18. Chemistry in service to mankind
Learning outcomes
Candidates should be able to:
- Explain addition and condensation polymers.
- Explain elastomers and fibres.
- Describe natural and synthetic polymers.
- Explain some synthetic polymers (polythene, PVC, Teflon, polystyrene, nylon, and bakelite).
- Explain types of dyes based on structure and method of application.
- Describe characteristics of drugs.
- Differentiate natural and synthetic drugs.
- Classify some common drugs.
- Be aware of the adverse effects of drug addiction.
- Explain insecticides, herbicides, and fungicides.
Chemistry plays a pivotal role in improving the quality of human life. From the development of life-saving drugs to the manufacture of polymers, dyes, and agricultural chemicals, chemistry contributes significantly to health, industry, and environmental management. This chapter explores how chemical substances are designed and utilized to serve mankind, focusing on polymers, dyes, drugs, and pesticides.
Polymers: The Giant Molecules
Polymers are giant molecules or macromolecules of high molecular mass made up of a large number of small repeating units called monomers.
Classification of Polymers
Polymers can be classified in different ways
- Based on Origin or Source
[A] Natural Polymers: formed naturally on the bodies of animals and plants.
e.g., Polysaccharides are polymers of monosaccharides,
Proteins are polymers of amino acids,
Nucleic acids are polymers of nucleotides, hence also called polynucleotides,
natural rubber, etc.
[B] Synthetic Polymers: produced artificially in the laboratory or industries.
E.g., Bakelite is a polymer of Phenol and Methanal,
Polyethene is a polymer of Ethene,
Polyvinyl chloride (PVC) is a polymer of Vinyl chloride,
Polystyrene is a polymer of Styrene, etc.
- Based on the mode of formation
[A] Addition Polymers: formed by the addition of whole monomer units without the elimination of any molecule
Such a process is called addition polymerization.
E.g., Polyethene, Polystyrene, Polyvinyl chloride (PVC), etc.
[B] Condensation Polymer: formed by reaction of monomers (generally having di or poly-functional groups) with elimination of small molecules like H2O (condensation reaction).
Such a process is called condensation polymerization.
- Did you know?
- Benzene can be prepared by the polymerization of Ethyne (acetylene) in a red-hot copper tube. This is addition polymerization. It is also called trimerisation as three ethyne molecules form a benzene molecule.
- Or,
- Based on Monomer units
Based on monomer units, polymers are classified as homopolymers and copolymers.
[A] Homopolymers: made up of only one kind of monomer unit
E.g., Polyethene is made up of Ethene monomer.
Polystyrene is made up of Styrene monomer.
Polyvinyl chloride (PVC) is made up of Vinyl chloride monomer only.
Formation of a homopolymer can be shown as:
[B] Copolymer: made up of two or more different kinds of monomer units.
e.g., Bakelite is made up of Phenol and Formaldehyde monomers.
Nylon-66 is made up of hexamethylene diamine and adipic acid monomers.
Formation of a copolymer can be shown as:
Elastomers: These are the polymers in which the polymer chains are held up by the weakest attractive forces. They are amorphous polymers having a high degree of elasticity. Weak forces permit the polymers to be stretched out about 10 times their normal length, but they return to their original position when the stretching force is withdrawn. In fact, these polymers consist of randomly coiled molecular chains having a few cross-links. When the stress is applied, these randomly coiled chains straighten out, and the polymer gets stretched. As soon as the stretching force is released, the polymer regains its original shape because weak forces do not allow the polymer to remain in the stretched form. E.g., natural rubber, ethylene propylene rubber, etc.
Fibers: These are polymers that have quite a strong interparticle force, such as H- bonds. They have high tensile strength and high modulus. They are threadlike polymers and can be woven into fabrics. E.g., Nylon, Dacron, and silk, etc.
Some Polymers
| Polymer | Monomer(s) | Structure | Uses |
| Polythene | Ethene (CH2=CH2) | – (CH2-CH2)2– | Plastic bags, bottles, containers |
| PVC (Polyvinyl chloride) | Vinyl chloride (Chloroethene, CH2=CHCl) | Water pipes, tanks, electrical insulation | |
| Teflon (PTFE) | Tetrafluoroethene (CF2=CF2) | Non-stick coating for utensils, high-frequency insulators | |
| Polystyrene | Styrene (C6H5−CH=CH2) | −(CH2−CH(C6H5))n− | Disposable cups, packaging materials |
| Nylon-66 | Hexamethylene diamine (H2N(CH2)6NH2) & Adipic acid HOOC(CH2)4COOH | Polyamide with amide linkages | Brushes, carpets, textiles |
| Bakelite | Phenol (C6H5OH) & Formaldehyde (HCHO) | Cross-linked thermosetting polymer | Electrical switches, handles, phone casings |
Dyes: Adding colour to life
Dyes are colored substances, generally organic compounds, used to impart color to textiles, silk, wool, foodstuffs, etc.
All coloured compounds can’t be dyes. For a chemical compound to act as a dye, it must possess the following characteristics;
- It must have a suitable, attractive color.
- It must be able tofix itself permanently to the material being dyed.
- It must not change by the action of water, air, dilute acid, bases, detergents, etc.
The colour is due to a specific functional group or part of the dye molecule called a chromophore (e.g., −N=N−, −NO2, quinoid rings).
Classification of Dyes
- Based on origin
Natural Dyes: extracted from nature, especially plants.
e.g., Indigo (a blue dye), Alizarin (a red dye), etc.
Synthetic Dyes: produced artificially in the laboratory or industry. e.g., Azo dyes ( p-hydroxyazobenzene, methyl orange, etc.)
2. Based on structure: dyes may be classified as:
a. Azo dyes
The dyes whose chromophore is azo group (-N=N-) are called Azo Dyes e.g.,
b.Triphenyl methane dyes
Central carbon atom with three phenyl rings
c. Phthalein dyes
d. nitro dyes
e. indigo dyes
f. anthraquinone dyes
3. Based on the method of application
a. Direct dyes
- dyes used for colouring the objects directly in their hot aqueous solutions
- stick to the fabric by H-bonds
- e.g., martius yellow, congo red used for colouring cotton, rayon, wool, nylon, silk, etc.
b. vat dyes
- Water-insoluble dyes, used in reduced colourless soluble form by treating with sodium bisulfite,
- fabrics are dipped into colourless dye solution and exposed to air or other oxidizing agents to reobtain the colour.
- e.g., indigo used for cotton.
c. mordant dye
- Requires a metal ion (called a mordant) to fix to fabric
- e.g., alizarin gives a rose red colour with Al and a blue colour with Ba.
d. disperse dyes
- insoluble in water but forms a colloidal solution (dispersion)
- fine particles of dyes dispersed into the fabric by immersing it in the colloidal solution
- e.g., anthraquinone derivatives used in synthetic fibres like orlon, nylon, polyester, terelene, etc.
e. developed dyes
- formed on the spot in the fabrics by the reaction of the precursors
- after their formation, these are attached to the fabrics by an irreversible chemical reaction.
- e.g., azoic dyeing is done this way in cotton, wool, nylon, etc.
Drugs: Healing Molecules
Drugs are substances used for the treatment of diseases, for relieving pain, or controlling population (contraceptives).
Characteristics of an Ideal Drug:
- Acts locally at the desired site.
- Is efficient and effective.
- Has minimum side effects.
- Does not injure host tissues.
- Does not cause resistance with long-term use.
Drugs are generally classified according to the purpose for which they are used.
- Antiseptics or disinfectants
Antiseptics are chemicals that can prevent the growth of or even kill microorganisms, and which can be applied to living tissues (cuts, wounds, etc.). e.g., phenol and its derivatives, tincture of iodine. Commonly used antiseptic, Dettol is a mixture of Chloroxylenol and Terpeneol.
Disinfectants or germicides are substances that can kill microorganisms but are also harmful to living tissues. e.g., Potassium permanganate solution above 1%, chlorine in water above 10 ppm, etc.
- Antipyretics
These are the drugs used to lower body temperature during high fevers. e.g, Aspirin, Paracetamol, Phenacetin, etc.
Analgesics
Analgesics are the drugs that are used to relieve pain. They are also called painkillers.
These can further be classified into narcotics and non-narcotics.
Narcotics are habit-forming. e.g., morphine, codeine, heroine etc.
Non-narcotics are not habit-forming. e.g. Aspirin, Analgin, Brufen, Paracetamol etc.
Anti-inflammatory
The drugs used to reduce the inflammation caused by some infections and injuries are called anti- inflammatory drugs, e.g., Aspirin, Brufen, Prednisolone, etc.
- Antimalarial
These are the drugs used in the treatment of malaria, e.g., Quinine, Chloroquine, etc.
- Antibiotics
These are the chemicals produced by some organisms (bacteria, fungi, etc.) which can be employed to destroy other organisms.
The first antibiotic discovered by Alexander Fleming was Penicillin.
Its basic structure is:
The antibiotics can further be classified as narrow-spectrum antibiotics and wide-spectrum antibiotics.
The Narrow-spectrum Antibiotics are effective against only very few organisms (or few diseases). e.g. Penicillin
The Wide (Broad) Spectrum Antibiotics are effective against a wide range of microorganisms. E.g., Chloramphenicol, Tetracycline, etc.
- Sulpha-drugs
Sulpha drugs constitute a group of synthetic drugs, which, like antibiotics, have antibacterial activities. But these are not derived from microorganisms but are chemically synthesized. E.g. Sulphanilamide, Sulphadiazine, etc.
- Tranquilizers and hypnotics
These drugs act on the central nervous system and help to reduce mental anxiety. They are used to treat mental diseases and also as sleeping pills. e.g., Reserpine, Serpasil, Hashish, Charas, etc.
Narcotic analgesics and certain tranquilizers can be habit-forming. Drug addiction leads to severe physical, mental, social, and economic consequences. Awareness and prevention are critical.
Pesticides
Various plants and animals (insects, weeds, rodents, etc.) cause huge destruction to the crops and agricultural products every year. These plants and animals, which are detrimental to the agricultural economy, are called ‘pests’.
The chemicals used for killing insects are called insecticides. E.g., DDT, DDD, BHC, Methoxychlor, etc. Malathion and parathion are biodegradable insecticides.
The chemicals used for killing algae and fungi are called Fungicides. E.g., copper sulphate, Bordeaux mixture (copper sulphate + lime), etc.
The chemicals used for killing weeds are called herbicides or Weedicides. E.g. 2,4-D (2,4-dichlorophenoxyacetic acid), 2,4,5-T(2,4,5-trichlorophenoxyacetic acid) etc.
These are all called Pesticides in general.
Rodents can be killed by Thalium sulphate, Alphanaphthyl, Thiourea, etc.
Note: While DDT was historically effective, it is non-biodegradable and has been banned or restricted in many countries due to environmental persistence and toxicity.
Chapter Summary
- Polymers are macromolecules made of repeating monomers. They are classified as natural/synthetic, addition/condensation, and homo/copolymers.
- Elastomers are highly elastic (e.g., rubber), while fibres are strong and threadlike (e.g., nylon).
- Common synthetic polymers include polythene, PVC, Teflon, polystyrene, nylon-66, and bakelite.
- Dyes require a chromophore and must be fast to light, water, and detergents. They are classified by origin, structure, and application method (direct, vat, mordant, disperse, developed).
- Drugs include antiseptics, disinfectants, antipyretics, analgesics, antibiotics, sulpha drugs, and tranquilizers. Each has a specific therapeutic role.
- Pesticides (insecticides, fungicides, herbicides, rodenticides) protect crops from pests.
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