldehydes and ketones are listed in Table 12.4. A homologous series is a family of compounds differing by one \(\mathrm{-CH_2-}\) unit at a time, so the table lets us see the effect of chain length alone.
Table 12.4 Boiling points of aldehydes and ketones
| Number of carbon atoms | Aldehyde | Boiling point | Ketone | Boiling point |
|---|---|---|---|---|
| 1 | Methanal | 252 K | —– | ——– |
| 2 | Ethanal | 294 K | ——- | ——— |
| 3 | Propanal | 319 K | Propanone | 329 K |
| 4 | Butanal | 348 K | Butan-2-one | 353 K |
| 5 | Pentanal | 376 K | Pentan-2-one | 375 K |
| 6 | Hexanal | 392 K | Hexan-2-one | 400 K |
As the chain grows, the boiling point rises steadily. The dipole–dipole force does not change much from one member to the next, but the growing hydrocarbon chain increases the surface over which weak van der Waals forces can act, and more energy is needed to pull the molecules apart. Notice also that the first two ketone entries are blank: a ketone needs a carbonyl carbon flanked by two carbon groups, so the smallest possible ketone has three carbon atoms.
Diagram
Fig. 12.6a – Line graph of boiling point in K (y-axis, 240 to 410) against number of carbon atoms (x-axis, 1 to 6), with one line for the aldehydes methanal to hexanal and a second line for the ketones propanone to hexan-2-one, each point labelled with the compound name
12.6.3 Solubility of Aldehydes and Ketones
The oxygen atom of the \(>\mathrm{C=O}\) group can involve itself in hydrogen bonding with a water molecule, as shown in Fig. 12.2. Here water supplies the hydrogen and the carbonyl oxygen supplies the lone pair, so the hydrogen bond that was impossible between two carbonyl molecules becomes possible between a carbonyl molecule and water.
Diagram
Fig. 12.2 – Hydrogen bonding in carbonyl compound and water: a >C=O group with δ⁻ on the oxygen and δ⁺ on the carbon, and a dotted hydrogen bond from that oxygen to the δ⁺ hydrogen of an H–O–H molecule
As a result of this, the lower aldehydes and ketones are water soluble; acetaldehyde and acetone are common examples. As the molecular mass increases, the proportion of the hydrocarbon part of the molecule increases, which cannot form hydrogen bonds, and the water solubility decreases.
12.6.4 Physical State, Boiling Points and Solubilities of Carboxylic Acids
Lower aliphatic carboxylic acids up to nine carbon atoms are colourless liquids with irritating odours. The higher homologues are colourless, odourless, wax-like solids and have low volatility. Boiling points of the lower carboxylic acids are listed in Table 12.5.
Table 12.5 Increasing boiling points of carboxylic acids
| Name | Formula | Boiling point in K |
|---|---|---|
| Formic acid | \(\mathrm{HCOOH}\) | 373 K |
| Acetic acid | \(\mathrm{CH_3COOH}\) | 391 K |
| Propionic acid | \(\mathrm{CH_3CH_2COOH}\) | 414 K |
| Butyric acid | \(\mathrm{CH_3CH_2CH_2COOH}\) | 437 K |
| Valeric acid | \(\mathrm{CH_3CH_2CH_2CH_2COOH}\) | 460 K |
Why Carboxylic Acids Boil So High
Carboxylic acids have higher boiling points than those of alkanes, ethers, alcohols, aldehydes and ketones of comparable mass, as Table 12.6 shows. The reason is that, in the liquid phase, carboxylic acids form a dimer in which two molecules are held together by two hydrogen bonds. The acidic hydrogen of one molecule forms a hydrogen bond with the carbonyl oxygen of the other molecule, as drawn in Fig. 12.3.
Diagram
Fig. 12.3 – Dimer of acetic acid: two acetic acid molecules facing each other in a ring, the O–H hydrogen of each molecule dotted across to the carbonyl oxygen of the other, with δ⁺ and δ⁻ marked; caption “two molecules held by two hydrogen bonds”
This doubles the size of the molecule, resulting in an increase in the intermolecular van der Waals forces, which in turn results in a high boiling point. In the case of acetic acid, dimers exist even in the gas phase (Fig. 12.3).
Table 12.6 Variation of boiling point with functional group
| Compound | Family | Molecular mass | Boiling point | Strength of intermolecular forces |
|---|---|---|---|---|
| \(\mathrm{CH_3\!-\!CH_2\!-\!CH_2\!-\!CH_3}\) | Alkane | 58 | 272 K | increases down the column |
| \(\mathrm{CH_3\!-\!O\!-\!CH_2\!-\!CH_3}\) | Ether | 60 | 281 K | |
| \(\mathrm{CH_3\!-\!CH_2\!-\!CHO}\) | Aldehyde | 58 | 322 K | |
| \(\mathrm{CH_3\!-\!CO\!-\!CH_3}\) | Ketone | 58 | 329 K | |
| \(\mathrm{CH_3\!-\!CH_2\!-\!CH_2\!-\!OH}\) | Alcohol | 60 | 370 K | |
| \(\mathrm{CH_3\!-\!COOH}\) | Carboxylic acid | 60 | 391 K |
Every compound in Table 12.6 has a molecular mass of 58 or 60, so mass is effectively held constant and the whole 119 K spread of boiling points is the work of the functional group alone.
Solubility of Carboxylic Acids
Lower aliphatic carboxylic acids containing up to four carbons are miscible with water due to the formation of intermolecular hydrogen bonds between carboxylic acid molecules and solvent water molecules. The solubility of carboxylic acids in water decreases with increase in molecular mass.
Higher carboxylic acids are practically insoluble in water due to the increased hydrophobic (water hating) interaction of the hydrocarbon part with water. Aromatic acids like benzoic acid are also practically insoluble in water at room temperature. Water insoluble carboxylic acids are soluble in less polar organic solvents like ether, alcohol, benzene and so on.
12.7 Polarity of Carbonyl Group
The polarity of a carbonyl group originates from the higher electronegativity of oxygen relative to carbon, as well as from resonance effects, as shown in Fig. 12.4.
Two separate influences are therefore at work on the same bond. The first is the inductive pull: because oxygen holds electrons more tightly than carbon does, the shared electrons of the \(\sigma\) bond are drawn towards oxygen. The second is resonance: the \(\pi\) electrons of the double bond can shift completely onto oxygen, giving a structure in which oxygen carries a full negative charge and carbon a full positive charge.
Diagram
Fig. 12.4 – Polarity of carbonyl group in four parts: (A) >C=O with an arrow on the bond showing electron shift to O, δ⁻ on O and δ⁺ on C; (B) the major resonance structure with the intact C=O and two lone pairs on O; (C) the minor resonance structure with a C–O single bond, O carrying a negative charge and three lone pairs, and C carrying a positive charge; (D) the resonance hybrid drawn with a dashed partial double bond, δ⁻ on O and δ⁺ on C
Structure (C) is described as the minor contributor because separating charge costs energy, while structure (B), with no separated charge, is the major contributor. The true molecule is neither one nor the other; it is the resonance hybrid (D), in which the oxygen carries a partial negative charge and the carbon a partial positive charge.
The carbonyl carbon has positive polarity, as seen in structures (A) and (D). Therefore, it is electron deficient. As a result, this carbon atom is electrophilic (electron loving) and is susceptible to attack by a nucleophile (\(\mathrm{Nu\!:^{-}}\)).
12.7.1 Reactivity of Aldehydes and Ketones
The reactivity of aldehydes and ketones is due to the polarity of the carbonyl group, which results in the electrophilicity of carbon. In general, aldehydes are more reactive than ketones toward nucleophilic attack. This can be well explained in terms of both the electronic effects and the steric effect.
Influence of Electronic Effects
Alkyl groups have an electron donating inductive effect (\(+\mathrm{I}\)). A ketone has two electron donating alkyl groups bonded to the carbonyl carbon, which are responsible for decreasing its positive polarity and its electrophilicity. In contrast, aldehydes have only one electron donating group bonded to the carbonyl carbon. This makes aldehydes more electrophilic than ketones.
The logic is worth stating plainly: electron density pushed towards the carbonyl carbon partly cancels its \(\delta+\) charge. Two alkyl groups push twice as much as one, so the ketone’s carbonyl carbon is the less hungry of the two for a nucleophile’s electron pair.
Steric Effects
Two bulky alkyl groups in a ketone come in the way of the incoming nucleophile. This is called steric hindrance to nucleophilic attack.
On the other hand, a nucleophile can easily attack the carbonyl carbon in an aldehyde, because it has one alkyl group and is less crowded, or sterically less hindered. Hence aldehydes are more easily attacked by nucleophiles. The two situations are compared in Fig. 12.7a.
Diagram
Fig. 12.7a – Side by side: a ketone R–C(=O)–R with δ⁻ on O and δ⁺ on C, arrows from both R groups pushing electrons towards the carbon and the two bulky R groups shown blocking an approaching nucleophile; and an aldehyde R–C(=O)–H with only one such arrow and a clear path for the approaching nucleophile
So the two effects point the same way. Electronically the aldehyde carbon carries more positive charge, and geometrically it is easier to reach — and reactivity towards nucleophilic addition depends on both.
Aromatic aldehydes are less reactive than aliphatic aldehydes in nucleophilic addition reactions. This is due to the electron-donating resonance effect of the aromatic ring, which makes the carbonyl carbon less electrophilic. The ring is able to feed \(\pi\) electron density into the carbonyl group in a way that a simple alkyl group cannot.
12.8 Chemical Properties of Aldehydes and Ketones
12.8.1 Laboratory Tests for Aldehydes and Ketones
Aldehydes are easily oxidized to carboxylic acids and therefore act as reducing agents toward mild oxidizing agents. Ketones do not have a hydrogen atom directly attached to the carbonyl carbon. Hence they are not oxidized by mild oxidizing agents.
The structural reason is worth pausing on. Oxidising an aldehyde means converting \(\mathrm{-CHO}\) into \(\mathrm{-COOH}\), and the hydrogen sitting on the carbonyl carbon is what makes this possible without breaking any carbon–carbon bond. A ketone has no such hydrogen, so oxidising it would mean breaking a strong C–C bond, and a mild oxidant cannot do that.
On the basis of this difference in reactivity, aldehydes and ketones are distinguished by the following tests. The whole scheme is set out as a decision path in FC 12.8a.
Flowchart
FC 12.8a – Distinguishing an aldehyde from a ketone: unknown carbonyl compound → add Schiff’s reagent; if pink/red/magenta colour appears → aldehyde, if no colour → not an aldehyde → boil with Tollens’ reagent; if silver mirror forms → aldehyde, if no mirror → boil with Fehling solution; if red precipitate of Cu₂O forms → aldehyde (note: α-hydroxy ketone also positive), if no precipitate → add freshly prepared sodium nitroprusside and basify with NaOH; if red colour appears → ketone
Tests Given by Only Aldehydes
Schiff test. When an alcoholic solution of an aldehyde is treated with a few drops of Schiff’s reagent, a pink or red or magenta colour appears. This confirms the presence of the aldehydic (\(\mathrm{-CHO}\)) group.
Tollens’ test or silver mirror test. When an aldehyde is boiled with Tollens’ reagent (ammoniacal silver nitrate), a silver mirror is formed. The aldehyde is oxidized to the carboxylate ion by Tollens’ reagent, and the \(\mathrm{Ag^{+}}\) ion is reduced to \(\mathrm{Ag}\).
\[\mathrm{R\!-\!CHO} + 2\,\mathrm{Ag(NH_3)_2^{+}} + 3\,\mathrm{OH^{-}} \xrightarrow{\ \Delta\ } \mathrm{R\!-\!COO^{-}} + 2\,\mathrm{Ag}\downarrow + 4\,\mathrm{NH_3}\uparrow + 2\,\mathrm{H_2O}\]
\[\text{(aldehyde)} \qquad \text{(Tollens' reagent)} \qquad \text{(carboxylate)} \quad \text{(Silver mirror)}\]
Two changes happen in the same equation, and it is worth naming them. The carbon of \(\mathrm{-CHO}\) gains an oxygen, so it is oxidized; the silver in \(\mathrm{Ag(NH_3)_2^{+}}\) goes from the \(+1\) state to free metal, so it is reduced. A reaction in which one species is oxidized while another is simultaneously reduced is a redox reaction.
Fehling test. When a mixture of an aldehyde and Fehling solution is boiled in hot water, a red precipitate of cuprous oxide is formed.
\[\mathrm{R\!-\!CHO} + 2\,\mathrm{Cu^{2+}} + 5\,\mathrm{OH^{-}} \xrightarrow{\ \text{boil}\ } \mathrm{R\!-\!COO^{-}} + \mathrm{Cu_2O}\downarrow + 3\,\mathrm{H_2O}\]
\[\text{(Aldehyde)} \quad \text{(Fehling solution)} \qquad \text{(carboxylate ion)} \quad \text{(red ppt)}\]
An aldehyde is oxidized to the carboxylate ion by Fehling solution, and the \(\mathrm{Cu^{2+}}\) ion is reduced to the \(\mathrm{Cu^{+}}\) ion. It may be noted that an \(\alpha\)-hydroxy ketone also gives this test positive, so a positive Fehling test alone does not prove that a compound is an aldehyde.
Laboratory Test for Ketonic Group: Sodium Nitroprusside Test
When a freshly prepared sodium nitroprusside solution is added to a ketone, the mixture is shaken well and basified by adding sodium hydroxide solution drop by drop, a red colour appears in the solution, which indicates the presence of the ketonic (\(>\mathrm{C=O}\)) group.
\[\mathrm{CH_3\!-\!CO\!-\!CH_3} + \mathrm{OH^{-}} \longrightarrow \mathrm{CH_3\!-\!CO\!-\!CH_2^{-}} + \mathrm{H_2O}\]
\[\text{(Acetone)}\]
\[[\mathrm{Fe(CN)_5NO}]^{2-} + \mathrm{CH_3\!-\!CO\!-\!CH_2^{-}} \longrightarrow [\mathrm{Fe(CN)_5NO(CH_3\!-\!CO\!-\!CH_2)}]^{3-}\]
\[\text{(Nitroprusside ion)} \qquad\qquad \text{(Red colouration)}\]
The anion of the ketone formed by the alkali reacts with the nitroprusside ion to form a red coloured complex, which indicates the presence of the ketonic group.
12.8.2 Chemical Reactions of Aldehydes and Ketones with Nucleophile
In all these reactions the nucleophilic reagent brings about the reaction by attacking the positively polarized electrophilic carbonyl carbon in aldehydes and ketones.
Addition of Hydrogen Cyanide (H–CN)
Hydrogen cyanide, a weak acid, adds across the carbon–oxygen double bond in aldehydes and ketones to produce compounds called cyanohydrins. The negative part of the reagent, \(\mathrm{{}^{-}CN}\), attacks the electrophilic carbon of the carbonyl group. The reaction requires either acid or base as a catalyst.
\[\mathrm{R(R')C{=}O} + \mathrm{HCN} \longrightarrow \mathrm{R(R')C(OH)\!-\!CN}\]
\[\text{(Aldehyde when R' = H; ketone when R' = alkyl/aryl)} \qquad \text{(cyanohydrin)}\]
For example,
\[\mathrm{CH_3\!-\!CHO} + \mathrm{H\!-\!CN} \longrightarrow \mathrm{CH_3\!-\!CH(OH)\!-\!CN}\]
\[\text{(Acetaldehyde)} \qquad \text{(acetaldehyde cyanohydrin)}\]
\[\mathrm{CH_3\!-\!CO\!-\!CH_3} + \mathrm{H\!-\!CN} \longrightarrow \mathrm{CH_3\!-\!C(CH_3)(OH)\!-\!CN}\]
\[\text{(Acetone)} \qquad \text{(Acetone cyanohydrin)}\]
Notice what has happened to the double bond: the \(\mathrm{{}^{-}CN}\) has joined the carbon and the \(\mathrm{H}\) has joined the oxygen, so the \(\pi\) bond is replaced by two new \(\sigma\) bonds. This is the general pattern of every nucleophilic addition in this section.
Cyanohydrin formation is a ‘step-up’ reaction, as a new carbon–carbon single bond is formed. The \(\mathrm{-C{\equiv}N}\) group can then be converted into \(\mathrm{-COOH}\), \(\mathrm{-CH_2\!-\!NH_2}\) and so on. Therefore, cyanohydrins are used as intermediates in step-up synthesis — that is, in building a molecule with one more carbon atom than the one you began with.
Addition of NaHSO₃ (Sodium Bisulphite)
Aldehydes and ketones react with a saturated aqueous solution of sodium bisulfite to give a crystalline precipitate of the sodium bisulfite adduct (addition compound). For example,
\[\mathrm{CH_3\!-\!CHO} + \mathrm{NaHSO_3} \longrightarrow \mathrm{CH_3\!-\!CH(OH)\!-\!SO_3Na}\]
\[\text{(Acetaldehyde)} \qquad \text{(Acetaldehyde sodium bisulfite adduct)}\]
\[\mathrm{CH_3\!-\!CO\!-\!CH_3} + \mathrm{NaHSO_3} \longrightarrow \mathrm{CH_3\!-\!C(CH_3)(OH)\!-\!SO_3Na}\]
\[\text{(Acetone)} \qquad \text{(Acetone sodium bisulfite adduct)}\]
Addition of Alcohols
An aldehyde reacts with one molecule of an anhydrous monohydric alcohol in the presence of dry hydrogen chloride to give an alkoxyalcohol known as a hemiacetal, which further reacts with one more molecule of anhydrous monohydric alcohol to give a geminal dialkoxy compound known as an acetal. “Geminal” means that both alkoxy groups sit on the same carbon atom.
Step 1:
\[\mathrm{R\!-\!CHO} + \mathrm{R'\!-\!OH} \ \underset{\text{dil. HCl}}{\overset{\text{dry HCl}}{\rightleftharpoons}}\ \mathrm{R\!-\!CH(OH)\!-\!OR'}\]
\[\text{(Aldehyde)} \qquad\qquad \text{(Hemiacetal) unstable}\]
Step 2:
\[\mathrm{R\!-\!CH(OH)\!-\!OR'} + \mathrm{R'\!-\!OH} \ \underset{\text{dil. HCl}}{\overset{\text{dry HCl}}{\rightleftharpoons}}\ \mathrm{R\!-\!CH(OR')_2} + \mathrm{H_2O}\]
\[\text{(Hemiacetal)} \qquad\qquad \text{(Acetal) stable}\]
For example, with ethanol:
Step 1:
\[\mathrm{CH_3\!-\!CHO} + \mathrm{H_5C_2\!-\!OH} \ \underset{\text{dil. HCl}}{\overset{\text{dry HCl}}{\rightleftharpoons}}\ \mathrm{H_3C\!-\!CH(OH)\!-\!OC_2H_5}\]
\[\text{(Acetaldehyde)} \qquad\qquad \text{(Hemiacetal) unstable}\]
Step 2:
\[\mathrm{H_3C\!-\!CH(OH)\!-\!OC_2H_5} + \mathrm{H_5C_2\!-\!OH} \ \underset{\text{dil. HCl}}{\overset{\text{dry HCl}}{\rightleftharpoons}}\ \mathrm{H_3C\!-\!CH(OC_2H_5)_2} + \mathrm{H_2O}\]
\[\text{(Hemiacetal, unstable)} \qquad \text{(1,1-Diethoxyethane) (stable)}\]
Similarly, ketones react with alcohol in the presence of an acid catalyst to form a hemiketal and a ketal.
Ketones react with 1,2- or 1,3-diols in the presence of dry hydrogen chloride to give five- or six-membered cyclic ketals, as shown in Fig. 12.8a. Because both \(\mathrm{-OH}\) groups belong to the same molecule, the second one is always close at hand once the first has reacted, which is why the ring closes so readily.
Diagram
Fig. 12.8a – A ketone R₂C=O plus ethane-1,2-diol (HO–CH₂–CH₂–OH) with dry HCl/dil. HCl giving a five-membered cyclic ketal: the former carbonyl carbon bearing both R groups and two ring oxygens joined through –CH₂–CH₂–; label ketone, ethane-1,2-diol, cyclic ketal
The reaction can be reversed by treating the cyclic ketal with aqueous HCl to regenerate the ketone.
Acetals and ketals are hydrolysed with aqueous mineral acids to give the corresponding aldehydes and ketones respectively.
An organic molecule containing both an alcohol and a carbonyl group can undergo an intramolecular reaction with dry HCl to form cyclic hemiacetals or hemiketals.
Addition of Grignard Reagent
Aldehydes and ketones on reaction with alkyl magnesium halide followed by acid hydrolysis give alcohols (refer to Chapter 11, sec. 11.4.1 d). The alkyl group of the Grignard reagent is the nucleophile here, and it attacks the carbonyl carbon exactly as \(\mathrm{{}^{-}CN}\) did.
Nucleophilic Addition–Elimination with Ammonia Derivatives
Aldehydes and ketones undergo addition–elimination with some ammonia derivatives (\(\mathrm{NH_2\!-\!Z}\)) to give a product containing \(\mathrm{C{=}N}\) bonds (imines). The reaction is reversible and takes place in a weakly acidic medium. The substituted imine is called a Schiff’s base.
\[>\mathrm{C{=}O} + \mathrm{NH_2\!-\!Z} \ \rightleftharpoons\ [\,>\mathrm{C(OH)\!-\!NH\!-\!Z}\,] \xrightarrow{\ -\mathrm{H_2O}\ } >\mathrm{C{=}N\!-\!Z} + \mathrm{H_2O}\]
\[\text{(Aldehyde or Ketone)} \qquad\qquad \text{(imine)}\]
The name of the reaction describes it exactly: the nitrogen first adds to the carbonyl carbon, and a molecule of water is then eliminated to create the new double bond.
Where \(\mathrm{Z} = \mathrm{-R}\), \(\mathrm{-Ar}\), \(\mathrm{-NH_2}\), \(\mathrm{-NHC_6H_5}\), \(\mathrm{-NHCONH_2}\), \(\mathrm{-NHC_6H_3(NO_2)_2}\).
Some important reactions of this type are listed in Table 12.7.
Table 12.7 Nucleophilic addition–elimination reactions of aldehydes and ketones with ammonia derivatives
| Sr. No. | Aldehyde (R′ = H) / Ketone (R′ ≠ H) | Ammonia derivative \(\mathrm{NH_2\!-\!Z}\) | Product (\(-\mathrm{H_2O}\)): imine, a crystalline derivative |
|---|---|---|---|
| 1 | \(\mathrm{R(R')C{=}O}\) | \(\mathrm{NH_2\!-\!OH}\) (Hydroxyl amine) | \(\mathrm{R(R')C{=}N\!-\!OH}\) (oxime) |
| 2 | \(\mathrm{R(R')C{=}O}\) | \(\mathrm{NH_2\!-\!NH_2}\) (Hydrazine) | \(\mathrm{R(R')C{=}N\!-\!NH_2}\) (hydrazone) |
| 3 | \(\mathrm{R(R')C{=}O}\) | \(\mathrm{NH_2\!-\!NH\!-\!C_6H_5}\) (Phenyl hydrazine) | \(\mathrm{R(R')C{=}N\!-\!NH\!-\!C_6H_5}\) (phenylhydrazone) |
| 4 | \(\mathrm{R(R')C{=}O}\) | \(\mathrm{NH_2\!-\!NH\!-\!CONH_2}\) (Semicarbazide) | \(\mathrm{R(R')C{=}N\!-\!NH\!-\!CONH_2}\) (semicarbazone) |
| 5 | \(\mathrm{R(R')C{=}O}\) | \(\mathrm{H_2N\!-\!NH\!-\!C_6H_3(NO_2)_2}\) (2,4-Dinitrophenyl hydrazine) | \(\mathrm{R(R')C{=}N\!-\!NH\!-\!C_6H_3(NO_2)_2}\) (2,4-Dinitrophenylhydrazone) |
For example,
\[\mathrm{CH_3\!-\!CHO} + \mathrm{NH_2\!-\!OH} \ \rightleftharpoons\ [\,\mathrm{H_3C\!-\!CH(OH)\!-\!NH\!-\!OH}\,] \xrightarrow{\ -\mathrm{H_2O}\ } \mathrm{H_3C\!-\!CH{=}N\!-\!OH}\]
\[\text{(Acetaldehyde)} \quad \text{(Hydroxyl amine)} \qquad\qquad \text{(Acetaldoxime)}\]
All aldehydes and ketones give similar reactions. The resulting products have high molecular mass and are crystalline solids. These reactions are, therefore, useful for characterization of the original aldehydes and ketones — a liquid that is hard to identify becomes a solid with a sharp melting point that can be matched against known values.
In strongly acidic medium, the nitrogen atom of the ammonia derivative \(\mathrm{H_2N\!-\!Z}\) is protonated to form the \(\mathrm{(H_3N^{+}\!-\!Z)}\) ion, which is no longer a nucleophile. This is why the medium must be only weakly acidic: a little acid helps the water leave, but too much destroys the attacking species itself.
Haloform Reaction
This reaction is given by acetaldehyde, all methyl ketones (\(\mathrm{CH_3\!-\!CO\!-\!R}\)) and all alcohols containing the \(\mathrm{CH_3\!-\!(CHOH)-}\) group. When an alcohol or methyl ketone is warmed with sodium hydroxide and iodine, a yellow precipitate is formed. Here the reagent sodium hypoiodite is produced in situ, that is, generated in the reaction mixture itself rather than added ready-made.
During the reaction, the sodium salt of a carboxylic acid is formed, which contains one carbon atom less than the substrate. The methyl group is converted into haloform. For example, acetone is oxidized by sodium hypoiodite to give the sodium salt of acetic acid and a yellow precipitate of iodoform.
\[\mathrm{H_3C\!-\!CO\!-\!CH_3} + 3\,\mathrm{NaOI} \xrightarrow[\ \Delta\ ]{\ \mathrm{NaOH},\ \mathrm{I_2}\ } \mathrm{CHI_3}\downarrow + \mathrm{H_3C\!-\!CO\!-\!ONa} + 2\,\mathrm{NaOH}\]
\[\text{(Acetone)} \quad \text{(Sodium hypoiodite)} \quad \text{(Iodoform)} \quad \text{(Sodium acetate)}\]
Three points are worth remembering about this test. First, if a \(\mathrm{C{=}C}\) bond is present in a given aldehyde, ketone or methyl ketone, it is not attacked by the hypohalite. Second, non-methyl ketones do not give a positive iodoform test. Third, secondary alcohols having the \(\mathrm{CH_3\!-\!CHOH-}\) group give a positive iodoform test, because the reagent first oxidizes them to a \(\mathrm{CH_3\!-\!CO-}\) group, which subsequently forms iodoform.
Aldol Condensation
Aldehydes containing at least one \(\alpha\)-hydrogen atom undergo a reaction in the presence of dilute alkali (dilute NaOH, KOH or \(\mathrm{Na_2CO_3}\)) as catalyst to form \(\beta\)-hydroxy aldehydes (aldol). This reaction is known as the aldol reaction. Formation of the aldol is an addition reaction.
The aldol formed from an aldehyde having \(\alpha\)-hydrogens undergoes subsequent elimination of a water molecule on warming, giving rise to an \(\alpha,\beta\)-unsaturated aldehyde.
\[2\,\mathrm{R\!-\!CH_2\!-\!CHO} \xrightarrow{\ \text{aq. NaOH}\ } \mathrm{R\!-\!CH_2\!-\!CH(OH)\!-\!CH(R)\!-\!CHO}\]
\[\text{(aldehyde)} \qquad\qquad \text{(aldol)}\]
\[\mathrm{R\!-\!CH_2\!-\!CH(OH)\!-\!CH(R)\!-\!CHO} \xrightarrow[\ \text{warm}\ ]{\ -\mathrm{H_2O}\ } \mathrm{R\!-\!CH_2\!-\!CH{=}C(R)\!-\!CHO}\]
\[\text{(aldol)} \qquad\qquad (\alpha,\beta\text{ - unsaturated aldehyde})\]
For example:
\[2\,\mathrm{CH_3\!-\!CHO} \xrightarrow{\ \text{dil. NaOH}\ } \mathrm{CH_3\!-\!CH(OH)\!-\!CH_2\!-\!CHO} \xrightarrow[\ -\mathrm{H_2O}\ ]{\ \Delta\ } \mathrm{CH_3\!-\!CH{=}CH\!-\!CHO}\]
\[\text{(Ethanal)} \qquad \text{(3-Hydroxybutanal, an Aldol)} \qquad \text{(But-2-enal)}\]
The overall reaction is called aldol condensation. It is a nucleophilic addition–elimination reaction.
Ketones containing at least two \(\alpha\)-hydrogens also undergo the aldol condensation reaction and give an \(\alpha,\beta\)-unsaturated ketone. For example:
\[2\,\mathrm{H_3C\!-\!CO\!-\!CH_3} \xrightarrow{\ \mathrm{Ba(OH)_2}\ } \mathrm{H_3C\!-\!C(CH_3)(OH)\!-\!CH_2\!-\!CO\!-\!CH_3}\]
\[\text{(Propanone)} \qquad \text{(4-Hydroxy-4-methylpentan-2-one, a Ketol)}\]
\[\mathrm{H_3C\!-\!C(CH_3)(OH)\!-\!CH_2\!-\!CO\!-\!CH_3} \xrightarrow[\ -\mathrm{H_2O}\ ]{\ \Delta\ } \mathrm{H_3C\!-\!C(CH_3){=}CH\!-\!CO\!-\!CH_3}\]
\[\text{(4-Methylpent-3-en-2-one)}\]
Flowchart
FC 12.8b – Aldol condensation in two stages: carbonyl compound with at least one α-hydrogen → dilute alkali removes an α-hydrogen → carbanion → attacks the carbonyl carbon of a second molecule (addition) → β-hydroxy carbonyl compound, the aldol → warm, –H₂O (elimination) → α,β-unsaturated carbonyl compound; branch: no α-hydrogen → no aldol reaction possible
Cross aldol condensation refers to the aldol condensation that takes place between two different aldehydes or ketones. If both aldehydes or ketones contain two \(\alpha\)-hydrogen atoms each, then a mixture of four products is formed. This happens because each molecule can act either as the one that loses an \(\alpha\)-hydrogen or as the one that is attacked, giving two self-condensation products and two cross products.
For example, a mixture of ethanal and propanal on reaction with dilute alkali followed by heating gives a mixture of four products (Fig. 12.5).
Diagram
Fig. 12.5 – Cross aldol condensation: ethanal (CH₃CHO) plus propanal (CH₃CH₂CHO) with i. dil. NaOH, ii. heat, branching into “self condensation” giving crotonaldehyde (but-2-enal) and 2-methylpent-2-enal, and “cross condensation” giving 2-methylbut-2-enal and pent-2-enal; label all four products
Ketones can also be used as one of the components in cross aldol condensation.
Cannizzaro Reaction
This reaction is given only by aldehydes having no \(\alpha\)-hydrogen atom. Such aldehydes undergo a self-oxidation and reduction reaction on heating with concentrated alkali. This is an example of a disproportionation reaction. In the Cannizzaro reaction, one molecule of an aldehyde is reduced to an alcohol and at the same time a second molecule is oxidized to a carboxylic acid salt.
For example, formaldehyde and benzaldehyde:
\[2\,\mathrm{H\!-\!CHO} + \mathrm{NaOH}\ (50\%) \xrightarrow{\ \Delta\ } \mathrm{H\!-\!CO\!-\!O^{-}Na^{+}} + \mathrm{CH_3\!-\!OH}\]
\[\text{(Formaldehyde)} \quad \text{(sodium hydroxide)} \quad \text{(Sodium formate)} \quad \text{(Methanol)}\]
\[2\,\mathrm{C_6H_5\!-\!CHO} \xrightarrow[\ \Delta\ ]{\ \mathrm{KOH}\ \text{in MeOH}\ } \mathrm{C_6H_5\!-\!CO\!-\!O^{-}K^{+}} + \mathrm{C_6H_5\!-\!CH_2\!-\!OH}\]
\[\text{(Benzaldehyde)} \qquad \text{(Potassium benzoate)} \quad \text{(Phenylmethanol / benzyl alcohol)}\]
Cross Cannizzaro reaction. When a mixture of formaldehyde and a non-enolisable aldehyde (an aldehyde with no \(\alpha\)-hydrogen) is treated with a strong base, formaldehyde is oxidized to formic acid while the other non-enolisable aldehyde is reduced to an alcohol. Formic acid forms sodium formate with NaOH. On acidification, sodium formate is converted into formic acid. For example:
\[\mathrm{H\!-\!CHO} + \mathrm{C_6H_5\!-\!CHO} \xrightarrow[\ \text{ii. } \mathrm{H_3O^{+}}\ ]{\ \text{i. conc. NaOH}\ } \mathrm{C_6H_5\!-\!CH_2\!-\!OH} + \mathrm{H\!-\!COOH}\]
\[\text{(Formaldehyde)} \quad \text{(Benzaldehyde)} \qquad \text{(Phenylmethanol)} \quad \text{(formic acid)}\]
Formaldehyde takes the oxidation role because its carbonyl carbon, carrying only hydrogens, gives up a hydride most readily of all.
12.8.3 Oxidation and Reduction Reactions of Aldehydes and Ketones
Oxidation by Dilute HNO₃, KMnO₄ and K₂Cr₂O₇
Aldehydes are oxidized to the corresponding carboxylic acids by an oxidant such as dilute nitric acid, potassium permanganate, and sodium or potassium dichromate in acidic medium.
\[\mathrm{R\!-\!CHO} \xrightarrow[\ \text{dil. } \mathrm{H_2SO_4}\ ]{\ \mathrm{K_2Cr_2O_7}\ } \mathrm{R\!-\!COOH}\]
\[\text{(Aldehyde)} \qquad \text{(Carboxylic acid)}\]
Ketones resist oxidation due to the strong CO–C bond, but they are oxidized by strong oxidizing agents such as \(\mathrm{CrO_3}\), alkaline \(\mathrm{KMnO_4}\) or hot concentrated \(\mathrm{HNO_3}\) to a mixture of carboxylic acids having a smaller number of carbon atoms than the starting ketone. Thus, oxidation of ketones is accompanied by breaking of the C–C bond.
\[\mathrm{R\!-\!CO\!-\!R'} \xrightarrow{\ \mathrm{CrO_3}\ } \mathrm{R\!-\!COOH} + \mathrm{R'\!-\!COOH}\]
\[\text{(Ketone)} \qquad\qquad \text{(Carboxylic acids)}\]
For example,
\[\mathrm{H_3C\!-\!CO\!-\!CH_3} \xrightarrow{\ \mathrm{CrO_3}\ } \mathrm{H_3C\!-\!COOH}\]
\[\text{(Acetone)} \qquad \text{(Acetic Acid)}\]
\[\mathrm{H_3C\!-\!CO\!-\!C_2H_5} \xrightarrow{\ \mathrm{CrO_3}\ } \mathrm{H_3C\!-\!COOH} + \mathrm{H_5C_2\!-\!COOH}\]
\[\text{(Butan-2-one)} \qquad \text{(Ethanoic acid)} \quad \text{(Propanoic acid)}\]
Because the C–C bond can break on either side of the carbonyl group, an unsymmetrical ketone gives a mixture of acids rather than a single product — which is why this oxidation is of little use in synthesis and mainly of interest as a contrast to the easy oxidation of aldehydes.
Clemmensen and Wolf-Kishner Reduction
The carbonyl group of aldehydes and ketones is reduced to the methylene group (\(\mathrm{-CH_2-}\)) on treatment with zinc–amalgam and concentrated hydrochloric acid (Clemmensen reduction), or with hydrazine followed by heating with sodium or potassium hydroxide in a high boiling solvent like ethylene glycol (Wolf-Kishner reduction). In both the reactions, oxygen is replaced by two hydrogen atoms.
Clemmensen reduction:
\[>\mathrm{C{=}O} + 4[\mathrm{H}] \xrightarrow[\ \Delta\ ]{\ \mathrm{Zn\text{-}Hg},\ \text{conc. HCl}\ } \mathrm{{-}CH_2{-}} + \mathrm{H_2O}\]
\[\text{(Carbonyl group in aldehydes and ketones)} \qquad \text{(Methylene group)}\]
For example:
\[\mathrm{H_3C\!-\!CO\!-\!CH_3} + 4[\mathrm{H}] \xrightarrow[\ \Delta\ ]{\ \mathrm{Zn\text{-}Hg},\ \text{conc. HCl}\ } \mathrm{H_3C\!-\!CH_2\!-\!CH_3} + \mathrm{H_2O}\]
\[\text{(Acetone)} \qquad\qquad \text{(propane)}\]
\[\mathrm{CH_3\!-\!CH_2\!-\!CHO} + 4[\mathrm{H}] \xrightarrow[\ \Delta\ ]{\ \mathrm{Zn\text{-}Hg},\ \text{conc. HCl}\ } \mathrm{CH_3\!-\!CH_2\!-\!CH_3} + \mathrm{H_2O}\]
\[\text{(Propanal)} \qquad\qquad \text{(Propane)}\]
Wolf-Kishner reduction:
\[>\mathrm{C{=}O} \xrightarrow[\ -\mathrm{H_2O}\ ]{\ \mathrm{H_2N\!-\!NH_2}\ } >\mathrm{C{=}N\!-\!NH_2} \xrightarrow[\ \Delta\ ]{\ \mathrm{KOH},\ \mathrm{HO\text{-}CH_2\text{-}CH_2\text{-}OH}\ } \mathrm{{-}CH_2{-}} + \mathrm{N_2}\]
\[\text{(Carbonyl group)} \qquad \text{(Hydrazone)} \qquad\qquad \text{(Methylene group)}\]
The first stage is nothing new — it is the addition–elimination of Table 12.7, with hydrazine as the ammonia derivative. The second stage expels the nitrogen as \(\mathrm{N_2}\) gas, and it is the escape of this very stable gas that drives the reaction forward.
For example:
\[\mathrm{C_2H_5\!-\!CHO} \xrightarrow[\ -\mathrm{H_2O}\ ]{\ \mathrm{H_2N\!-\!NH_2}\ } \mathrm{C_2H_5\!-\!CH{=}N\!-\!NH_2} \xrightarrow[\ \Delta\ ]{\ \mathrm{KOH},\ \mathrm{HO\text{-}CH_2\text{-}CH_2\text{-}OH}\ } \mathrm{CH_3\!-\!CH_2\!-\!CH_3} + \mathrm{N_2}\]
\[\text{(Propanal)} \qquad \text{(Hydrazone)} \qquad\qquad \text{(Propane)}\]
\[\mathrm{C_6H_5\!-\!CO\!-\!C_2H_5} \xrightarrow[\ -\mathrm{H_2O}\ ]{\ \mathrm{H_2N\!-\!NH_2}\ } \mathrm{C_6H_5\!-\!C(C_2H_5){=}N\!-\!NH_2} \xrightarrow[\ \Delta\ ]{\ \mathrm{KOH},\ \mathrm{HO\text{-}CH_2\text{-}CH_2\text{-}OH}\ } \mathrm{C_6H_5\!-\!CH_2\!-\!C_2H_5} + \mathrm{N_2}\]
\[\text{(Ethyl phenyl ketone)} \qquad \text{(Hydrazone)} \qquad\qquad \text{(n-Propyl benzene)}\]
Wolf-Kishner reduction is used to synthesize straight chain alkyl substituted benzenes, which is not possible by the Friedel–Crafts alkylation reaction.
12.8.4 Electrophilic Substitution Reactions
Aromatic aldehydes and ketones undergo electrophilic substitution reactions such as nitration, sulfonation and halogenation. The aldehydic (\(\mathrm{-CHO}\)) and ketonic (\(>\mathrm{C=O}\)) groups are electron-withdrawing by inductive as well as resonance effects. They deactivate the benzene ring at the ortho- and para- positions. This results in the formation of the meta-product.
Deactivation means the ring reacts more slowly than benzene itself, because electron density has been pulled out of it. That withdrawal is felt most strongly at the ortho and para positions, leaving the meta position comparatively the richest in electrons and therefore the site the incoming electrophile attacks.
For example,
\[\mathrm{C_6H_5\!-\!CHO} + \mathrm{HO\!-\!NO_2} \xrightarrow{\ \text{conc. } \mathrm{H_2SO_4}\ } \mathrm{3\text{-}NO_2\!-\!C_6H_4\!-\!CHO} + \mathrm{H_2O}\]
\[\text{(Benzaldehyde)} \quad \text{(conc. } \mathrm{HNO_3}\text{)} \qquad \text{(m-Nitrobenzaldehyde)}\]
Diagram
Fig. 12.8b – Nitration of benzaldehyde: a benzene ring bearing –CHO reacting with conc. HNO₃ and conc. H₂SO₄ to give the same ring with –NO₂ at the meta position, plus H₂O; mark the ortho, meta and para positions on the starting ring
12.9 Chemical Properties of Carboxylic Acids
12.9.1 Acidic Character of Carboxylic Acids
The carboxyl group (\(\mathrm{-COOH}\)) imparts acidic character to carboxylic acids. A carboxyl group is made of an \(\mathrm{-OH}\) group bonded to a carbonyl group. In aqueous solution the H atom in the OH of the carboxyl group dissociates as a proton, and the carboxylate ion is formed as the conjugate base.
\[\mathrm{R\!-\!COOH} + \mathrm{H_2O} \rightleftharpoons \mathrm{R\!-\!COO^{-}} + \mathrm{H_3O^{+}}\]
\[\text{(carboxylate ion)}\]
The conjugate base is simply what is left of the acid after the proton has gone. How strong an acid is depends on how comfortable that leftover species is, because a stable conjugate base means the equilibrium above lies further to the right.
The carboxylate ion is resonance stabilized by two equivalent resonance structures, as shown in Fig. 12.9a.
Diagram
Fig. 12.9a – Resonance in the carboxylate ion: structure (i) R–C(=O)–O⁻ with a curly arrow moving the π electrons and the lone pair, structure (ii) R–C(–O⁻)=O as the equivalent mirror image, and the resonance hybrid drawn with two identical dashed partial C–O bonds each carrying half a negative charge
The carboxylate ion has two resonance structures (i) and (ii), and both of them are equivalent to each other (refer to Std. XI Chemistry Textbook, Chapter 14). This gives good resonance stabilization to the carboxylate ion, which in turn gives acidic character to carboxylic acids. Because the two structures are identical in energy, the negative charge is shared equally between the two oxygen atoms instead of being trapped on one.
Influence of Electronic Effects on Acidity of Carboxylic Acids
All the carboxylic acids do not have the same \(\mathrm{p}K_a\) value. The structure of ‘R’ in R-COOH has an influence on the acid strength of carboxylic acids. Various haloacetic acids illustrate this point very well (Tables 12.8 and 12.9).
Table 12.8: \(\mathrm{p}K_a\) values of haloacetic acids
| Acid | \(\mathrm{p}K_a\) | Acid strength |
|---|---|---|
| \(\mathrm{F\!-\!CH_2\!-\!COOH}\) | 2.56 | decreases down the column |
| \(\mathrm{Cl\!-\!CH_2\!-\!COOH}\) | 2.86 | |
| \(\mathrm{Br\!-\!CH_2\!-\!COOH}\) | 2.90 | |
| \(\mathrm{I\!-\!CH_2\!-\!COOH}\) | 3.18 | |
| \(\mathrm{CH_3\!-\!COOH}\) | 4.76 |
Halogens are electronegative atoms and exert an electron withdrawing inductive effect (\(-\mathrm{I}\) effect). The negatively charged carboxylate ion in the conjugate base of a haloacetic acid gets stabilized by the \(-\mathrm{I}\) effect of the halogen, which is responsible for diffusing the native charge, as drawn in Fig. 12.9b.
Diagram
Fig. 12.9b – Conjugate base of a haloacetic acid, X–CH₂–COO⁻, with arrowheads placed on the C–X and C–C bonds pointing towards the halogen to show the electron withdrawing –I effect pulling density away from the negatively charged carboxylate end
The higher the electronegativity of the halogen, the greater is the stabilization of the conjugate base, the stronger is the acid, and the smaller is the \(\mathrm{p}K_a\) value. This is exactly the order in Table 12.8: fluorine, the most electronegative, gives the lowest \(\mathrm{p}K_a\), and acetic acid, with no halogen at all, the highest.
Acidity of Aromatic Carboxylic Acids
Benzoic acid is the simplest aromatic acid. From the \(\mathrm{p}K_a\) value of benzoic acid (4.2) we understand that it is stronger than acetic acid (\(\mathrm{p}K_a\) 4.76). The \(sp^2\) hybrid carbon of the aromatic ring exerts an electron withdrawing inductive effect (\(-\mathrm{I}\) effect), which stabilizes the conjugate base and increases the acid strength of aromatic acids. An \(sp^2\) carbon holds its electrons more tightly than an \(sp^3\) carbon does, because it has a greater share of s character, and this is what makes the ring carbon a withdrawing group rather than a donating one.
Table 12.9 illustrates that the more the number of electron withdrawing substituents, the higher is the acid strength.
Table 12.9 \(\mathrm{p}K_a\) values of chloroacetic acids
| Name | Structure | \(\mathrm{p}K_a\) | Acid strength |
|---|---|---|---|
| Monochloroacetic acid | \(\mathrm{Cl\!-\!CH_2\!-\!COOH}\) | 2.86 | increases down the column |
| Dichloroacetic acid | \(\mathrm{Cl_2CH\!-\!COOH}\) | 1.26 | |
| Trichloroacetic acid | \(\mathrm{Cl_3C\!-\!COOH}\) | 0.6 |
Electron-withdrawing groups like \(\mathrm{-Cl}\), \(\mathrm{-CN}\) and \(\mathrm{-NO_2}\) increase the acidity of substituted benzoic acids, while electron-donating groups like \(\mathrm{-CH_3}\), \(\mathrm{-OH}\), \(\mathrm{-OCH_3}\) and \(\mathrm{-NH_2}\) decrease the acidity of substituted benzoic acids. The three compounds in Fig. 12.9c make the comparison directly.
Diagram
Fig. 12.9c – Three benzene rings side by side: 4-nitrobenzoic acid (COOH at C1, NO₂ at C4, pKa = 3.41), benzoic acid (COOH only, pKa = 4.2), and 4-methylbenzoic acid (COOH at C1, CH₃ at C4, pKa = 4.4); label each with its name and pKa value
12.9.2 Laboratory Tests for Carboxyl (–COOH) Group
The presence of the \(\mathrm{-COOH}\) group in carboxylic acids is identified by the following tests.
Litmus Test (valid for water soluble substances)
The aqueous solution of an organic compound containing the \(\mathrm{-COOH}\) group turns blue litmus red, which indicates the presence of an acidic functional group. It may be noted that aqueous solutions of water soluble phenols also turn blue litmus red, so this test alone cannot separate the two.
Sodium Bicarbonate Test
When sodium bicarbonate is added to an organic compound containing the \(\mathrm{-COOH}\) group, a brisk effervescence of carbon dioxide gas is evolved. A water insoluble acid goes into solution and gives a precipitate on acidification with concentrated HCl. This indicates the presence of the \(\mathrm{-COOH}\) group.
\[\mathrm{R\!-\!COOH} + \mathrm{NaHCO_3}\text{(aq)} \xrightarrow[\ -\mathrm{H_2O}\ ]{\ -\mathrm{CO_2}\ } \mathrm{R\!-\!COONa}\text{(aq)}\]
\[\text{(water insoluble)}\]
\[\mathrm{R\!-\!COONa}\text{(aq)} \xrightarrow{\ \mathrm{HCl}\ } \mathrm{R\!-\!COOH}\downarrow + \mathrm{NaCl}\text{(aq)}\]
Phenol does not evolve \(\mathrm{CO_2}\) gas with sodium bicarbonate. Hence carboxylic acid and phenol are distinguished by this test.
Ester Test
One drop of concentrated sulfuric acid is added to a mixture of the given organic compound containing the \(\mathrm{-COOH}\) group and one mL of ethanol; the reaction mixture is heated for 5 minutes in a hot water bath. After this, the hot solution is poured into a beaker containing water. A fruity smell of ester confirms the presence of the carboxylic acid.
\[\mathrm{R\!-\!COOH} + \mathrm{C_2H_5OH} \xrightarrow[\ \text{warm}\ ]{\ \mathrm{H^{+}}\ } \mathrm{R\!-\!COO\!-\!C_2H_5} + \mathrm{H_2O}\]
\[\text{(ester)}\]
12.9.3 Formation of Acyl Chloride
Reaction with \(\mathrm{PCl_3}\), \(\mathrm{PCl_5}\), \(\mathrm{SOCl_2}\). Carboxylic acids on heating with \(\mathrm{PCl_3}\), \(\mathrm{PCl_5}\) or \(\mathrm{SOCl_2}\) give the corresponding acyl chlorides. Thionyl chloride (\(\mathrm{SOCl_2}\)) is preferred because the byproducts formed are in the gaseous state, so they can easily escape from the reaction mixture. In this reaction the \(\mathrm{-OH}\) group of \(\mathrm{-COOH}\) is replaced by \(\mathrm{-Cl}\).
\[\mathrm{R\!-\!COOH} + \mathrm{SOCl_2} \xrightarrow{\ \Delta\ } \mathrm{R\!-\!COCl} + \mathrm{SO_2}\uparrow + \mathrm{HCl}\uparrow\]
\[\text{(Carboxylic acid)} \qquad \text{(acyl chloride)}\]
\[3\,\mathrm{R\!-\!COOH} + \mathrm{PCl_3} \xrightarrow{\ \Delta\ } 3\,\mathrm{R\!-\!COCl} + \mathrm{P(OH)_3}\]
\[\mathrm{R\!-\!COOH} + \mathrm{PCl_5} \xrightarrow{\ \Delta\ } \mathrm{R\!-\!COCl} + \mathrm{POCl_3} + \mathrm{HCl}\]
12.9.4 Reaction with Ammonia: Formation of Amide
Carboxylic acids react with ammonia to form the ammonium carboxylate salt, which on further strong heating at high temperature decomposes to give the acid amide.
\[\mathrm{R\!-\!COOH} + \mathrm{NH_3} \rightleftharpoons \mathrm{R\!-\!COO^{-}NH_4^{+}}\]
\[\text{(Carboxylic acid)} \qquad \text{(ammonium carboxylate)}\]
\[\mathrm{R\!-\!COO^{-}NH_4^{+}} \xrightarrow[\ -\mathrm{H_2O}\ ]{\ \Delta\ } \mathrm{R\!-\!CONH_2}\]
\[\text{(Acid amide)}\]
The first step is an ordinary acid–base reaction; the amide appears only in the second step, when heating drives a molecule of water out of the salt.
Acid amides can also be prepared by reacting acid chloride with ammonia.
\[\mathrm{R\!-\!COCl} + \mathrm{NH_3} \longrightarrow \mathrm{R\!-\!CONH_2} + \mathrm{HCl}\]
\[\text{(Acyl chloride)} \qquad \text{(Acid amide)}\]
12.9.5 Formation of Acid Anhydride
Mono carboxylic acids on heating with a strong dehydrating agent like \(\mathrm{P_2O_5}\) or concentrated \(\mathrm{H_2SO_4}\) give acid anhydrides. The reaction is reversible. Anhydrides are readily hydrolyzed back to acids on reaction with water.
\[2\,\mathrm{R\!-\!CO\!-\!O\!-\!H} \ \underset{+\mathrm{H_2O}}{\overset{\mathrm{P_2O_5},\ \Delta}{\rightleftharpoons}}\ \mathrm{R\!-\!CO\!-\!O\!-\!CO\!-\!R}\]
\[\text{(Carboxylic acid)} \qquad\qquad \text{(Acid anhydride)}\]
A better yield of acid anhydride is obtained by heating sodium carboxylate with acyl chloride.
\[\mathrm{R\!-\!COO^{-}Na^{+}} + \mathrm{R\!-\!COCl} \xrightarrow{\ \Delta\ } \mathrm{R\!-\!CO\!-\!O\!-\!CO\!-\!R} + \mathrm{NaCl}\]
\[\text{(sodium carboxylate)} \quad \text{(acyl chloride)} \qquad \text{(acid anhydride)}\]
Acyl chloride and the sodium salt of the acid are prepared by reacting the carboxylic acid separately with thionyl chloride and sodium hydroxide respectively.
\[\mathrm{R\!-\!COOH} \xrightarrow{\ \mathrm{SOCl_2}\ } \mathrm{R\!-\!COCl}\]
\[\mathrm{R\!-\!COOH} \xrightarrow{\ \mathrm{NaOH}\ } \mathrm{R\!-\!COO^{-}Na^{+}}\]
\[\mathrm{R\!-\!COCl} + \mathrm{R\!-\!COO^{-}Na^{+}} \xrightarrow{\ \Delta\ } \mathrm{R\!-\!CO\!-\!O\!-\!CO\!-\!R} + \mathrm{NaCl}\]
\[\text{(Anhydride)}\]
12.9.6 Decarboxylation of Carboxylic Acids
Sodium salts of carboxylic acids on heating with soda lime give hydrocarbons which contain one carbon atom less than the carboxylic acid. Soda lime is a mixture of sodium hydroxide and calcium oxide; the calcium oxide keeps the mixture dry and easy to handle at the high temperature required.
\[\mathrm{R\!-\!COO^{-}Na^{+}} + \mathrm{NaOH} \xrightarrow[\ \Delta\ ]{\ \mathrm{CaO}\ } \mathrm{R\!-\!H} + \mathrm{Na_2CO_3}\]
For example,
\[\mathrm{CH_3\!-\!COO^{-}Na^{+}} + \mathrm{NaOH} \xrightarrow[\ \Delta\ ]{\ \mathrm{CaO}\ } \mathrm{CH_4} + \mathrm{Na_2CO_3}\]
\[\text{(Sodium acetate)} \qquad\qquad \text{(Methane)}\]
The carbon that is lost is the carboxyl carbon itself, which departs in the carbonate, so a two-carbon acid salt gives a one-carbon hydrocarbon.
12.9.7 Reduction of Carboxylic Acids
Carboxylic acids are reduced to primary alcohols by a powerful reducing agent like lithium aluminium hydride. A carboxylic acid can also be reduced by diborane; diborane does not reduce \(\mathrm{-COOR}\), \(\mathrm{-NO_2}\) or \(\mathrm{-X}\). Note that sodium borohydride (\(\mathrm{NaBH_4}\)) does not reduce the \(\mathrm{-COOH}\) group.
\[\mathrm{R\!-\!COOH} + \mathrm{LiAlH_4} \xrightarrow{\ \text{dry ether}\ } \mathrm{R\!-\!CH_2OH}\]
The seven reactions of this section are collected in FC 12.9a, which is worth memorising as a single picture of what the carboxyl group can be turned into.
Flowchart
FC 12.9a – Reactions of a carboxylic acid R–COOH: → (H₂O) → R–COO⁻ + H₃O⁺, acidic character; → (NaHCO₃) → R–COONa + CO₂↑, laboratory test; → (C₂H₅OH, conc. H₂SO₄) → ester; → (SOCl₂ or PCl₃ or PCl₅) → acyl chloride R–COCl; → (NH₃, then Δ) → amide R–CONH₂; → (P₂O₅, Δ) → anhydride R–CO–O–CO–R; → (as sodium salt, NaOH/CaO, Δ) → hydrocarbon R–H + Na₂CO₃; → (LiAlH₄, dry ether) → primary alcohol R–CH₂OH
12.10 Summary
Aldehydes, ketones and carboxylic acids are all built around the carbon–oxygen double bond, \(>\mathrm{C=O}\), called the carbonyl group. In an aldehyde the carbonyl carbon carries at least one hydrogen, giving the formyl group \(\mathrm{-CHO}\); in a ketone it carries two alkyl or aryl groups; in a carboxylic acid it carries an \(\mathrm{-OH}\) group, giving the carboxyl group \(\mathrm{-COOH}\).
Each family is classified by the carbon skeleton attached to the carbonyl group. Aldehydes and ketones are aliphatic or aromatic; aliphatic ketones are further simple (symmetrical) or mixed (unsymmetrical). Formaldehyde and formic acid are counted as aliphatic by exception. Carboxylic acids are aliphatic or aromatic, and are mono, di or tri carboxylic acids by the number of \(\mathrm{-COOH}\) groups. An aromatic compound whose \(\mathrm{-COOH}\) is not joined directly to the ring is a side-chain aromatic acid.
In the IUPAC system an aldehyde is an alkanal, a ketone an alkanone, and an acid an alkanoic acid; two groups give -dial, -dione and -dioic acid. A group on an alicyclic ring gives carbaldehyde or cycloalkane carboxylic acid. As a lower-priority substituent, \(\mathrm{-CHO}\) becomes the prefix formyl and a ketonic \(>\mathrm{C=O}\) becomes oxo. Trivial names use \(\alpha\), \(\beta\), \(\gamma\) for positions, counting from the carbon next to the carbonyl carbon.
Aldehydes and ketones are prepared by oxidation or dehydrogenation of primary and secondary alcohols respectively, by ozonolysis of alkenes, and by hydration of alkynes with 40% sulfuric acid and 1% mercuric sulfate.
From an acyl chloride, hydrogen with a palladium catalyst poisoned by barium sulfate gives an aldehyde (Rosenmund reduction), while dialkyl cadmium gives a ketone. From a nitrile, \(\mathrm{SnCl_2}\)/HCl followed by hydrolysis gives an aldehyde (Stephen reaction), and a Grignard reagent followed by hydrolysis gives a ketone. DIBAl-H, \(\mathrm{AlH(i\text{-}Bu)_2}\), reduces a nitrile to an aldehyde without touching C=C or C≡C bonds, and reduces an ester to an aldehyde at 195 K.
Aromatic aldehydes are made from methyl arenes by the Etard reaction with chromyl chloride in \(\mathrm{CS_2}\), by \(\mathrm{CrO_3}\) in acetic anhydride at 273–278 K, and by side chain chlorination of toluene followed by hydrolysis at 373 K, which is the commercial route to benzaldehyde. Gatterman–Koch formylation puts \(\mathrm{-CHO}\) directly on the ring using CO and HCl with anhydrous \(\mathrm{AlCl_3}\). Aromatic ketones come from Friedel–Crafts acylation.
Carboxylic acids are obtained by acid hydrolysis of nitriles and amides, of acyl chlorides and anhydrides, and of esters. Alkaline hydrolysis of an ester gives the carboxylate salt, which is acidified to free the acid; this reaction is called saponification and it is how soaps are made.
Oxidation of an alkyl benzene with dilute \(\mathrm{HNO_3}\), alkaline or acidic \(\mathrm{KMnO_4}\), or chromic acid converts the whole side chain, however long, into one \(\mathrm{-COOH}\) group; a tertiary alkyl substituent is not oxidized. Alkenes are oxidized by \(\mathrm{KMnO_4}\) in dilute \(\mathrm{H_2SO_4}\) to acids. A Grignard reagent added to dry ice and then hydrolysed gives an acid with one carbon more than the halide used.
The carbonyl bond is polar, so aldehydes and ketones have dipole–dipole attraction and dissolve in water by hydrogen bonding through the carbonyl oxygen; solubility falls as the hydrocarbon part grows. Carboxylic acids form dimers held by two hydrogen bonds, which doubles the effective molecular size and gives them the highest boiling points. For comparable mass: carboxylic acids > alcohols > ketones > aldehydes > ethers > alkanes, following H-bond > dipole–dipole > van der Waals.
The polarity of \(>\mathrm{C=O}\) arises from the electronegativity of oxygen and from resonance, leaving the carbon electron deficient and electrophilic, open to attack by nucleophiles. Aldehydes are more reactive than ketones because a ketone’s two alkyl groups both reduce the positive charge on the carbon (\(+\mathrm{I}\) effect) and crowd it sterically. Aromatic aldehydes are less reactive than aliphatic ones because the ring donates electron density by resonance.
Aldehydes are detected by the Schiff test (magenta colour), Tollens’ test (silver mirror) and the Fehling test (red \(\mathrm{Cu_2O}\) precipitate, also given by \(\alpha\)-hydroxy ketones). Ketones are detected by the sodium nitroprusside test (red colour). In Tollens’ and Fehling’s tests the aldehyde is oxidized to the carboxylate ion while \(\mathrm{Ag^{+}}\) or \(\mathrm{Cu^{2+}}\) is reduced.
With nucleophiles, aldehydes and ketones add HCN to give cyanohydrins (a step-up reaction that builds a new C–C bond), add \(\mathrm{NaHSO_3}\) to give crystalline adducts used for purification, add alcohols to give hemiacetals and acetals (hemiketals and ketals from ketones; cyclic ketals from 1,2- or 1,3-diols, useful as protecting groups), and add Grignard reagents to give alcohols.
With ammonia derivatives \(\mathrm{NH_2\!-\!Z}\) in weakly acidic medium they undergo addition–elimination, losing water to give imines: oximes, hydrazones, phenylhydrazones, semicarbazones and 2,4-dinitrophenylhydrazones. These crystalline solids are used to characterize the parent compound. In strongly acidic medium the derivative is protonated and stops being a nucleophile.
Acetaldehyde, all methyl ketones and all alcohols containing the \(\mathrm{CH_3\!-\!(CHOH)-}\) group give the haloform reaction with \(\mathrm{NaOH}\) and \(\mathrm{I_2}\), producing yellow iodoform and a carboxylate with one carbon less. Non-methyl ketones give a negative test, and a \(\mathrm{C{=}C}\) bond present in the molecule is not attacked.
Carbonyl compounds with at least one \(\alpha\)-hydrogen (two for ketones) undergo the aldol reaction with dilute alkali to give a \(\beta\)-hydroxy aldehyde or ketone, which on warming loses water to give an \(\alpha,\beta\)-unsaturated product. The overall change is aldol condensation, a nucleophilic addition–elimination. Cross aldol condensation between two different partners, each with two \(\alpha\)-hydrogens, gives a mixture of four products.
Aldehydes with no \(\alpha\)-hydrogen undergo the Cannizzaro reaction with concentrated alkali, a disproportionation in which one molecule is reduced to an alcohol and another oxidized to the carboxylate salt. In the cross Cannizzaro reaction with a non-enolisable aldehyde, formaldehyde is the one oxidized.
Aldehydes are oxidized to carboxylic acids by dilute \(\mathrm{HNO_3}\), \(\mathrm{KMnO_4}\) or \(\mathrm{K_2Cr_2O_7}\). Ketones resist oxidation, and strong oxidants such as \(\mathrm{CrO_3}\) break a C–C bond to give a mixture of shorter acids. Both aldehydes and ketones are reduced to \(\mathrm{-CH_2-}\) by Zn–Hg with concentrated HCl (Clemmensen) or by hydrazine with KOH in ethylene glycol (Wolf-Kishner), the latter being the route to straight-chain alkyl benzenes that Friedel–Crafts alkylation cannot give.
Aromatic aldehydes and ketones undergo electrophilic substitution such as nitration, sulfonation and halogenation. The \(\mathrm{-CHO}\) and \(>\mathrm{C=O}\) groups withdraw electrons inductively and by resonance, deactivating the ortho and para positions, so the meta product is formed.
Carboxylic acids are acidic because the carboxylate ion left behind is stabilized by two equivalent resonance structures. Lower \(\mathrm{p}K_a\) means a stronger acid. Electron withdrawing groups (\(-\mathrm{I}\)) such as halogens, \(\mathrm{-NO_2}\) and \(\mathrm{-CN}\) stabilize the conjugate base and raise acid strength; electron donating groups such as \(\mathrm{-CH_3}\), \(\mathrm{-OH}\), \(\mathrm{-OCH_3}\) and \(\mathrm{-NH_2}\) lower it. Benzoic acid (\(\mathrm{p}K_a\) 4.2) is stronger than acetic acid (\(\mathrm{p}K_a\) 4.76) because of the \(-\mathrm{I}\) effect of the \(sp^2\) ring carbon.
The \(\mathrm{-COOH}\) group is identified by the litmus test, by brisk effervescence of \(\mathrm{CO_2}\) with sodium bicarbonate — which phenols do not give — and by the fruity smell of an ester with ethanol and concentrated \(\mathrm{H_2SO_4}\).
Carboxylic acids give acyl chlorides with \(\mathrm{SOCl_2}\), \(\mathrm{PCl_3}\) or \(\mathrm{PCl_5}\), thionyl chloride being preferred because its byproducts are gases; amides with ammonia followed by heating; anhydrides with \(\mathrm{P_2O_5}\) or by heating sodium carboxylate with an acyl chloride; hydrocarbons with one carbon less on decarboxylation of their sodium salts with soda lime; and primary alcohols on reduction with \(\mathrm{LiAlH_4}\) or diborane, though not with \(\mathrm{NaBH_4}\).
The chapter as a whole moves through one connected cycle of interconversions, summarised in FC 12.S1.
Flowchart
FC 12.S1 – The chapter as one interconversion cycle: primary alcohol → (oxidation/dehydrogenation) → aldehyde → (oxidation) → carboxylic acid; secondary alcohol → (oxidation) → ketone; carboxylic acid → (SOCl₂) → acyl chloride → (H₂/Pd–BaSO₄) → aldehyde, or → (R₂Cd) → ketone; carboxylic acid → (LiAlH₄) → primary alcohol, closing the loop; side branches: aldehyde/ketone → (HCN) → cyanohydrin, → (NaHSO₃) → adduct, → (R′OH) → acetal/ketal, → (NH₂–Z) → imine, → (dil. alkali, α-H present) → aldol → α,β-unsaturated carbonyl compound, → (conc. alkali, no α-H) → Cannizzaro products, → (Zn–Hg/HCl or H₂N–NH₂/KOH) → –CH₂–; carboxylic acid → (NH₃, Δ) → amide, → (P₂O₅) → anhydride, → (soda lime, Δ) → hydrocarbon
12.11 Objective Questions
- In the resonating structures A and B shown in Fig. E12.1, the number of unshared electrons in the valence shell present on oxygen respectively are
- 2, 4
- 2, 6
- 4, 6
- 6, 4
Diagram
Fig. E12.1 – Two resonating structures: (A) a carbonyl group drawn as >C=O with its lone pairs on oxygen; (B) the charge-separated form with a C–O single bond, a positive charge on carbon and a negative charge on oxygen
- In the Wolf-Kishner reduction, alkyl aryl ketones are reduced to alkyl benzenes. During this change, ketones are first converted into
- acids
- alcohols
- hydrazones
- alkenes
- Aldol condensation is
- electrophilic substitution reaction
- nucleophilic substitution reaction
- elimination reaction
- addition–elimination reaction
- Which one of the compounds shown in Fig. E12.2 has the lowest acidity?
Diagram
Fig. E12.2 – Four benzene rings each bearing –COOH: (a) with –NO₂ at the meta position; (b) with –Cl at the ortho position; (c) unsubstituted benzoic acid; (d) with –OCH₃ at the para position
- Diborane reduces
- ester group
- nitro group
- halo group
- acid group
- Benzaldehyde does NOT show a positive test with
- Schiff reagent
- Tollens’ reagent
- Sodium bisulphite solution
- Fehling solution
12.12 Check Your Concepts
Assertion: Formaldehyde does not undergo aldol condensation. Reason: Formaldehyde has no \(\alpha\)-hydrogen atom. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.
A ketone contains the same carbonyl group as an aldehyde, yet it is not oxidized by Tollens’ reagent. Explain the structural reason for this difference.
Explain why benzaldehyde is less reactive than ethanal towards nucleophilic addition reactions.
Propanal is warmed with dilute NaOH and the product is then heated. Write both products and name each of them.
Distinguish between the members of each of the following pairs using a single chemical test: (a) ethanal and propanone; (b) benzoic acid and phenol; (c) pentan-2-one and pentan-3-one.
Complete the following reaction and name the product: \(\mathrm{CH_3\!-\!CO\!-\!CH_3} + \mathrm{NH_2\!-\!NH\!-\!CONH_2} \longrightarrow\) ?
Arrange the following in decreasing order of boiling point and justify the order: ethanoic acid, propan-1-ol, propanone, butane.
Why is the Rosenmund reduction carried out with palladium poisoned by barium sulfate rather than with pure palladium? What would be obtained if pure palladium were used?
An organic compound A gives a silver mirror with Tollens’ reagent and a yellow precipitate with \(\mathrm{I_2}\) and NaOH. On oxidation, A gives an acid containing the same number of carbon atoms as A. Identify A and explain each observation.
Why does a carboxylic acid have a higher boiling point than an alcohol of comparable molecular mass, even though both can form hydrogen bonds?
Explain how the sodium bicarbonate test distinguishes a carboxylic acid from a phenol, and write the equation involved.
Which of \(\mathrm{CH_3COOH}\), \(\mathrm{F\!-\!CH_2\!-\!COOH}\) and \(\mathrm{I\!-\!CH_2\!-\!COOH}\) would you expect to be the strongest acid? Justify your answer using the idea of conjugate base stability.
Predict the major organic product formed when benzaldehyde is treated with concentrated \(\mathrm{HNO_3}\) in the presence of concentrated \(\mathrm{H_2SO_4}\), and explain why that particular position of the ring is attacked.
What is the action of DIBAl-H on (a) pent-3-enenitrile and (b) an ester at 195 K? State the special advantage of this reagent in each case.
Match each reagent in column I with the observation it gives in column II.
| Column I | Column II |
|---|---|
| a. Tollens’ reagent | i. red colour in solution |
| b. Fehling solution | ii. magenta colour |
| c. sodium nitroprusside with NaOH | iii. silver mirror |
| d. Schiff’s reagent | iv. red precipitate of cuprous oxide |
Why is cyanohydrin formation described as a ‘step-up’ reaction? Name two functional groups into which the \(\mathrm{-C{\equiv}N}\) group of a cyanohydrin can subsequently be converted.
State what happens when acetic acid is treated with each of the following, writing the equation in each case: (a) thionyl chloride, (b) \(\mathrm{P_2O_5}\) on heating, (c) ammonia followed by strong heating.
Explain why a cyclic ketal is used as a protecting group for a ketone, and state how the ketone is recovered at the end of the synthesis.
Complete the sequence and identify A and B: \[\mathrm{C_6H_5\!-\!CH_3} \xrightarrow{\ \mathrm{Cl_2},\ h\nu\ } \mathbf{A} \xrightarrow[\ 373\ \text{K}\ ]{\ \mathrm{H_3O^{+}}\ } \mathbf{B}\] State also why this route is used commercially.
Why does the oxidation of an unsymmetrical ketone such as butan-2-one with \(\mathrm{CrO_3}\) give a mixture of two acids rather than a single product?
Assertion: Sodium borohydride cannot be used to convert a carboxylic acid into a primary alcohol. Reason: Sodium borohydride is a milder reducing agent than lithium aluminium hydride. State whether both statements are correct and whether the reason explains the assertion.
n-Propylbenzene cannot be prepared satisfactorily by Friedel–Crafts alkylation. Name the reaction studied in this chapter that can be used instead, and write the sequence of steps starting from ethyl phenyl ketone.
Arrange the following in increasing order of reactivity towards nucleophilic addition and give one reason for the position of each: propanone, benzaldehyde, ethanal, methanal.
An aldehyde X has no \(\alpha\)-hydrogen. State which of the following reactions X can and cannot undergo, giving a reason in each case: (a) aldol condensation, (b) Cannizzaro reaction, (c) reaction with 2,4-dinitrophenylhydrazine, (d) Tollens’ test.
Explain why the entire alkyl side chain of an alkyl benzene is oxidized to a single \(\mathrm{-COOH}\) group by alkaline \(\mathrm{KMnO_4}\), while a tertiary alkyl substituent is left untouched.
What are aromatic ketones?
Is phenyl acetic acid an aromatic carboxylic acid?
Write the reaction showing the conversion of ethanenitrile into ethanol.
Predict the product of the following reaction: \[\mathrm{CH_3\!-\!CH_2\!-\!COOCH_3} \xrightarrow[\ \text{ii. } \mathrm{H_3O^{+}}\ ]{\ \text{i. } \mathrm{AlH(i\text{-}Bu)_2}\ } ?\]
Name the product obtained by reacting toluene with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride.
Write the reaction showing the conversion of benzonitrile into benzoic acid.
Name the product obtained by the oxidation of 1,2,3,4-tetrahydronaphthalene with acidified potassium permanganate.
What is formalin?
Arrange the following compounds in the increasing order of their boiling points: formaldehyde, ethane, methyl alcohol.
Acetic acid is prepared from methyl magnesium bromide and dry ice in the presence of dry ether. Name the compound which serves not only as a reagent but also as a cooling agent in the reaction.
Observe the following equation for the reaction of Tollens’ reagent with an aldehyde. How do we know that a redox reaction has taken place? Explain. \[\mathrm{R\!-\!CHO} + 2\,\mathrm{Ag(NH_3)_2^{+}} + \mathrm{OH^{-}} \xrightarrow{\ \Delta\ } \mathrm{R\!-\!COO^{-}} + 2\,\mathrm{Ag}\downarrow + 4\,\mathrm{NH_3} + 2\,\mathrm{H_2O}\]
Formic acid is stronger than acetic acid. Explain.
What is the action of hydrazine on cyclopentanone in the presence of KOH in ethylene glycol?
Write the reaction showing the conversion of acetaldehyde into acetaldehyde dimethyl acetal.
Aldehydes are more reactive toward nucleophilic addition reactions than ketones. Explain.
Write the reaction showing the action of the following reagents on propanenitrile:
- dilute NaOH
- dilute HCl.
- Arrange the carboxylic acids shown in Fig. E12.3 in increasing order of their acidic strength and justify your answer.
Diagram
Fig. E12.3 – Three acids: cyclohexanecarboxylic acid (a six-membered all-carbon ring bearing –COOH); an oxygen-containing six-membered ring with –COOH on the carbon adjacent to the ring oxygen; and an oxygen-containing six-membered ring with –COOH on a carbon one position further from the ring oxygen
- Write a note on:
- Cannizaro reaction
- Stephen reaction.
- What is the action of the following reagents on toluene?
- Alkaline \(\mathrm{KMnO_4}\), dilute HCl and heat
- \(\mathrm{CrO_2Cl_2}\) in \(\mathrm{CS_2}\)
- Acetyl chloride in the presence of anhydrous \(\mathrm{AlCl_3}\).
- Write the IUPAC names of the structures shown in Fig. E12.4.
Diagram
Fig. E12.4 – Two structures: (a) a cyclohexanone ring carrying a methyl substituent; (b) an open chain with a –COOH group at each end joined directly to one another
Write the reaction showing the conversion of p-bromoisopropyl benzene into p-isopropyl benzoic acid (3 steps).
Write the reaction showing the aldol condensation of cyclohexanone.
Draw and complete the reaction scheme of Fig. E12.5, which starts with acetaldehyde. In each empty box, write the structural formula of the organic compound that would be formed.
Diagram
Fig. E12.5 – Reaction scheme with CH₃CHO at the centre: an arrow labelled HCN leading left to an empty box; an arrow labelled “reduction” leading right to an empty box; an arrow labelled “dilute H₂SO₄, heat” leading down from the HCN product to an empty box; an arrow labelled “Tollens’ reagents” leading down from CH₃CHO to an empty box; an arrow labelled “Conc. H₂SO₄, heat” leading down to the given compound CH₂=CHCO₂H; and an arrow labelled “Cold dilute KMnO₄/H⁺” leading right from it to a final empty box