18. Chemistry in service to mankind

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:

1. Based on Origin or Source

[A] Natural Polymers: formed naturally in 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.

2. 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.

3. 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 hexamethylenediamine 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out about 10 times their normal length, but they return to their original position when the stretching force is withdrawn.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.

Fibres

These are polymers that have quite strong interparticle forces, 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

Dyes are coloured substances, generally organic compounds, used to impart colour 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 colour.
• It must be able to fix itself permanently to the material being dyed.
• It must not change by the action of water, air, dilute acid, bases, detergents, etc.

Classification of Dyes

1. 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

a. Azo dyes

The dyes whose chromophore is azo group (−N=N−) are called Azo Dyes, e.g.,

b. Triphenylmethane 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 oxidising 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 are substances used for the treatment of diseases, for relieving pain, or controlling population (contraceptives).

Drugs are generally classified according to the purpose for which they are used.

1. 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. A 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.

2. Antipyretics

These are the drugs used to lower body temperature during high fevers. e.g., Aspirin, Paracetamol, Phenacetin, etc.

3. 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, heroin, etc.

Non-narcotics are not habit-forming. e.g., Aspirin, Analgin, Brufen, Paracetamol, etc.

4. 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.

5. Antimalarial

These are the drugs used in the treatment of malaria, e.g., Quinine, Chloroquine, etc.

6. 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.

7. Sulpha-drugs

Sulpha drugs constitute a group of synthetic drugs, which, like antibiotics, have antibacterial activities. But these are not derived from microorganisms and are chemically synthesised instead. E.g., sulphanilamide, Sulphadiazine, etc.

8. Tranquillisers 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.

Various plants and animals (insects, weeds, rodents, etc.) cause huge destruction to crops and agricultural products every year. These plants and animals, which are detrimental to the agricultural economy, are called ‘pests‘.

Insecticides

The chemicals used for killing insects are called insecticides. E.g., DDT, DDD, BHC, Methoxychlor, etc. Malathion and parathion are biodegradable insecticides.

Fungicides

The chemicals used for killing algae and fungi are called Fungicides. E.g., copper sulphate, Bordeaux mixture (copper sulphate + lime), etc.

Herbicides (Weedicides)

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 Thallium sulphate, Alphanaphthyl, Thiourea, etc.