Chemistry in the service of mankind

This unit is a survey of the four families of manufactured chemicals that changed ordinary life: polymers, dyes, drugs and pesticides. Nothing here is hard, but there is a great deal to know — monomers, linkages, classes, examples — and the examiner asks for exactly those names and examples. Read it as a set of tables to be learned rather than as theory to be derived.

18.1 Polymers: addition and condensation; elastomers and fibres; natural and synthetic

Polymers are also classified by behaviour:

and by origin: natural (starch, cellulose, proteins, natural rubber, DNA), semi-synthetic (rayon, cellulose acetate, vulcanised rubber) and synthetic (polythene, PVC, nylon, bakelite).

18.2 Synthetic polymers: polythene, PVC, Teflon, polystyrene, nylon and bakelite

Repeat units, written out:

• Polythene: –(CH₂–CH₂)ₙ–
• PVC: –(CH₂–CHCl)ₙ–
• Teflon: –(CF₂–CF₂)ₙ–
• Polystyrene: –(CH₂–CH(C₆H₅))ₙ–
• Nylon-6,6: –[NH–(CH₂)₆–NH–CO–(CH₂)₄–CO]ₙ– (an amide linkage)

n H₂N(CH₂)₆NH₂ + n HOOC(CH₂)₄COOH → –[NH(CH₂)₆NHCO(CH₂)₄CO]ₙ– + 2n H₂O

18.3 Dyes

According to Witt's theory, a dye molecule needs two kinds of group:

• A chromophore — the colour-bearing group, always unsaturated: –N=N– (azo), –NO₂, –NO, >C=O, –C=C– or a quinonoid ring.
• An auxochrome — a group such as –OH, –NH₂, –NHR, –NR₂, –SO₃H or –COOH that deepens the colour and, more importantly, binds the dye to the fibre.

A compound with a chromophore but no auxochrome is merely coloured (a chromogen), not a dye: azobenzene is orange but washes straight out of cloth.

Classification by structure

Classification by method of application

Indigo (nīl) was one of the oldest trade dyes of the Terai and the north Indian plains, and is still the vat dye that colours denim.

18.4 Drugs

Characteristics of a good drug

• It should be effective at a small dose against the specific disease.
• It should show selective toxicity: destroy the pathogen without harming the patient's own cells.
• It should have minimum side effects and should not be habit-forming.
• It should be stable on storage, cheap, and easy to administer and absorb.
• Its products should be excreted, not stored in the body.

Natural and synthetic drugs

Classification of common drugs

Habit-forming drugs and drug addiction. Some drugs act on the central nervous system and produce dependence: the user needs ever-larger doses (tolerance), and stopping causes withdrawal symptoms.

Addiction damages the liver, brain, kidneys and heart; injecting drugs spreads HIV and hepatitis B through shared needles. In Nepal the Narcotic Drugs (Control) Act, 2033 B.S. (1976) prohibits the cultivation, production, sale, trafficking and consumption of cannabis, opium and coca products, and makes addiction itself punishable; treatment is through counselling, rehabilitation centres and community support.

18.5 Pesticides: insecticides, herbicides and fungicides

A pesticide is any chemical used to kill or control organisms that damage crops, stored food or health. The three groups examined are named after their target.

Why they are regulated. DDT does not break down; it dissolves in fat and is passed up the food chain, so its concentration multiplies at every step — biomagnification.

Pesticide residues on vegetables, poisoning of the farmers who spray without protection, death of bees and fish, and the growth of resistant pests are the other costs. In Nepal, pesticides are registered and controlled by the Plant Quarantine and Pesticide Management Centre under the Pesticide Act, 2048 B.S. (1991). More than twenty pesticides are banned outright — among them DDT, aldrin, dieldrin, endrin, BHC, methyl parathion and endosulfan — and paraquat, chlorpyrifos and phorate were added in recent years. Rapid residue-testing laboratories, such as the one at the Kalimati fruit and vegetable market in Kathmandu, screen incoming produce before it reaches the retail stalls.

Practice questions

Group A — Multiple choice (1 mark each)

1. Which of the following is a condensation polymer? [1]apolythenebPVCcnylon-6,6dTeflon
2. The monomer of Teflon is [1]aCH₂=CHClbCF₂=CF₂cCH₂=CH₂dC₆H₅CH=CH₂
3. In Witt's theory, –N=N– is a [1]aauxochromebchromophorecmordantdleuco group
4. Indigo is applied to cotton as a [1]aacid dyebbasic dyecvat dyeddisperse dye
5. Which is used as an antacid? [1]aaspirinbdiazepamcMg(OH)₂dchloroquine
6. 2,4-D is a [1]afungicidebherbicidecinsecticidedrodenticide

Group B — Short answer (5 marks each)

1. Distinguish between addition and condensation polymerisation, with two examples of each. [5]
2. Give the monomer, type of polymerisation and one use of polythene, PVC, polystyrene and bakelite. [5]
3. What is a dye? Explain Witt's theory with methyl orange as the example, and classify dyes on the basis of their method of application. [5]
4. Define a drug. State four characteristics of a good drug and distinguish between an antiseptic and a disinfectant with examples. [5]
5. A sample of polythene has an average molar mass of 42,000 g mol⁻¹. Find its degree of polymerisation. [5]
6. What are habit-forming drugs? Name the four types with one example each and state two harmful effects of drug addiction. [5]

Group C — Long answer (8 marks each)

1. (a) Classify polymers on the basis of the mode of polymerisation, of physical properties and of origin, giving two examples of each class. [5](b) Write the equation for the formation of nylon-6,6 and calculate the mass of nylon obtained from 29.2 g of adipic acid with excess diamine. [3]
2. (a) Classify dyes on the basis of structure, with an example of each. [4](b) What are pesticides? Distinguish between insecticides, herbicides and fungicides with two examples each, and give two harmful effects of using persistent pesticides such as DDT. [4]