Question

Synthetic detergents have advantage over usual soaps as far as cleansing power is concerned. But use of synthetic detergents over a long time creates environmental pollution. How can the pollution caused by synthetic detergents be minimised? Classify the detergents according to their chemical nature.

Answer 1

Synthetic detergents are cleansing agents which have all the properties of soaps, but which actually do not contain any soap. These can be used both in soft and hard water as they given foam even in hard water. Detergents can be classified into three groups according their chemical nature.

(i) Anionic Detergents: These are sodium salts of sulphonated long-chain alcohols or hydrocarbons. Alkyl hydrogen sulphates formed by treating long-chain alcohols with concentrated sulphuric acid are neutralised with alkali to form anionic detergents. Similarly alkyl benzene sulphonates are obtained by neutralising alkyl benzene sulphonic acids with alkali.

\[\ce{\underset{Lauryl alcohol}{CH3(CH2)10CH2OH} ->[H2SO4] \underset{Lauryl hydrogensulphate}{CH3(CH2)10CH2OSOH} ->[NaOH(aq)] \underset{(Anionic detergent)}{\underset{Sodium laurylsulphate}{CH3(CH2)10CH2OS\overset{_}{O}3\overset{+}{N}a}}}\]

In anionic detergents, the anionic part of the molecule is involved in the cleansing action.

(ii) Cationic detergents: These are quaternary ammonium salts of amines with acetates, chlorides or bromides as anions.

Cetyltrimethyl ammonium bromide

(iii) Non-ionic detergents: These do not contain any ion in their constitution, e.g., detergent formed by steric acid and polyethylene glycol.

\[\ce{\underset{Stearic acid}{CH3(CH2)16COOH} + \underset{Polyethleneglycol}{HO(CH2CH2O)nCH2CH2OH} ->[-H2O] CH3(CH2)16COO(CH2CH2O)nCH2CH2OH}\]

Pollution by synthetic detergents can be minimized by reducing the branching of hydrocarbon chain or using unbranched hydrocarbons.