DIAZO COUPLING

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Arene diazonium ions are weak electrophiles. They react with highly reactive aromatic compounds i.e., with phenols and 30 amines to produce azo compounds.This electrophilic aromatic substitution reaction is called Diazo coupling reaction.

Examples :

DIAZO COUPLING

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Important points :

Coupling of arene diazonium cation & phenol takes place most rapidly in slightly alkaline solution.

(pH < 10)

Reason : Appreciable amount of phenol is present as phenoxide (more reactive than phenol due to more resonance

stabilisation) under these conditions.

If the solution is too alkaline (pH >10), arene diazonium salt itself reacts with hydroxide ion to form unreactive diazohydroxide or diazotate ion..

OH

HA

−→←

Arenediazonium ion (couples)

Ar N+

N

Ar N

N OH

OH

HA

−→← Ar N

N ODiazohydroxide (does not couple)

Diazotate ion (does not couple)

DIAZO COUPLING

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Coupling of arene diazonium cation and amines takes place more rapidly in slightly acidic (pH = 5 to 7) solution. Reason : 1) Arene diazonium is with max concentration under these conditions 2) Excessive amount of amine must not have converted to unreactive aminium salt

HA

–OH→←

NRR

N+

R

R H

Amine Aminium salt(couples ) (does not couple)

If the pH of solution is < 5, rate of Amine coupling is low.

Coupling with phenols and Aniline derivatives occurs almost exclusively at para position if it is free. If not, coupling occurs at ortho position.