Class 11 Chemistry — Chapter 308: Chemistry in Everyday Life

Key concepts: Chemistry in Everyday Life (Class 11 Chemistry)

This chapter shows chemistry doing everyday work: curing disease, keeping food edible and getting clothes clean. It begins with drugs, defined as low molar mass chemicals that interact with a biological target, and the four ways of classifying them — by pharmacological effect, by drug action, by chemical structure and by molecular target. It then explains how a drug actually works, either by inhibiting an enzyme at its active or allosteric site or by acting as an agonist or antagonist at a receptor, and surveys the main therapeutic classes: antacids, antihistamines, tranquilisers, narcotic and non-narcotic analgesics, antimicrobials, antiseptics, disinfectants and antifertility drugs. The last two sections deal with chemicals added to food — artificial sweeteners, preservatives and antioxidants — and with cleansing agents, covering the saponification of oils to soap, the three families of synthetic detergents and the problem of biodegradability.

Drug and medicine

A drug is a chemical of low molar mass, usually 100 to 500 u, that interacts with a macromolecular target and produces a biological response. It is a medicine only when that response is useful in therapy and the dose is correct; above the therapeutic dose the same chemical is toxic or habit forming.

Four ways of classifying drugs

By pharmacological effect (all analgesics together, most useful to a doctor), by drug action (the biochemical process affected, such as blocking histamine), by chemical structure (drugs sharing a skeleton such as the sulphonamides), and by molecular target (drugs acting on one enzyme or receptor, most useful to a medicinal chemist).

Enzymes as drug targets

Enzymes are protein catalysts that hold a substrate at an active site and speed up its reaction. A competitive inhibitor resembles the substrate and blocks the active site, while an allosteric or non-competitive inhibitor binds elsewhere and changes the shape of the active site. Weak, reversible bonding is preferred because covalent binding inhibits the enzyme permanently.

Receptors, agonists and antagonists

Receptors are proteins embedded in the cell membrane whose binding site faces outwards and receives chemical messengers. An agonist mimics the natural messenger and switches the receptor on, whereas an antagonist occupies the site and blocks the message. Small differences between receptors let a drug be made selective and so reduce side effects.

Antacids and antihistamines

Antacids such as magnesium hydroxide and aluminium hydroxide neutralise excess stomach acid, while ranitidine and cimetidine go further by blocking histamine at H₂ receptors so that less HCl is secreted. Antihistamines such as cetirizine and brompheniramine block H₁ receptors and relieve allergy symptoms.

Tranquilisers and analgesics

Tranquilisers such as equanil, chlordiazepoxide and barbiturates treat stress, anxiety and mental disease by acting on the central nervous system. Analgesics relieve pain: non-narcotic ones such as aspirin and paracetamol are non-addictive, while narcotic ones such as morphine and codeine relieve severe pain but cause sleep, stupor and addiction.

Antimicrobials and antibiotics

Antimicrobials kill or stop the growth of micro-organisms. Bactericidal antibiotics such as penicillin, ofloxacin and the aminoglycosides kill bacteria, whereas bacteriostatic ones such as erythromycin, tetracycline and chloramphenicol only arrest growth. Penicillin G is narrow spectrum; chloramphenicol and tetracycline are broad spectrum.

Antiseptics and disinfectants

Both kill or prevent the growth of micro-organisms, but antiseptics such as dettol, iodoform and bithionol are safe on living tissue while disinfectants are applied to floors, drains and instruments. The same chemical can be either: 0.2% phenol is an antiseptic, 1% phenol is a disinfectant.

Artificial sweeteners

Saccharin is about 550 times sweeter than sucrose and is excreted unchanged, so it is valuable to diabetic and obese people. Aspartame is roughly 100 times sweeter but decomposes on heating, limiting it to cold foods. Sucralose is a chlorinated sugar stable to cooking, and alitame is so sweet that controlling the sweetness of a food is difficult.

Preservatives and antioxidants

Preservatives such as sodium benzoate, salt, sugar and vegetable oils prevent spoilage by micro-organisms. Antioxidants such as BHA, BHT and sulphur dioxide are oxidised in preference to the food and scavenge free radicals, delaying the oxidation of fats and oils that makes them rancid.

Soaps and saponification

A soap is the sodium or potassium salt of a long chain fatty acid, made by boiling an oil or fat with alkali; this alkaline hydrolysis is called saponification and also yields glycerol. Types include toilet soaps, transparent soaps, medicated soaps, shaving soaps and laundry soaps. In hard water soap forms an insoluble Ca and Mg scum.

Synthetic detergents and biodegradability

Anionic detergents (alkylbenzene sulphonates, sodium lauryl sulphate) clean laundry and are used in toothpaste; cationic detergents (quaternary ammonium salts) are used in hair conditioners and are germicidal; non-ionic detergents are esters of polyethylene glycol used in liquid dishwashing. Straight-chain detergents are biodegradable, branched ones are not and cause river foam.