Question

Write a short note on coupling reactions.

Answer 1

The reaction involves the retention of diazo groups.

1. Arenediazonium salts, when treated with certain reactive aromatic compounds such as phenols or aromatic amines, give azo compounds.
2. These have an extended conjugated system of double bonds in which two aromatic rings are joined through the azo group \[\ce{- N = N -}\]. This reaction is called azo coupling.
3. Azo compounds are brightly coloured and are used as dyes.
4. This is an example of an electrophilic aromatic substitution reaction. Here, the electrophiles are positively charged diazonium ions.
5. Substitution usually occurs para to the ring-activating group.
   e.g. Benzenediazonium chloride reacts with phenol in mild alkaline medium to give p-hydroxyazobenzene (orange dye).
6. Azo coupling with β-naphthol in NaOH is used as a confirmatory test for primary aromatic amines.
7. Benzenediazonium chloride reacts with aniline in a mild alkaline medium to give p-aminoazobenzene (yellow dye).⁣