Question

Benzenediazonium chloride when treated with phenols gives azo dyes, to get best result the pH of the medium should be

• A. Around 4
• B. Around 8
• C. Around 10
• D. 12

Answer 1

The correct option is B

Around 8

Couplings of arenediazonium cations and phenols take place most rapidly in slightly alkaline medium because

(i) acidic medium (pH < 7) will suppress the ionisation of $C_6{H}_{5}OH$ to the more ractive $C_6{H}_5{O}^{-}$, while the alkaline medium (pH > 7) will enhance ionisation of phenol to phenoxide ion.

(ii)Strongly alkanline medium (pH > 10)causes the arenediazonium salt to react with the $O{H}^{-}$ ion to form a relatively unreactive diazohydroxide or diazoate ion.