Which of the following is not associated with the coupling reaction of benzene diazonium chloride?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Phenol produces yellow dye in coupling reaction.
In a coupling reaction, the benzene diazonium ion acts as a weak electrophile and attacks the para position of a strongly activated ring (phenol or aniline). The two rings are joined by an –N=N– (azo) group. These azo compounds have an extended conjugated system, so they are coloured and used as dyes.
With phenol (in basic medium, where phenol is present as the more reactive phenoxide ion): p-hydroxyazobenzene, an orange dye.
\( \displaystyle \ce{C6H5N2^+Cl^- + C6H5OH + OH^- -> C6H5N=N-C6H4OH + Cl^- + H2O} \)
With aniline (in mildly acidic medium): p-aminoazobenzene, a yellow dye.
\( \displaystyle \ce{C6H5N2^+Cl^- + C6H5NH2} \xrightarrow{\ce{H^+}} \ce{C6H5N=N-C6H4NH2 + HCl} \)
Option (1) Phenol in basic condition undergoes coupling reaction. — Associated with the coupling reaction. In alkali, phenol forms the phenoxide ion, whose ring is strongly activated for attack by the diazonium ion.
Option (2) Aniline in acidic condition undergoes coupling reaction. — Associated with the coupling reaction. Coupling with aniline is done in a mildly acidic medium.
Option (3) Phenol produces yellow dye in coupling reaction. — Not associated with the coupling reaction. Phenol gives p-hydroxyazobenzene, an orange dye, not a yellow one.
Option (4) Aniline produces p-aminoazobenzene. — Associated with the coupling reaction. The diazonium ion attacks the para position of aniline, giving the yellow dye p-aminoazobenzene.
NCERT (2026-27 edition):
Reactions Involving Retention of Diazo Group
