JEE Chemistry Chemistry In Everyday Life Notes Set 03

Dyes: Colour and Constitution

A dye is a coloured substance that can be fixed to a fabric so that it is fast to light, water, acid and alkali (washing soda and soap are alkaline). Not every coloured compound is a dye: azobenzene is red but is not a dye, while p-aminoazobenzene (aniline yellow) is. Witt's theory says a dye has a chromophore, a group with multiple bonds such as \( \ce{-N=N-} \), \( \ce{-NO_2} \), \( \ce{-N=O} \) or \( \ce{>C=O} \), conjugated through alternating single and double bonds, which absorbs part of white light so that the complementary colour is seen (a dye that looks red absorbs green). An auxochrome such as -OH, \( \ce{-NH_2} \), \( \ce{-NR_2} \), -COOH or \( \ce{-SO_3H} \) is a colour-assisting group: it deepens the colour and, being acidic or basic, helps bind the dye to the fibre. Groups such as -OCH3 and -N(CH3)2 are auxochromes, while \( \ce{>C=N-} \) is a chromophore.

The blue used as laundry blue to remove yellowish tint is ultramarine, an inorganic aluminosilicate, not indigo. Blue prints are made with iron compounds. Red ink uses eosin.

Drugs and Chemotherapy

A drug is a chemical that acts on the body to treat disease; an ideal drug acts at the desired site, is efficient and safe, has minimum side effects and does not lead to resistance. Each drug has an optimum dose, below which it does nothing and above which it is harmful. Chemotherapy, a term of Paul Ehrlich (1913), is the use of chemicals to destroy infecting micro-organisms or cancer cells without harming the host. Main classes:

Aspirin is made by acetylating salicylic acid with acetic anhydride or acetyl chloride; it should not be taken on an empty stomach, and its calcium and sodium salts are more soluble and less harmful. Methyl salicylate (oil of wintergreen, iodex) is an ester used in pain balms. A placebo is an inactive sugar pill given for its psychological effect. Interferon is a protein made by virus-infected cells. Measles is viral, cholera and typhoid are bacterial, and sickle-cell anaemia is a molecular disease caused by one amino-acid change in haemoglobin. Aspartame, about 100 times sweeter than sugar, is the artificial sweetener of soft drinks, saccharin about 300 times; vitamin E is a fat-soluble antioxidant for edible oils, and sodium benzoate is a food preservative.

Pesticides, Soaps and Detergents

Insecticides: DDT, BHC and its gamma isomer lindane (gammexane, 99% gamma-BHC) and methoxychlor are chlorinated; parathion, malathion and baygon are organophosphates or carbamates; 2,4-D is a herbicide (weed killer). Soaps are sodium or potassium salts of long-chain fatty acids, such as \( \ce{C_{17}H_{35}COONa} \), made by saponification of fats, and they form insoluble calcium and magnesium scum in hard water. Synthetic detergents are sodium alkyl sulphates (from lauryl alcohol with \( \ce{H_2SO_4} \) then NaOH) or sodium alkylbenzene sulphonates, \( \ce{C_{12}H_{25}-C_6H_4-SO_3Na} \), whose calcium and magnesium salts are soluble, so they work in hard water. Detergents with a straight, unbranched alkyl chain are biodegradable; branched chains resist bacterial attack and cause foaming in water. Pheromones are chemicals secreted by an animal, often an insect, to influence others of its species.

Rocket Propellants

A propellant is a fuel and oxidiser together, which burns to eject gas at high speed; the thrust follows from Newton's third law. Solid propellants: composite (polyurethane or polybutadiene fuel with ammonium perchlorate oxidiser) as in SLV-3 and PSLV first stage, and double base (nitrocellulose with nitroglycerine). Liquid propellants: biliquid (a liquid fuel plus a liquid oxidiser), such as unsymmetrical dimethylhydrazine (UDMH) or monomethylhydrazine with liquid \( \ce{N_2O_4} \), or liquid hydrogen with liquid oxygen, or kerosene with liquid oxygen. A hybrid propellant has a solid fuel and a liquid oxidiser. Helium is inert and is never a fuel. The specific impulse rises with the square root of the combustion temperature.