Chemistry in the Service of Mankind

Advantages of using pesticides:

1. Pest control: They kill or repel insects, weeds, rodents and fungi that damage crops, so crop loss is greatly reduced.
2. Higher yield and quality: By protecting plants from pests and diseases, they give better yield and cleaner, marketable produce.
3. Control of disease vectors: Pesticides (insecticides) help control mosquitoes, flies and other vectors, thus checking diseases like malaria and dengue.
4. Cheap and quick: They give fast, large-scale protection at relatively low cost, which is useful in commercial farming.

Disadvantages of using pesticides:

1. Environmental pollution: They pollute soil, water and air; they kill useful organisms (bees, earthworms, predators) along with the pests.
2. Bioaccumulation and biomagnification: Being non-biodegradable (e.g., DDT), they accumulate in living tissues and get concentrated along the food chain, reaching harmful levels in humans.
3. Health hazards: They cause acute poisoning and long-term effects such as cancer, nerve damage, and hormonal and reproductive disorders in farmers and consumers.
4. Resistance and resurgence: Continuous use makes pests resistant to the pesticide, and killing natural enemies leads to reappearance of pests in greater numbers.
5. Residues in food: Traces left on crops and in soil reduce food quality and safety.

(a) Paracetamol

Paracetamol is an antipyretic that lowers body temperature in fever. Penicillin is an antibiotic and diazepam is a tranquilliser.

Drug addiction is the habitual and compulsive use of drugs (such as heroin, cannabis or alcohol) for which a person develops physical and mental dependence, so he cannot stop using them.

It harms society in several ways. Addicts lose their health and ability to work, so productivity falls and family income drops. They often spend all their money on drugs, and many turn to theft and violence, so crime increases. Families break down and children are neglected. Sharing needles spreads diseases such as HIV and hepatitis. Treatment and policing place a heavy burden on the community and the nation.

One adverse effect: damage to the brain, liver and lungs, leading to loss of memory and early death.

Addition polymers are formed when unsaturated monomers (containing a C=C double bond) join together repeatedly by addition across the double bond, without the elimination of any small molecule. The monomers must be unsaturated. The polymer has the same empirical formula as the monomer.

Example: Ethene → Polythene

$n\,CH_2=CH_2 \xrightarrow{\text{heat, pressure, catalyst}} (-CH_2-CH_2-)_n$

Condensation polymers are formed when two bifunctional monomers (each having two reactive groups such as –OH, –COOH, –NH₂) react repeatedly with the elimination of a small molecule like water, alcohol or HCl.

Example: Nylon-6,6, formed from adipic acid and hexamethylenediamine with elimination of water.

$n\,HOOC-(CH_2)_4-COOH + n\,H_2N-(CH_2)_6-NH_2 \rightarrow [-CO-(CH_2)_4-CO-NH-(CH_2)_6-NH-]_n + 2n\,H_2O$

(i) Homo-polymer vs Co-polymer

(ii) Monomers and their molecular formulae

a) Polystyrene

• Monomer: Styrene (vinyl benzene)
• Molecular formula: C₆H₅–CH=CH₂ (i.e. C₈H₈)

b) Bakelite

• Monomers: Formaldehyde and Phenol
• Formulae:
  • Formaldehyde: HCHO (CH₂O)
  • Phenol: C₆H₅OH (C₆H₆O)

i) Antipyretics

Example: Paracetamol (acetaminophen) Use: Used to reduce fever (and relieve mild pain) by acting on the hypothalamus to lower body temperature.

ii) Pesticides

Example: DDT (dichlorodiphenyltrichloroethane) Use: Used to kill or control insect pests (insecticide) in agriculture and to control disease-carrying insects like mosquitoes.

iii) Bakelite

Example: Bakelite (a thermosetting phenol–formaldehyde resin) Use: Used for making electrical switches, plugs, sockets and handles of utensils, because it is hard, rigid and a poor conductor of heat and electricity.

iv) Nylon-6,6

Example: Nylon-6,6 (a polyamide formed from hexamethylenediamine and adipic acid) Use: Used for making ropes, tyre cords, parachutes and fabrics, because it is strong, elastic and resistant to abrasion.

Homopolymer: A polymer formed by the polymerisation of only one type of monomer is called a homopolymer. For example, polythene is formed by the polymerisation of ethene monomers only.

$n\,\text{CH}_2=\text{CH}_2 \longrightarrow (-\text{CH}_2-\text{CH}_2-)_n$

Co-polymer: A polymer formed by the polymerisation of two or more different types of monomers is called a co-polymer. For example, nylon-6,6 is formed from hexamethylenediamine and adipic acid.

$n\,\text{H}_2\text{N}-(\text{CH}_2)_6-\text{NH}_2 + n\,\text{HOOC}-(\text{CH}_2)_4-\text{COOH} \longrightarrow \text{Nylon-6,6}$

(i) Tranquilizer — Used to treat mental stress and anxiety; for example, equanil is used to control anxiety and tension.

(ii) Antiseptics — Used to kill or prevent the growth of microorganisms on living tissues; for example, dettol (dilute solution) is applied on cuts and wounds to prevent infection.

i) Aspirin (acetylsalicylic acid, 2-acetoxybenzoic acid): a benzene ring carrying a –COOH group and an acetoxy group (–O–CO–CH₃) on adjacent (ortho) carbons.

$\mathrm{HOOC{-}C_6H_4{-}O{-}CO{-}CH_3}\qquad(\text{molecular formula } C_9H_8O_4)$

ii) DDT (1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane): two $p$-chlorophenyl rings and a $-CCl_3$ group attached to the same carbon.

$\mathrm{(p\text{-}Cl{-}C_6H_4)_2CH{-}CCl_3}\qquad(\text{molecular formula } C_{14}H_9Cl_5)$

Each chlorine on the rings is in the para position (carbon 4) relative to the carbon bonded to the CH group.

• Soap and detergents: used for cleaning clothes and washing the body.
• Fertilisers (e.g. urea): supply nitrogen to plants and raise crop yield.
• Medicines (e.g. paracetamol): used to reduce fever and pain.
• Plastics (e.g. polythene): used for packaging, bags and pipes.

(Cement, paints, dyes and cosmetics are other acceptable answers.)

i) Insecticide: malathion (or DDT), used to kill insects and mosquitoes on crops and in homes.

ii) Synthetic polymer: polythene (or nylon-6,6), used for making plastic bags, pipes and packaging films.

iii) Antiseptic: tincture of iodine (or dettol, boric acid), applied on cuts and wounds to kill germs and prevent infection.

iv) Fungicide: Bordeaux mixture ($CuSO_4 + Ca(OH)_2$), sprayed on plants such as potato, tomato and grapes to control fungal diseases.

(c) Teflon

$(-CF_2-CF_2-)_n$ is polytetrafluoroethene (PTFE), made by polymerising tetrafluoroethene, $CF_2=CF_2$. Its trade name is Teflon.

Condensation polymerization is the process in which many bifunctional monomers join repeatedly through condensation reactions, with the elimination of a small molecule such as $H_2O$, HCl or $NH_3$ at each step, to form a polymer.

Monomers of Bakelite (a phenol–formaldehyde resin):

• Phenol: $C_6H_5OH$, a benzene ring carrying one –OH group.
• Formaldehyde (methanal): $HCHO$, that is $H-\overset{O}{\overset{\|}{C}}-H$.

$n\,C_6H_5OH + n\,HCHO \xrightarrow{\text{acid or base}} \text{Bakelite} + n\,H_2O$

Phenol is reactive at its two ortho positions and its para position, so formaldehyde links the rings through –CH₂– bridges into a cross-linked, thermosetting network.

• Natural polymer: starch (or cellulose, proteins, natural rubber). Starch is a polymer of glucose obtained from plants.
• Synthetic polymer: polythene (or PVC, nylon-6,6, Teflon). Polythene is made by addition polymerisation of ethene: $nCH_2{=}CH_2 \rightarrow (-CH_2-CH_2-)_n$.