Chapter 16: Chemistry in Everyday Life
For centuries, people boiled animal fat with wood-ash lye and obtained soap, without having the faintest idea of what was happening in the pot. It worked, so nobody asked why. In the early nineteenth century a French chemist named Michel Eugène Chevreul decided to ask. He spent years patiently taking apart animal fats in his laboratory, and what he found changed both the kitchen and the washing area.
Chevreul showed that a fat is not a single substance at all. It is a combination of long-chain acids joined to glycerol, and boiling that fat with alkali simply breaks those joints apart — releasing the fatty acids as their salts, which we call soap, and setting the glycerol free. He isolated and named several of these fatty acids, including stearic and oleic acid. The same insight explained something else people had always noticed but never understood: why old butter and stale oil smell so unpleasant. If a fat can be split apart in a pot, it can also be split apart slowly by the moisture, air and microbes in a food packet.
One careful set of experiments, and three everyday things — cooking oil, spoiled butter and a bar of soap — turned out to be the same chemistry seen from different angles.
Diagram
Fig. 16.0 – A single illustration split into three panels sharing one central triglyceride structure: panel 1 an oil bottle and butter dish labelled “food fat”, panel 2 a soap bar with lather labelled “saponification”, panel 3 a spoiled butter block with wavy odour lines labelled “rancidity”; arrows from the central structure to each panel.
Keep these questions in mind as you read. This chapter will give you everything you need to reason out each one for yourself.
16.1 Basics of Food Chemistry
Chemistry All Around Us
Life, the atmosphere, the earth and the universe have all evolved over billions of years to reach their present state. That evolution has not stopped; it continues even now, and every stage of it is accompanied by a great variety of chemical changes.
Look around and you will find chemistry at work in the most ordinary events. The weathering of rocks, lightning in the sky, the eruption of volcanoes, photosynthesis in a green leaf, the ripening of fruit, the fermentation of dough and the release of fragrance by a blooming flower are all natural phenomena driven by intricate chemistry.
Chemistry is equally busy inside us. A large number of life processes occurring within and across our body are chemical processes, controlled with remarkable precision.
Long before laboratories existed, human civilizations in different parts of the world discovered that certain plant, animal and mineral products were useful — for food, for healing, for cleaning and for preserving. This knowledge was built by observation and experience, and passed down through generations.
With the advent of modern science, scientists were able to isolate the chemicals present in these natural materials and determine their structures. Once a structure was known, it could be copied and modified. Synthetic organic chemistry — the deliberate building of molecules in the laboratory — led to a major advancement in science. The synthesis of natural molecules, and of new molecules carrying structural variations, revolutionised the materials now used in every walk of human life. This influence is visible in all the basic needs: food, clothing, shelter and much beyond.
In this chapter we shall look at three of these areas — food chemistry, medicinal chemistry and the chemistry of cleansing materials — keeping to compounds with simple structural features, so that the reasoning stays clear.
16.1.1 Nutrients and Their Digestion
What Food Provides
Food provides nutrients, and these nutrients are used by the body as its source of energy. Nutrients do more than supply energy: they regulate growth, and they maintain and repair body tissues.
The nutrients comprise carbohydrates, lipids, proteins, vitamins, minerals and water. Different food groups supply them in different proportions. Grains, fruits and vegetables provide carbohydrates and vitamins; meat, fish, eggs, dairy products and pulses provide proteins and vitamins; and lipids (fats and oils) are provided by vegetable oils, dairy products and animal fats.
This is the chemical basis of what you have long been told is a balanced diet — no single food group carries every nutrient the body needs.
Why Most Nutrients Are Large Molecules
Most nutrients are organic macromolecules. A macromolecule is simply a very large molecule, built by joining many small units together.
Proteins and carbohydrates are polymeric materials. A polymer is a large molecule made by linking a great many small repeating units, called monomers, one after another — rather like a long chain built from identical links.
Such large molecules cannot pass through the wall of the intestine as they are. They must first be cut back into their small units. This cutting-up is what we call digestion.
As a result of food digestion, the polymeric proteins and carbohydrates ultimately break down into their monomers — namely, \(\alpha\)-amino acids and glucose respectively — under the influence of enzymes. Enzymes are protein molecules made by living cells that act as highly specific biological catalysts: they speed up a particular reaction enormously without themselves being consumed.
Why Cooking Makes Food Easier to Digest
Cooking makes food easy to digest, and the reason is chemical rather than magical. During the cooking process, the high polymers of carbohydrates or proteins are hydrolysed to smaller polymers. Hydrolysis means a bond is broken by reaction with water — the water molecule splits and its fragments add to the two ends produced.
Cooking also changes the physical nature of the food mixture. The uncooked food mixture is described as a heterogeneous suspension — large particles, clearly separate from the liquid, which would settle out on standing. On cooking, this becomes colloidal matter, in which the dispersed particles are far smaller and stay dispersed. You met suspensions and colloids earlier while studying colloidal state; here you are seeing the same classification applied to a bowl of cooked rice or dal.
Because of the smaller size of the resulting constituent nutrient molecules, cooked food is easier to digest than uncooked food. Smaller particles present a much larger surface area to the digestive enzymes, so the enzymes can act faster.
16.1.2 Food Quality Chemistry
Quality of food is an important aspect of food chemistry. Food quality is described in terms of parameters such as flavour, smell, texture, colour and microbial spoilage. Notice that four of these five are properties you can judge with your senses — and every one of them is the outward sign of some chemical change happening inside the food.
Enzymes are present naturally in all foods. This single fact explains a great deal, because an enzyme does not stop working when a fruit is picked or an animal product is packed. It keeps catalysing reactions in the stored food.
How Stored Food Changes
The quality of foods changes on shelving — that is, during storage — mostly due to three causes: enzyme action, chemical reactions with the environment, and the action of microorganisms.
Some of these effects are beneficial, and we deliberately encourage them. The setting of milk into curd and the raising of flour dough to make bread are both brought about by the deliberate action of microorganisms.
Most changes brought about by microorganisms and by interaction with the environment, however, adversely affect the food quality. The task of food chemistry is therefore to allow the useful changes and block the harmful ones.
Browning of Cut Fruits and Vegetables
When fruits such as banana and apple, or vegetables such as potato and bottle gourd, are peeled and sliced, sooner or later they turn brown.
The cutting action damages the cells, and this damage releases chemicals that were earlier kept apart inside the intact cell. With the pH prevailing in the fruit or vegetable, the polyphenols released are oxidised by oxygen in air, owing to the action of an enzyme, to form quinones. A polyphenol is simply an aromatic compound carrying more than one phenolic \(-\mathrm{OH}\) group on the ring, and a quinone is the oxidised form in which two of these have become \(\mathrm{C{=}O}\) groups. The change is shown in Fig. 16.3.
Diagram
Fig. 16.3 – Enzymatic oxidation of a poly phenol to a quinone: a benzene ring bearing two adjacent O–H groups, reacting with ½O₂ over an arrow marked “enzyme”, giving a ring with two C=O groups (a quinone) plus H₂O; label the left structure “a poly phenol” and the right structure “a quinone”.
Written as an equation, the change is:
\[\text{poly phenol} + \tfrac{1}{2}\,\mathrm{O_2} \xrightarrow{\ \text{enzyme}\ } \text{quinone} + \mathrm{H_2O}\]
The quinones formed do not stop there. Quinones undergo further reactions including polymerization, giving brown coloured products named tannins. It is these tannin polymers, not the quinone itself, that give the cut surface its familiar brown patch. The full sequence is summarised in FC 16.1.
Flowchart
FC 16.1 – Enzymatic browning of cut fruit: cutting/peeling → cell damage → polyphenols and enzyme released and mixed → polyphenols + ½O₂ (enzyme) → quinones → polymerization → tannins (brown colour); branch from “polyphenols + O₂ (enzyme)”: add reducing agent (SO₂ or ascorbic acid) OR lower the pH (lemon juice/citric acid, vinegar) → reaction slowed → browning delayed.
This browning reaction can be slowed down using reducing agents such as \(\mathrm{SO_2}\), ascorbic acid (vitamin C), or by change of pH by adding an edible acid such as lemon juice (citric acid) or vinegar. A reducing agent works by opposing the oxidation itself; changing the pH works by moving the medium away from the pH at which the enzyme is most active.
Rancidity of Oils and Fats
On keeping for a long time, oils and fats develop an unpleasant or rancid smell and a disagreeable taste.
To see why, recall what a fat is. Fats are triesters of fatty acids (long chain carboxylic acids) and glycerol (propane-1,2,3-triol). An ester is the product of a carboxylic acid and an alcohol; since glycerol carries three \(-\mathrm{OH}\) groups, one glycerol molecule can esterify three fatty acid molecules, giving a triglyceride.
There are two quite different chemical routes to rancidity, and it is worth keeping them separate.
The first cause: hydrolysis. One cause of rancidity is the release of fatty acids produced during hydrolysis of fats, brought about by water present in the food. The ester linkages are simply cut by water, as shown in Fig. 16.4.
Diagram
Fig. 16.4 – Hydrolysis of a triglyceride: on the left, a glycerol backbone H₂C–O–CO–R₁, HC–O–CO–R₂, H₂C–O–CO–R₃ labelled “Triglyceride”, plus 3H₂O, with the arrow labelled “Hydrolysis”; on the right, glycerol (H₂C–OH, HC–OH, H₂C–OH) labelled “Glycerol” plus the three free acids HO–CO–R₁, HO–CO–R₂, HO–CO–R₃ labelled “Free Fatty Acids”.
In condensed form:
\[\text{Triglyceride} + 3\,\mathrm{H_2O} \xrightarrow{\ \text{Hydrolysis}\ } \text{Glycerol} + 3\ \text{Free fatty acids}\]
Note the coefficient carefully: three molecules of water are needed, because three ester linkages have to be broken.
The hydrolysis of fats occurs rapidly in the presence of certain microorganisms and is an enzyme catalysed reaction. That is why a moist, warm, unrefrigerated fat spoils so much faster than a dry, cold one.
Which fatty acids are released decides what the spoiled food smells like. Rancidity of milk and butter is due to the release of four, six and eight carbon fatty acids (butanoic, hexanoic and octanoic acids) on hydrolysis. Chocolate develops an oily or fatty flavour due to release of palmitic, stearic and oleic acids on hydrolysis. Lauric acid, on hydrolysis, gives a soapy flavour to coconut oil.
The second cause: oxidation. The second cause of rancidity of oils and fats is oxidation by molecular oxygen in the air. Many vegetable oils have one or more \(\mathrm{C{=}C}\) double bonds in the fatty acid part of their structure. These are called mono or poly unsaturated fats — “mono” for one double bond, “poly” for more than one.
The unsaturated fat molecules break down during the oxidation and form volatile aldehydes and carboxylic acids, which give the unpleasant rancid taste. This is called oxidative rancidity.
It is caused by a free radical reaction initiated by light (photo oxidation) or catalysed by either enzymes or metal ions. A free radical is a species carrying an unpaired electron; it is highly reactive and, once formed, it generates further radicals, so the reaction sustains itself rather than dying out.
Polyunsaturated oils containing a greater number of \(\mathrm{C{=}C}\) double bonds usually become rancid very quickly, because each double bond is another site at which the attack can begin. High temperature increases the rate of air oxidation of unsaturated fats.
Extensive oxidation can lead to some polymerization, with a consequent increase in viscosity and browning. This is exactly the thick, dark, sticky film you see on an oil that has been heated too many times.
The two routes to rancidity are compared in FC 16.2.
Flowchart
FC 16.2 – Two routes to rancidity. Route A (hydrolytic): fat + water present in food → enzyme/microorganism catalysed hydrolysis → glycerol + free fatty acids → off smell and taste (butanoic/hexanoic/octanoic from butter; lauric from coconut oil). Route B (oxidative): unsaturated fat with C=C → initiation by light, or catalysis by enzymes or metal ions → free radical reaction with O₂ of air → volatile aldehydes + carboxylic acids → rancid taste; extensive oxidation → polymerization → increased viscosity and browning.
Saturated, Unsaturated and Trans Fats
You have noted earlier that fats are triglycerides of fatty acids. Animal fats mostly contain saturated fatty acids, while vegetable oils contain unsaturated fatty acids as well.
Why saturated fats are solids. Long chains of tetrahedral carbon atoms in a saturated fatty acid get packed closely together. Because every carbon is \(sp^3\) and the chain has no rigid double bond in it, the chain can straighten out and lie neatly alongside its neighbours. Moreover, van der Waals forces between the long saturated chains are sufficiently strong to convert saturated fats into the solid form at room temperature. Van der Waals forces are individually weak, but they act along the entire length of a long chain, and hundreds of such contacts add up.
Why unsaturated fats are liquids. The long carbon chains of unsaturated fatty acids contain one or more \(\mathrm{C{=}C}\) double bonds. This produces one or more ‘kinks’ in the chain, as you can see in Fig. 16.1, which prevent the molecules from packing closely together. The van der Waals forces between the unsaturated chains are therefore weak. The melting points of unsaturated fats are, in consequence, lower.
Diagram
Fig. 16.1 – Molecular shapes of fats (a schematic representation): (a) a saturated fat molecule, three straight parallel zig-zag chains attached through O–CO groups to a glycerol backbone; (b) an unsaturated fat molecule, with one chain showing a C=C double bond producing a distinct kink that pushes the chains apart.
Natural fats are mixtures of triglycerides. They do not have sharp melting points, and usually melt over a range of temperatures — one of the practical signs that you are dealing with a mixture and not a pure compound. The more unsaturated the fat, the lower is its melting point and the less crystalline it is. Some examples of fats are given in Table 16.1.
Table 16.1: Natural fats and their physical states
| Mainly Saturated fats | Mainly mono-unsaturated fats | Mainly poly-unsaturated fats |
|---|---|---|
| Coconut fat/oil, butter fat, lard, margarine, vanaspati ghee | Olive oil, peanut oil, canola oil | Safflower oil, sunflower oil, soyabean oil, corn oil, fish oil |
| solid | liquid | liquid |
Diagram
Fig. 16.5 – Schematic graph, melting point on the y-axis against number of C=C double bonds (0, 1, 2, 3) on the x-axis for four fatty acids of equal chain length; a steeply falling curve, with a horizontal dashed line marked “room temperature” cutting between the 0 and 1 points; small chain sketches beside each point showing the increasing number of kinks.
Cis and trans forms. A \(\mathrm{C{=}C}\) can have geometrical isomers, cis and trans. Geometrical isomerism arises because rotation about a double bond is not free, so the groups attached are locked in one arrangement or the other.
In the cis form of an unsaturated fatty acid the two hydrogens on the two double bonded carbons are on the same side of the double bond, whereas they are on the opposite sides in the trans isomer. This is drawn in Fig. 16.6.
Diagram
Fig. 16.6 – Cis and trans arrangement about a C=C in a fatty acid chain: left, the cis isomer with both H atoms on the same side and the chain bent into a pronounced kink; right, the trans isomer with the H atoms on opposite sides and the chain running almost straight; label the H positions on both.
The cis isomer is the most common form of unsaturated fats. The trans form occurs only in animal fats and in processed unsaturated fats. Trans fats are difficult to metabolize and may build up to dangerous levels in fatty tissue.
Fats and cholesterol transport. Fats in the form of lipoprotein are used in the body for transport of cholesterol. A lipoprotein is a combination of lipid and protein that allows a water-insoluble lipid to travel through the aqueous bloodstream.
Excessive low density lipoprotein (LDL) results in deposition of cholesterol in blood vessels, which in turn results in the increased risk of cardio vascular disease. There is some evidence that eating large amounts of saturated or trans unsaturated fats increases the tendency of cholesterol getting deposited in blood vessels. Cis fats do not cause formation of such deposits and decrease the chance of developing coronary heart disease.
Omega-3 and Omega-6 Fatty Acids
Two categories of the natural unsaturated fats of concern include those containing either Omega-3 or Omega-6 fatty acids. These names are given for the position of the double bond in the long carbon chain of the fatty acid.
Omega denotes the last carbon of the carbon chain. So this numbering starts from the end farthest from the \(-\mathrm{COOH}\) group, which is the opposite of the IUPAC numbering you normally use. Omega-3 fatty acids have a \(\mathrm{C{=}C}\) bond between the third and fourth carbon from the end of a carbon chain.
For example: Linolenic Acid (9,12,15-octadecatrienoic acid). Its structure with both numbering schemes marked is shown in Fig. 16.7.
Diagram
Fig. 16.7 – Structure of linolenic acid (9,12,15-octadecatrienoic acid): HO–CO– at the left end with the carbon next to it marked α, an 18-carbon zig-zag chain with cis double bonds at positions 9, 12 and 15 numbered from the acid end, and the terminal carbon at the right end marked ω, with a curved arrow marking three carbons in from the ω end.
Antioxidants as Food Additives
An antioxidant is a substance that delays the onset of oxidation or slows down the rate of oxidation of food stuff. It is used to extend the shelf life of food.
Antioxidants react with oxygen-containing free radicals and thereby prevent oxidative rancidity. This is the key to how they work: recall that oxidative rancidity is a free radical reaction that keeps itself going by producing fresh radicals. An antioxidant intercepts those radicals, so the chain is broken before large amounts of volatile aldehydes and carboxylic acids can form.
A natural antioxidant. For example, vitamin E (tocopherol) is a very effective natural antioxidant which is added to pack edible oils. Its structure appears in Fig. 16.8.
Diagram
Fig. 16.8 – Structure of Vitamin E (Tocopherol): a chroman ring system carrying a phenolic HO– group and three ring CH₃ substituents, with a long saturated branched hydrocarbon side chain bearing CH₃ branches; label the phenolic OH as the antioxidant site and the long chain as the fat-soluble portion.
Two parts of this structure do two different jobs. The phenolic \(-\mathrm{OH}\) group in its structure is responsible for its antioxidant activity, while the long chain of saturated carbons makes it fat soluble. Fat solubility matters, because an antioxidant is of no use in oil unless it dissolves in the oil where the radicals are being formed.
It is found in foods such as wheat germ, nuts, seeds, green leafy vegetables and oils like safflower oil.
Synthetic antioxidants. For economic reasons, synthetic antioxidants are used as additives to increase the shelf-life of packed foods. Common structural units found in synthetic antioxidants are the phenolic \(-\mathrm{OH}\) group and the tertiary butyl group. For example BHT, which is 3,5-di-tert-butyl-4-hydroxytoluene, drawn in Fig. 16.9.
Diagram
Fig. 16.9 – Structure of BHT (3,5-di-tert-butyl-4-hydroxytoluene): a benzene ring with an OH group, two tert-butyl groups —C(CH₃)₃ on the carbons flanking the OH, and a CH₃ group on the ring; label the compound “BHT”.
Notice the family resemblance to vitamin E: a phenolic \(-\mathrm{OH}\) group again, and bulky hydrocarbon groups again. The phenolic \(-\mathrm{OH}\) supplies the antioxidant action, and the bulky tert-butyl groups sit on either side of it.
In examinations, this sub-topic is commonly tested through “odd one out” items (spotting the compound that is not an antioxidant), match-the-following (compound to its function), and assertion–reason questions of the type Assertion: polyunsaturated oils turn rancid faster than saturated fats. Reason: they contain more \(\mathrm{C{=}C}\) double bonds available to free radical attack. Answering these well needs the cause-and-effect chain, not the names alone.
16.2 Compounds with Medicinal Properties
Drug and Medicine — Two Words That Are Not the Same
A chemical which interacts with biomolecules such as carbohydrates, lipids, proteins and nucleic acid and produces a biological response is called a drug.
Not every biological response is a helpful one. A drug having a therapeutic and useful biological response is used as a medicine. So “drug” is the wider word, describing what a chemical does inside the body, while “medicine” describes how we choose to use it.
A medicine contains a drug as its active ingredient. Besides this, it contains some additional chemicals which make the drug suitable for its use as a medicine. A tablet, for instance, must hold together, dissolve at the right place and carry an accurately measured dose — and the substances that make this possible are not themselves therapeutic.
Medicines are used in the diagnosis, prevention and treatment of a disease.
Drugs, being foreign substances in a body, often give rise to undesirable, adverse side effects. This is not a flaw in one particular medicine; it follows from what a drug is. A molecule that interacts strongly with biomolecules can rarely be trusted to interact with only the one target we intend.
Drug design is an important branch of medicinal chemistry which aims at the synthesis of new molecules having a better biological response — better meaning more effective against the disease and less troublesome elsewhere in the body.
There is an increasing trend in current research in medicinal chemistry to take cognizance of traditional medical knowledge, such as Ayurvedic medicine or natural materials, to discover new drugs. A plant that generations have used for a particular ailment is, in effect, the result of a very long experiment; modern chemistry asks which molecule in that plant is responsible.
In this section, we consider some simple compounds having medicinal properties, and the active ingredients of some natural materials that are traditionally known to possess medicinal properties.
16.2.1 Analgesics and Antipyretics
‘Algesis’ is a Greek word meaning ‘feeling of pain’. Drugs which give relief from pain are called analgesics.
About half of the analgesics are anti-inflammatory drugs, which kill pain by reducing inflammation or swelling. They do not switch off the sensation directly; they remove the swelling that is causing it — which is why they work well for a sprain or a toothache.
Antipyretics are used to reduce fever.
From Willow Bark to Aspirin
The pain-killing and fever-reducing properties of an extract of the bark of the willow plant, commonly found in Europe, have been known for centuries. Here again, traditional use came first and chemistry followed.
In the nineteenth century its active ingredient, salicylic acid (2-hydroxybenzoic acid), was isolated and purified. Once a pure compound was in hand, chemists could modify it.
The compound aspirin, which is the acetyl derivative of salicylic acid, was found to have fewer side effects than salicylic acid itself. Both structures are shown in Fig. 16.10.
Diagram
Fig. 16.10 – Structures of salicylic acid and aspirin: left, a benzene ring bearing an OH group and a COOH group on adjacent carbons, labelled “Salicylic acid”; right, the same ring with the OH replaced by O–CO–CH₃ and the COOH retained, labelled “Aspirin (prodrug)”.
Aspirin is one of the most widely used analgesics to date. It however retains the stomach irritating side effects of salicylic acid — the acetylation reduced the trouble but did not remove it.
A less problematic pain-killer is paracetamol, which is a phenol. Its molecular structure appears in Fig. 16.11.
Diagram
Fig. 16.11 – Molecular structure of paracetamol: a benzene ring with an HO– group at one position and, at the para position, an N–H attached to a C=O bearing a CH₃ group (an amide, –NH–CO–CH₃).
16.2.2 Antimicrobials
The name antimicrobial is an umbrella term for any drug that inhibits or kills microbial cells, which include bacteria, fungi and viruses. Under this umbrella sit three groups that students often confuse, and the difference between them is mainly a question of where they are used.
Disinfectants are non-selective antimicrobials, which kill a wide range of microorganisms including bacteria. Disinfectants are used on non-living surfaces — for example, floors, instruments, sanitary ware and many others. Being non-selective means they do not distinguish between harmful and harmless organisms, and this is acceptable precisely because nothing living is meant to be there.
Antiseptics are used to sterilise surfaces of living tissue when the risk of infection is very high, such as during surgery, on wounds and so on. Because they are applied to living tissue, they must be far gentler than a disinfectant.
Antibiotics are a type of antimicrobial designed to target bacterial infections within or on the body. The words “within the body” are the crucial ones: an antibiotic is meant to travel inside a living body and act there.
The whole classification is summarised in FC 16.3.
Flowchart
FC 16.3 – Classification of antimicrobials: antimicrobial (kills or inhibits bacteria, fungi, viruses) → branch 1: disinfectant → non-selective → used on non-living surfaces (floors, instruments, sanitary ware); branch 2: antiseptic → used on living tissue surfaces (wounds, surgery); branch 3: antibiotic → targets bacterial infection within or on the body → sub-branch by range: broad spectrum / narrow spectrum / limited spectrum → sub-branch by origin: synthetic / semisynthetic / microbial.
Antiseptics and Disinfectants
Commonly used antiseptics include inorganics like iodine and boric acid, or organics like iodoform and some phenolic compounds.
Tincture of iodine — a 2 to 3 percent solution of iodine in an alcohol–water mixture — and iodoform serve as powerful antiseptics and find use in application on wounds. A “tincture” is simply a solution in alcohol or in an alcohol–water mixture.
A dilute aqueous solution of boric acid is a weak antiseptic used for the eyes. Note the pattern: for the most delicate living tissue, the weakest antiseptic.
Phenols as antiseptics and disinfectants. Various phenols are used as antiseptics and disinfectants. A dilute aqueous solution of phenol, known as carbolic acid, was one of the first antiseptics used in medicine in the late nineteenth century. It was however found to be corrosive — that is, it attacked the living tissue it was meant to protect.
The remedy came from chemistry rather than from abandoning phenols. Many chloro derivatives of phenols have been realized as more potent antiseptics than phenol itself. Because they are more potent, they can be used at much lower concentrations, which reduces their corrosive effects. This is a general and very useful idea: increasing potency allows a lower dose, and a lower dose means less collateral damage.
Two of the most common phenol derivatives in use are trichlorophenol (TCP) and chloroxylenol. The latter is the active ingredient (4.8 % W/V) of the popular antiseptic dettol. The other ingredients of dettol are isopropyl alcohol, pine oil, castor oil soap, caramel and water — again illustrating that a medicine is an active ingredient plus the substances that make it usable.
Thymol, obtained from the oil of thyme (a spice plant), is an excellent non-toxic antiseptic. p-Chlorobenzyl phenol is used as a disinfectant in all purpose cleaners. These four phenolic compounds are drawn in Fig. 16.12.
Diagram
Fig. 16.12 – Structures of four antiseptic and disinfectant phenols: (a) chloroxylenol — benzene ring with OH, two CH₃ groups and one Cl; (b) 2,4,6-trichlorophenol — benzene ring with OH and three Cl atoms at the 2, 4 and 6 positions; (c) thymol — benzene ring with OH, a CH₃ group and an isopropyl group; (d) p-chloro-o-benzylphenol — benzene ring with OH, a Cl atom and a benzyl (CH₂–C₆H₅) group. Label each structure with its name.
Antibiotics
Antibiotics are purely synthetic or obtained from microorganisms — bacteria, fungi or molds. So there are two quite different origins, and the story of each is worth following.
The synthetic line: Ehrlich, salvarsan and the sulpha drugs. Arsenic compounds were known to be highly poisonous to humans since long. Paul Ehrlich, a German bacteriologist, investigated arsenic based organic compounds in order to produce less toxic substances for the treatment of syphilis. The aim was to keep the poison lethal for the microorganism while making it tolerable for the patient.
He discovered the first effective treatment of syphilis, the synthetic antibiotic named salvarsan. He was awarded the Nobel prize for medicine (1908) for this discovery.
Ehrlich then noticed a similarity in the structures of salvarsan and azodyes, both of which are shown in Fig. 16.13. Recognising a structural resemblance between an effective drug and a completely different class of compound is exactly the kind of observation on which drug design is built.
Diagram
Fig. 16.13 – Structures of salvarsan and an azodye: (a) salvarsan — two benzene rings, each carrying an NH₂ group and an OH group, joined through an As=As linkage, labelled “Salvarsan”; (b) azodye — a benzene ring bearing an HO– group joined through an N=N linkage to a second benzene ring, labelled “Azodye”. Highlight the As–As and N=N linkages to show the structural similarity.
With further investigations he succeeded in the synthesis of an effective diazo antibacterial, prontosil, in 1932.
Subsequently it was discovered that prontosil gets converted into a simpler compound, sulphanilamide, in our body. In other words, prontosil is not itself the working molecule — the body makes the working molecule from it. This gave further direction to research in drug design, which led to the discovery of a wide range of sulpha drugs, analogues to sulphanilamide. One of the most effective is sulphapyridine. All three structures are given in Fig. 16.14, and the whole line of development is traced in FC 16.4.
Diagram
Fig. 16.14 – Structures of prontosil, sulphanilamide and sulphapyridine: (a) prontosil — a benzene ring carrying an H₂N–SO₂– group joined through an N=N linkage to a second ring bearing two NH₂ groups; (b) sulphanilamide — a benzene ring with H₂N– at one end and –SO₂NH₂ at the para position; (c) sulphapyridine — a benzene ring with H₂N– at one end and –SO₂–NH– at the para position, the NH being attached to a pyridine ring. Label each structure.
Flowchart
FC 16.4 – Development of the sulpha drugs: arsenic compounds known to be highly poisonous → Ehrlich searches for less toxic arsenic based organic compounds → salvarsan, first effective treatment of syphilis (Nobel prize for medicine, 1908) → structural similarity noticed between salvarsan and azodyes → synthesis of prontosil, a diazo antibacterial (1932) → prontosil converted in the body → sulphanilamide → analogues synthesised → wide range of sulpha drugs → sulphapyridine, one of the most effective.
The microbial line: penicillin and chloramphenicol.
Note carefully what those thirteen years represent. Noticing that a fungus kills bacteria is one thing; purifying the active ingredient, proving it works in patients and producing it in quantity is a separate and much longer piece of work. The general structure of penicillin is given in Fig. 16.15 and chloramphenicol in Fig. 16.16.
Diagram
Fig. 16.15 – General structure of penicillin: a four-membered β-lactam ring fused to a five-membered sulphur-containing ring; the five-membered ring bears two CH₃ groups and a COOH group, and the β-lactam ring carries an R–CO–NH– side chain with the variable group R marked.
Diagram
Fig. 16.16 – Structure of chloramphenicol: a benzene ring bearing an O₂N– group, attached to a CH(OH)–CH(NH–CO–CHCl₂)–CH₂OH chain; label the nitro group, the two hydroxyl groups and the dichloroacetamide group.
Notice in Fig. 16.15 that the side chain is written simply as R. Different groups in that position give different members of the penicillin family, all built on the same core — which is what makes semisynthetic antibiotics possible.
Spectrum of an antibiotic. Antibiotics can be of three types: broad spectrum (effective against a wide range of bacteria), narrow spectrum (effective against one group of bacteria) or limited spectrum (effective against a single organism).
A disadvantage of broad spectrum antibiotics is that they also kill the useful bacteria in the alimentary canal. The drug cannot tell a harmful bacterium from a helpful one, and the alimentary canal is full of helpful ones.
Today many broad spectrum, narrow spectrum and limited spectrum antibiotics are known. They are synthetic, semisynthetic or of microbial origin.
16.2.3 Traditional Knowledge in Medicine
Since ancient times, in India, many grandma remedies have been practised for curing ailments. The ancient medicinal system of India has documented the medicinal uses of innumerable Indian plants.
There is an increasing trend in modern medicinal chemistry to make use of this traditional knowledge, from various parts of the world, to isolate active ingredients from medicinal plants and further develop new drugs. The chemist’s question is always the same: within this plant, which single compound is doing the work? Once that compound is isolated, its structure can be determined, its purity controlled and its dose measured — three things a raw plant extract can never offer.
Table 16.2 lists a few medicinal plants, their medicinal property and the active ingredients present in them.
Table 16.2: Active ingredients of some medicinal plants
| Plant | Medicinal property | Name of the active ingredient |
|---|---|---|
| Turmeric | antiseptic | Curcumin |
| Wintergreen | analgesic | Methyl salicylate |
| Cinnamon | antimicrobial for colds | Cinnamaldehyde |
| Clove | antimicrobial, analgesic | Eugenol |
| Citrus fruits | antioxidant | Vitamin C (ascorbic acid) |
| Indian gooseberry (amla) | antidiabetic, antimicrobial, antioxidant | Vitamin C, Gallic acid |
Diagram
Fig. 16.17 – Structures of the active ingredients listed in Table 16.2: (a) curcumin — two substituted aromatic rings, each bearing an OH and an OCH₃ group, joined by a long conjugated chain containing two C=O groups; (b) methyl salicylate — benzene ring with an OH group and a –CO–O–CH₃ ester group on the adjacent carbon; (c) cinnamaldehyde — benzene ring attached to –CH=CH–CHO; (d) eugenol — benzene ring bearing HO– and H₃CO– groups and an allyl (–CH₂–CH=CH₂) side chain; (e) ascorbic acid — five-membered lactone ring bearing two OH groups on the C=C, with a side chain carrying two more OH groups; (f) gallic acid — benzene ring with three OH groups and a COOH group. Label each with its name.
Two things are worth noticing in Fig. 16.17. First, several of these natural active ingredients carry a phenolic \(-\mathrm{OH}\) group — the same group you met in vitamin E and BHT as the seat of antioxidant activity, and in the chlorophenol antiseptics. Second, methyl salicylate is the methyl ester of salicylic acid, the very compound isolated from willow bark; so the analgesic property of wintergreen and the analgesic property of aspirin trace back to the same chemical skeleton.
Questions from this sub-topic are most often set as match-the-following (plant to active ingredient, or active ingredient to medicinal property) and as one-sentence answers, so learn the entries of Table 16.2 as pairs rather than as isolated names.
16.3 Cleansing Agents
Cleansing agents are substances which are used to remove stain, dirt or clutter on surfaces. They may be natural or synthetically developed.
16.3.1 Types of Cleansing Agents
Commercially, cleansing agents are mainly of two types depending upon chemical composition: soaps and synthetic detergents. The difference is not in what they do — both remove greasy dirt by the same mechanism — but in what they are made of, and that difference decides how they behave in hard water.
Soaps
Soaps are sodium or potassium salts of long chain fatty acids. They are obtained by alkaline hydrolysis of natural oils and fats with \(\mathrm{NaOH}\) or \(\mathrm{KOH}\). This is called the saponification reaction.
Chemically, oils are triesters of long chain fatty acids and propane-1,2,3-triol, commonly known as glycerol or glycerin. You met this structure earlier in this chapter, when the same triester was being split by water during hydrolytic rancidity. Here the splitting agent is an alkali instead of water, and the products are correspondingly different: the fatty acid comes off not as the free acid but as its sodium or potassium salt.
Saponification of oil produces soap and glycerol. The structural change is shown in Fig. 16.18.
Diagram
Fig. 16.18 – Saponification: on the left, a glycerol backbone CH₂–O–CO–R, CH–O–CO–R, CH₂–O–CO–R labelled “oil/fat”, plus 3 NaOH, with the arrow labelled “saponification”; on the right, CH₂–OH, CH–OH, CH₂–OH labelled “glycerin” plus 3 R–CO–O⁻Na⁺ labelled “soap”.
In condensed form:
\[\text{Oil/fat (triester)} + 3\,\mathrm{NaOH} \xrightarrow{\ \text{saponification}\ } \text{Glycerin} + 3\,\mathrm{RCOONa}\]
Look carefully at the coefficient 3. One molecule of oil carries three ester linkages, so three formula units of alkali are needed, and three molecules of soap are produced along with one molecule of glycerol.
Choosing the Right Soap
The quality of soap depends upon the nature of the oil and the alkali used, and this is why not every soap is meant for every job.
Potassium soaps are soft to the skin. Therefore toilet soaps are prepared by using better grades of oil and \(\mathrm{KOH}\). Care is taken to remove the excess of alkali, since alkali left behind in the bar is exactly what would irritate the skin.
Laundry soaps are made using \(\mathrm{NaOH}\). These also contain fillers like sodium rosinate (a lathering agent), sodium silicate, borax, sodium phosphate and trisodium phosphate. Fillers make the bar cheaper and improve lathering, but they are not chosen with skin in mind.
Hard Water and Soap
Soaps are water soluble, which is why a bar of soap lathers readily in rain water or distilled water. In hard water, however, they form scum and become inactive.
The reason is the dissolved matter in the water. Hard water contains dissolved salts of calcium and magnesium, which react with soap, precipitating the calcium or magnesium salt of the fatty acid. This precipitate is the scum, and it sticks to the fabric being washed:
\[2\,\mathrm{RCOONa}\text{(aq)} + \mathrm{CaCl_2}\text{(aq)} \rightarrow (\mathrm{RCOO})_2\mathrm{Ca}\text{(s)} + 2\,\mathrm{NaCl}\text{(aq)}\]
Notice the state symbols. The soap goes in as a dissolved species, marked (aq), and comes out as a solid, marked (s). Once the fatty acid part has been locked up in an insoluble calcium salt, it can no longer do any cleaning — hence the word “inactive”. Worse, the scum settles on the cloth, so the washing leaves a dull grey deposit behind.
Washing soda (\(\mathrm{Na_2CO_3}\)) precipitates the dissolved calcium salts as the carbonate and helps the soap action by softening the water:
\[\mathrm{CaCl_2}\text{(aq)} + \mathrm{Na_2CO_3}\text{(aq)} \rightarrow \mathrm{CaCO_3}\text{(s)} + 2\,\mathrm{NaCl}\text{(aq)}\]
The logic here is worth grasping, because it is a general strategy. The calcium is removed before the soap is added, by a cheap reagent that sacrifices itself. Once the calcium ions are out of solution as \(\mathrm{CaCO_3}\), there is nothing left to precipitate the soap, and the soap can lather and clean normally.
Diagram
Fig. 16.21 – Schematic graph: volume of lather on the y-axis against volume of cleansing-agent solution added on the x-axis; four curves — soap in distilled water (rises immediately), soap in borewell water (flat portion first, then rises), detergent in distilled water and detergent in borewell water (both rise immediately and nearly overlap); mark the flat portion of the soap-in-borewell-water curve as “scum forming”.
Synthetic Detergents
Synthetic detergents are man made cleansing agents designed to be used even in hard water. That single design requirement explains everything about them: their polar head is chosen so that its calcium and magnesium salts do not precipitate out.
There are three types of synthetic detergents — anionic detergents, cationic detergents and nonionic detergents — classified by the nature of the polar part of the molecule.
i. Anionic detergents are sodium salts of long chain alkyl sulfonic acids, or of long chain alkyl substituted benzene sulphonic acids. The cleaning part of the molecule carries a negative charge, which is where the name comes from.
ii. Cationic detergents are quaternary ammonium salts having one long chain alkyl group. A quaternary ammonium ion is a nitrogen atom bonded to four carbon groups, which leaves it permanently positively charged. Here the cleaning part of the molecule is the positive ion.
iii. Nonionic detergents are ethers of polyethylene glycol with alkyl phenol, or esters of polyethylene glycol with long chain fatty acid. These carry no charge at all; their polar part is a chain of repeating \(-\mathrm{OCH_2CH_2}-\) units, which is polar enough to dissolve in water without being ionic. The three types are compared schematically in Fig. 16.20.
Diagram
Fig. 16.20 – Schematic comparison of the three types of synthetic detergent: three molecules drawn side by side, each with the same long zig-zag hydrocarbon tail but different heads — (a) a circular head marked with a negative sign labelled “anionic”, (b) a circular head marked with a positive sign labelled “cationic”, (c) an uncharged head drawn as a short chain of –O–CH₂CH₂– repeat units ending in –OH, labelled “nonionic”.
Table 16.3 displays some synthetic detergents.
Table 16.3: Synthetic detergents
| Type | Example | Use |
|---|---|---|
| Anionic detergent | (sodium lauryl sulphate) \(\mathrm{CH_3(CH_2)_{10}CH_2OSO_3^{-}Na^{+}}\) | Household detergent, additive in toothpaste |
| Cationic detergent | \(\mathrm{CH_3(CH_2)_{15}\!-\!N^{+}(CH_3)_3\,Br^{-}}\) (cetyltrimethyl ammonium bromide) | hair conditioner, germicide |
| Nonionic detergent | \(\mathrm{CH_3(CH_2)_{16}\!-\!COO(CH_2CH_2O)_n\,CH_2CH_2OH}\) (an ester) | liquid dishwash |
| Nonionic detergent | \(\mathrm{C_9H_{19}\!-\!C_6H_4\!-\!O\!-\!(CH_2CH_2O)_n\,CH_2CH_2OH}\) (an ether) | liquid detergent |
Mechanism of Cleansing Action
Soaps and detergents bring about the cleansing of dirty, greasy surfaces by the same mechanism. This is worth stating clearly at the start, because the two differ in chemical composition but not at all in how they clean.
Dirt is held at the surface by means of oily matter and, therefore, cannot get washed away with water. This is the whole problem. Water is a polar solvent and grease is nonpolar, so water cannot wet the greasy film, cannot lift it, and simply runs off.
The molecules of soaps and detergents solve this by having two parts. One part is polar, called the head, and the other part is a long nonpolar chain of carbons, called the tail. A single soap molecule is drawn in Fig. 16.19.
Diagram
Fig. 16.19 – A single soap molecule: a small circle at one end labelled “polar head (hydrophilic)” carrying the –COO⁻Na⁺ group, joined to a long zig-zag hydrocarbon chain labelled “nonpolar tail (hydrophobic)”.
The polar head, being hydrophilic, can dissolve in water, which is a polar solvent. The nonpolar tail, being hydrophobic, dissolves in oil, fat or grease. The words are worth remembering literally: hydro is water, philic means liking, phobic means avoiding.
So a single molecule has one end that is happy in water and another end that is happy in grease. Neither end can force the other to change its preference, and out of this tug-of-war the cleaning action arises.
The molecules of soap or detergent are arranged around the oily droplet such that the nonpolar tails point towards the central oily drop, while the polar heads are directed towards the water, as shown in Fig. 16.2. Thus, micelles of soap or detergent are formed surrounding the oil drops, which are removed in the washing process.
Diagram
Fig. 16.2 – Soap micelle at work: a greasy dirt particle at the centre of a cluster of soap molecules, with hydrophobic tails pointing inwards into the grease and hydrophilic heads pointing outwards into the surrounding water; label “greasy dirt particle”, “hydrophobic tail”, “hydrophilic head”, and show a layer of foam at the top of the water.
Once the grease is enclosed in a micelle, its outside surface is entirely polar heads. As far as the surrounding water is concerned, it is now dealing with a polar particle, not with grease — so the droplet can be carried away in the rinse water, taking the dirt held in it along. The complete sequence is set out in FC 16.5.
Flowchart
FC 16.5 – Mechanism of cleansing action of soap: greasy dirt sticking to the surface → water alone cannot wet the grease (polar water, nonpolar grease) → soap/detergent added → each molecule has a polar hydrophilic head and a nonpolar hydrophobic tail → tails dissolve into the oily droplet, heads remain in the water → molecules arrange around the droplet → micelle formed, grease enclosed with polar heads outside → micelle now dispersible in water → agitation lifts the droplet off the surface → rinsed away in the washing process → surface left clean.
In examinations, this section is most often tested through “give two differences” questions (soap versus synthetic detergent, disinfectant versus antiseptic), through an equation for saponification, and through an explanation of the cleansing mechanism supported by a flow chart — so practise drawing the micelle and labelling the head and tail correctly.
16.4 Summary
Chemistry underlies the ordinary events around us and the life processes within us. This chapter examined three areas of it: food chemistry, medicinal chemistry and the chemistry of cleansing materials.
Food provides nutrients, which act as the body’s source of energy and also regulate growth and maintain and repair body tissues. The nutrients comprise carbohydrates, lipids, proteins, vitamins, minerals and water.
Most nutrients are organic macromolecules. Proteins and carbohydrates are polymeric; on digestion, under the influence of enzymes, they break down into their monomers — \(\alpha\)-amino acids and glucose respectively.
Cooking hydrolyses the high polymers of carbohydrates and proteins to smaller polymers, and converts the uncooked heterogeneous suspension into colloidal matter. Because the resulting nutrient molecules are smaller, cooked food is easier to digest than uncooked food.
Food quality is described in terms of flavour, smell, texture, colour and microbial spoilage. Quality changes on storage mainly through enzyme action, chemical reactions with the environment and the action of microorganisms. A few such changes are deliberately used, such as the setting of milk into curd and the raising of dough for bread.
Reactions of stored food with the environment are controlled by keeping oxygen and microorganisms away (air-tight containers, evacuation, filling with \(\mathrm{N_2}\)), by refrigeration, which lowers the reaction rate, and by boiling, which denatures the enzymes.
Browning of cut fruits and vegetables occurs because cutting damages cells, releasing polyphenols that are oxidised by air in the presence of an enzyme to quinones:
\[\text{poly phenol} + \tfrac{1}{2}\,\mathrm{O_2} \xrightarrow{\ \text{enzyme}\ } \text{quinone} + \mathrm{H_2O}\]
Quinones polymerize further to brown products called tannins. The browning is slowed by reducing agents such as \(\mathrm{SO_2}\) and ascorbic acid, or by lowering the pH with lemon juice (citric acid) or vinegar.
Fats are triesters of long chain fatty acids and glycerol (propane-1,2,3-triol). Rancidity arises by two separate routes.
Hydrolytic rancidity: water present in the food, with microorganisms and enzymes, hydrolyses the fat to glycerol and free fatty acids.
\[\text{Triglyceride} + 3\,\mathrm{H_2O} \xrightarrow{\ \text{Hydrolysis}\ } \text{Glycerol} + 3\ \text{Free fatty acids}\]
The smell produced depends on which acids are released: butanoic, hexanoic and octanoic acids from milk and butter; palmitic, stearic and oleic acids give chocolate an oily flavour; lauric acid gives coconut oil a soapy flavour.
Oxidative rancidity: molecular oxygen attacks the \(\mathrm{C{=}C}\) bonds of unsaturated fats in a free radical reaction initiated by light or catalysed by enzymes or metal ions, giving volatile aldehydes and carboxylic acids. Polyunsaturated oils turn rancid quickly, high temperature speeds the process, and extensive oxidation causes polymerization with increased viscosity and browning.
Saturated fatty acid chains pack closely and their van der Waals forces are strong enough to make saturated fats solid at room temperature. A \(\mathrm{C{=}C}\) bond puts a kink in the chain, preventing close packing, weakening the forces and lowering the melting point — so unsaturated fats are liquids.
Natural fats are mixtures of triglycerides, have no sharp melting point and melt over a range; the more unsaturated the fat, the lower its melting point and the less crystalline it is.
In the cis isomer the two hydrogens on the doubly bonded carbons lie on the same side, and in the trans isomer on opposite sides. The cis form is the common form of unsaturated fats; the trans form occurs only in animal fats and processed unsaturated fats, is difficult to metabolize, and may build up to dangerous levels in fatty tissue.
Fats are transported as lipoproteins. Excessive low density lipoprotein (LDL) deposits cholesterol in blood vessels and raises the risk of cardiovascular disease; saturated and trans unsaturated fats increase this tendency, while cis fats do not form such deposits and decrease the chance of coronary heart disease.
Omega numbering counts carbons from the last carbon of the chain. Omega-3 fatty acids have a \(\mathrm{C{=}C}\) bond between the third and fourth carbon from that end; linolenic acid (9,12,15-octadecatrienoic acid) is an example. Omega-3 fats raise HDL, the good cholesterol, while omega-6 fats are considered to carry a risk of high blood pressure. Walnuts, flaxseeds, chia seeds, soyabeans and cod liver oil are rich in omega-3 fatty acids.
An antioxidant delays the onset of oxidation or slows its rate, and so extends the shelf life of food. Antioxidants react with oxygen-containing free radicals and thereby prevent oxidative rancidity.
Vitamin E (tocopherol) is a natural antioxidant added to packed edible oils: its phenolic \(-\mathrm{OH}\) group provides the antioxidant activity and its long saturated chain makes it fat soluble. Synthetic antioxidants, used for economic reasons, commonly contain a phenolic \(-\mathrm{OH}\) group and a tertiary butyl group; BHT is 3,5-di-tert-butyl-4-hydroxytoluene.
A drug is a chemical that interacts with biomolecules and produces a biological response; a drug with a therapeutic and useful response is used as a medicine, in which it is the active ingredient. Being foreign substances, drugs often give adverse side effects. Drug design aims at synthesising new molecules with a better biological response, increasingly drawing on traditional medical knowledge.
Analgesics relieve pain, about half of them by reducing inflammation or swelling; antipyretics reduce fever. Salicylic acid (2-hydroxybenzoic acid), the active ingredient of willow bark, was isolated in the nineteenth century. Aspirin, its acetyl derivative, has fewer side effects but still irritates the stomach; paracetamol, a phenol, is a less problematic pain-killer.
Antimicrobial is the umbrella term for drugs that inhibit or kill bacteria, fungi and viruses. Disinfectants are non-selective and are used on non-living surfaces; antiseptics sterilise the surfaces of living tissue; antibiotics target bacterial infections within or on the body.
Common antiseptics include the inorganics iodine and boric acid and the organics iodoform and phenolic compounds. Tincture of iodine is a 2–3 percent solution of iodine in an alcohol–water mixture; dilute boric acid solution is a weak antiseptic for the eyes.
Dilute phenol (carbolic acid) was among the first antiseptics but proved corrosive. Its chloro derivatives are more potent, so they act at much lower concentrations with reduced corrosive effect: trichlorophenol (TCP) and chloroxylenol, the latter being the active ingredient (4.8 % W/V) of dettol. Thymol from oil of thyme is a non-toxic antiseptic, and p-chlorobenzyl phenol is a disinfectant in all purpose cleaners.
Antibiotics are purely synthetic or obtained from microorganisms. Paul Ehrlich developed the arsenic-based synthetic antibiotic salvarsan for syphilis and received the Nobel prize for medicine in 1908; noticing its similarity to azodyes, he synthesised prontosil in 1932, which the body converts into sulphanilamide, leading to the family of sulpha drugs, of which sulphapyridine is among the most effective.
Alexander Fleming discovered the antibacterial properties of a penicillium fungus in 1929; penicillin, established as a drug over the next thirteen years, was the first antibiotic of microbial origin, and chloramphenicol, isolated in 1947, is another.
Antibiotics are broad spectrum (a wide range of bacteria), narrow spectrum (one group) or limited spectrum (a single organism). Broad spectrum antibiotics have the disadvantage of also killing the useful bacteria of the alimentary canal.
Modern medicinal chemistry draws on traditional knowledge to isolate active ingredients from medicinal plants: curcumin from turmeric (antiseptic), methyl salicylate from wintergreen (analgesic), cinnamaldehyde from cinnamon, eugenol from clove, vitamin C from citrus fruits, and vitamin C with gallic acid from amla.
Cleansing agents remove stain, dirt or clutter from surfaces and are of two types by composition: soaps and synthetic detergents.
Soaps are sodium or potassium salts of long chain fatty acids, made by alkaline hydrolysis of oils and fats with \(\mathrm{NaOH}\) or \(\mathrm{KOH}\) — the saponification reaction, which yields soap and glycerol:
\[\text{Oil/fat (triester)} + 3\,\mathrm{NaOH} \xrightarrow{\ \text{saponification}\ } \text{Glycerin} + 3\,\mathrm{RCOONa}\]
Potassium soaps are soft to the skin, so toilet soaps use better grades of oil with \(\mathrm{KOH}\) and the excess alkali is removed; laundry soaps use \(\mathrm{NaOH}\) with fillers such as sodium rosinate, sodium silicate, borax, sodium phosphate and trisodium phosphate.
Soaps become inactive in hard water because dissolved calcium and magnesium salts precipitate the fatty acid salt as scum, which sticks to fabric:
\[2\,\mathrm{RCOONa}\text{(aq)} + \mathrm{CaCl_2}\text{(aq)} \rightarrow (\mathrm{RCOO})_2\mathrm{Ca}\text{(s)} + 2\,\mathrm{NaCl}\text{(aq)}\]
Washing soda (\(\mathrm{Na_2CO_3}\)) softens water by precipitating the dissolved calcium salts as the carbonate, and so restores the soap’s action.
Synthetic detergents are man made cleansing agents that work even in hard water. Anionic detergents are sodium salts of long chain alkyl sulfonic acids or alkyl substituted benzene sulphonic acids; cationic detergents are quaternary ammonium salts with one long chain alkyl group; nonionic detergents are ethers of polyethylene glycol with alkyl phenol or esters of polyethylene glycol with a long chain fatty acid.
Soaps and detergents clean by the same mechanism. Each molecule has a polar hydrophilic head, which dissolves in water, and a long nonpolar hydrophobic tail, which dissolves in oil, fat or grease. Around an oily droplet the tails point inwards and the heads outwards, forming a micelle that is carried away in the washing process.
Flowchart
FC 16.6 – The chapter at a glance: Chemistry in everyday life → three branches. Branch 1, food chemistry: nutrients → digestion to monomers → cooking → food quality → loss of quality by browning (polyphenol → quinone → tannin) and by rancidity (hydrolytic → free fatty acids; oxidative → volatile aldehydes and carboxylic acids) → control by antioxidants, low temperature, exclusion of oxygen and light. Branch 2, medicinal chemistry: drug → medicine → analgesics and antipyretics (salicylic acid → aspirin; paracetamol) and antimicrobials → disinfectants / antiseptics / antibiotics (broad, narrow, limited spectrum) → active ingredients from traditional medicinal plants. Branch 3, cleansing agents: soaps (saponification of oils and fats; scum in hard water) and synthetic detergents (anionic / cationic / nonionic; work in hard water) → both clean by micelle formation.
16.5 Check Your Concepts
Assertion: Polyunsaturated oils become rancid much faster than saturated fats. Reason: They contain a greater number of \(\mathrm{C{=}C}\) double bonds, which are the sites attacked in a free radical reaction with atmospheric oxygen. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.
Stored food is protected in three different ways — by filling the container with \(\mathrm{N_2}\) gas, by refrigeration and by boiling. Identify which requirement of a chemical reaction each of these three methods removes or reduces.
Butter kept too long develops a sharp, sour smell, whereas coconut oil kept too long develops a soapy flavour. Both are undergoing the same reaction. Explain why the complaint is different in the two cases.
Distinguish between hydrolytic rancidity and oxidative rancidity on the basis of (i) the species attacking the fat, (ii) the bond attacked and (iii) the products formed.
Edible oils are often sold in dark or opaque containers rather than clear ones. Which cause of rancidity does this packaging guard against, and how?
Oil that has been used for deep frying several times becomes thick, dark and unpleasant in smell. Name the two changes in the oil responsible for this, and state why repeated heating makes them worse.
Natural fats do not have sharp melting points but melt over a range of temperature. What does this tell you about their composition?
Arrange the following triglycerides in the expected order of increasing melting point and justify your order in terms of chain shape and van der Waals forces: (i) one made from a fatty acid with three cis \(\mathrm{C{=}C}\) bonds, (ii) one made from a fully saturated fatty acid of the same chain length, (iii) one made from a fatty acid with a single trans \(\mathrm{C{=}C}\) bond of the same chain length.
Peanut oil is a liquid at room temperature but vanaspati ghee made from such oils is a solid. What structural change is responsible, and why does it alter the physical state?
Assertion: Cis unsaturated fats reduce the chance of developing coronary heart disease. Reason: They do not cause the deposition of cholesterol in blood vessels. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.
A fatty acid is named 11,14-eicosadienoic acid, where “eicosa” denotes a chain of twenty carbon atoms. Determine whether it belongs to the omega-3 or the omega-6 family, showing clearly how you count from the omega end.
Both vitamin E and BHT are used as antioxidants although one is natural and the other synthetic. State the structural feature they have in common, explain what that feature does, and state why vitamin E is particularly suitable for addition to edible oils.
What would you expect to happen to a packet of fried snacks stored for several months if no antioxidant had been added to the oil used? Name the type of rancidity involved and the class of products responsible for the change in taste.
Uncooked food is described as a heterogeneous suspension and cooked food as colloidal matter. Explain how this change in physical state, together with the change in molecular size, makes cooked food easier to digest.
Match the following.
| A group | B group |
|---|---|
| a. Cetyltrimethyl ammonium bromide | i. Antiseptic obtained from a spice plant |
| b. Sodium lauryl sulphate | ii. Antioxidant added to packed edible oil |
| c. Thymol | iii. Cationic detergent |
| d. Vitamin E | iv. Anionic detergent |
| v. Antibiotic of microbial origin |
“Every medicine contains a drug, but not every drug is used as a medicine.” Explain this statement.
Besides the active ingredient, a tablet contains several other chemicals. Why are they added, and why are they not described as drugs?
Aspirin was prepared from salicylic acid to reduce its side effects, yet aspirin still irritates the stomach. Name the functional group that was changed in going from salicylic acid to aspirin, name the group that was left unchanged, and relate your answer to the persisting side effect.
A dilute solution of phenol was one of the earliest antiseptics but is no longer preferred, while its chloro derivatives are widely used. Explain why the chloro derivatives are the better choice, using the idea of potency and concentration.
Tincture of iodine is applied to a cut on the hand, but a dilute solution of boric acid is used for the eyes. Explain the reason for using different antiseptics in these two situations.
Chloroxylenol is used in an antiseptic while p-chlorobenzyl phenol is used in an all purpose floor cleaner, although both are chloro derivatives of phenols. What decides whether a compound is used as an antiseptic or as a disinfectant?
Assertion: A broad spectrum antibiotic may cause digestive problems. Reason: It is effective against a wide range of bacteria and therefore also kills the useful bacteria of the alimentary canal. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.
An antiseptic applied to a wound does not disturb the bacteria of the alimentary canal, but an antibiotic taken by mouth does. Explain this difference in terms of where each of them acts.
Prontosil is not itself the active antibacterial compound. Explain what happens to it in the body, name the compound formed, and state why this discovery was important for drug design.
Arrange the following in the correct historical sequence and write one line on the significance of each step: sulphapyridine, salvarsan, prontosil, sulphanilamide.
Modern medicinal chemistry increasingly turns to traditional knowledge of medicinal plants. State what a chemist tries to obtain from such a plant, and give two advantages of using the isolated active ingredient rather than the raw plant extract.
Name the active ingredient responsible for the medicinal property of each of the following, and state the property: turmeric, clove, cinnamon, wintergreen.
Complete and balance the following reaction, name both products and name the reaction: \[\mathrm{C_3H_5(OOCC_{17}H_{35})_3} + 3\,\mathrm{NaOH} \rightarrow\]
Toilet soaps are made with \(\mathrm{KOH}\) and better grades of oil, while laundry soaps are made with \(\mathrm{NaOH}\) and contain fillers. Give two reasons why a laundry soap should not be used for bathing.
A soap solution gives very little lather with borewell water and leaves a grey deposit on the cloth. Write the equation for the reaction responsible, explain why the deposit forms, and state how the addition of washing soda restores the lather.
You are given two unlabelled solutions, one of soap and one of a synthetic detergent, along with a supply of hard water. Describe a simple test that would identify each, stating clearly the observation on which your conclusion rests.
Classify each of the following as an anionic, cationic or nonionic detergent, stating the structural feature that decides your answer, and name one use of each: (i) \(\mathrm{CH_3(CH_2)_{15}N^{+}(CH_3)_3Br^{-}}\), (ii) \(\mathrm{CH_3(CH_2)_{10}CH_2OSO_3^{-}Na^{+}}\), (iii) \(\mathrm{C_9H_{19}\text{-}C_6H_4\text{-}O\text{-}(CH_2CH_2O)_nCH_2CH_2OH}\).
A greasy plate cannot be cleaned with water alone but is cleaned easily with soap solution. Explain this using the two parts of a soap molecule, and state which part of the molecule faces the water in the aggregate that is formed.
Detergents with heavily branched hydrocarbon chains were later replaced by those with straight chains. Explain the environmental reason behind this change, and identify which principle of green chemistry it illustrates.
Explain the following.
- Cooking makes food easy to digest.
- On cutting, some fruits and vegetables turn brown.
- Vitamin E is added to packed edible oil.
- Browning of cut apple can be prolonged by applying lemon juice.
- A dilute solution (4.8 % w/v) of 2,4,6-trichlorophenol is employed as an antiseptic.
- Turmeric powder can be used as an antiseptic.
Identify the functional groups in the following molecules.
- Aspirin
- Paracetamol
- Penicillin
- Chloramphenicol
- Sulphanilamide
- Glycerin
Diagram
Fig. 16.22 – Structures to accompany Question A-Q3, arranged A to F: (A) aspirin — benzene ring with COOH and O–CO–CH₃ on adjacent carbons; (B) paracetamol — benzene ring with HO– and, at the para position, N(H)–CO–CH₃; (C) penicillin — β-lactam ring fused to a sulphur-containing five-membered ring bearing COOH, with an R–CO–NH– side chain; (D) chloramphenicol — O₂N–C₆H₄–CH(OH)–CH(NH–CO–CHCl₂)–CH₂OH; (E) sulphanilamide — benzene ring with NH₂ and SO₂NH₂ at the para positions; (F) glycerin — CH₂–OH, CH–OH, CH₂–OH.
Give two differences between the following.
- Disinfectant and antiseptic
- Soap and synthetic detergent
- Saturated and unsaturated fats
- Rice flour and cooked rice
Explain with examples.
- Antiseptics
- Disinfectants
- Cationic detergents
- Anionic detergents
- Non-ionic detergents
Name two drugs which reduce body pain.
Explain the mechanism of cleansing action of soap with a flow chart.
What is meant by a broad spectrum antibiotic and a narrow spectrum antibiotic?
Name the painkiller obtained from acetylation of salicylic acid.
Name the class of drug often called a painkiller.
Who discovered penicillin?
Draw the structures of chloroxylenol and salvarsan.
Write the molecular formula of butylated hydroxy toluene.
What is tincture of iodine?
Draw the structure of BHT.
Write a chemical equation for saponification.
Write the molecular formula and name of the compound shown in Fig. 16.23.
Diagram
Fig. 16.23 – Structure for Question A-Q10 (I): a benzene ring bearing a COOH group and, on the adjacent carbon, an OCOCH₃ group.
- Write two examples of each of the following.
- Analgesics
- Antiseptics
- Antibiotics
- Disinfectants
- What do you understand by an antioxidant?
16.6 Objective Questions
- Oxidative rancidity is a ……. reaction.
- addition
- substitution
- free radical
- combination
- Saponification is carried out by …….
- oxidation
- alkaline hydrolysis
- polymerisation
- free radical formation
- Aspirin is chemically named as …….
- salicylic acid
- acetyl salicylic acid
- chloroxylenol
- thymol
- Find the odd one out from the following.
- dettol
- chloroxylenol
- paracetamol
- trichlorophenol
- Arsenic based antibiotic is …….
- azodye
- prontosil
- salvarsan
- sulphapyridine
- The chemical used to slow down the browning action of cut fruit is …….
- \(\mathrm{SO_3}\)
- \(\mathrm{SO_2}\)
- \(\mathrm{H_2SO_4}\)
- \(\mathrm{Na_2CO_3}\)
- The chemical responsible for the rancid flavour of fats is …….
- butyric acid
- glycerol
- protein
- saturated fat
- Health benefits are obtained by consumption of …….
- saturated fats
- trans fats
- mono unsaturated fats
- all of these
- Match the pairs.
| A group | B group |
|---|---|
| a. Paracetamol | i. Antibiotic |
| b. Chloramphenicol | ii. Synthetic detergent |
| c. BHT | iii. Soap |
| d. Sodium stearate | iv. Antioxidant |
| v. Analgesic |