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Question
Write a short note on coupling reactions.
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Solution
The reaction involves the retention of diazo groups.
- Arenediazonium salts, when treated with certain reactive aromatic compounds such as phenols or aromatic amines, give azo compounds.
- 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.
- Azo compounds are brightly coloured and are used as dyes.
- This is an example of an electrophilic aromatic substitution reaction. Here, the electrophiles are positively charged diazonium ions.
- 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).
- Azo coupling with β-naphthol in NaOH is used as a confirmatory test for primary aromatic amines.
- Benzenediazonium chloride reacts with aniline in a mild alkaline medium to give p-aminoazobenzene (yellow dye).
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