Which of the following compound will undergo azo coupling reaction with benzene diazonium chloride?
(A) Aniline
(B) Phenol
(C) p-Toluidine
(D) Nitrobenzene
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A), (B) and (C) only
The benzenediazonium ion, \( \ce{C6H5N2+} \), is a weak electrophile. It attacks only aromatic rings made strongly electron rich by an –OH or –\( \ce{NH2} \) group. The product is an azo compound, \( \ce{Ar-N=N-Ar'} \), and the reaction is called coupling; it normally takes place at the position para to the activating group.
(A) Aniline — Correct. The –\( \ce{NH2} \) group activates the ring; in mildly acidic medium aniline gives yellow p-aminoazobenzene:
\( \displaystyle \ce{C6H5N2+Cl^- + C6H5NH2} \xrightarrow{\ce{H+}} \ce{C6H5-N=N-C6H4-NH2} + \ce{HCl} \)
(B) Phenol — Correct. In mildly alkaline medium phenol gives orange p-hydroxyazobenzene:
\( \displaystyle \ce{C6H5N2+Cl^- + C6H5OH} \xrightarrow{\ce{OH^-}} \ce{C6H5-N=N-C6H4-OH} + \ce{Cl^- + H2O} \)
(C) p-Toluidine — Correct. The ring still carries the strongly activating –\( \ce{NH2} \) group. The para position is occupied by –\( \ce{CH3} \), so the diazonium ion couples at the position ortho to –\( \ce{NH2} \):
\( \displaystyle \ce{C6H5N2+Cl^- + CH3C6H4NH2} \xrightarrow{\ce{H+}} \ce{C6H5-N=N-C6H3(CH3)NH2} + \ce{HCl} \)
(D) Nitrobenzene — Incorrect. The –\( \ce{NO2} \) group withdraws electrons and deactivates the ring, so the weak diazonium electrophile cannot attack it.