Which of the following compounds does not undergo an azo coupling rea…

The correct answer is Option (4) → Nitrobenzene

Azo coupling is an electrophilic substitution: the benzenediazonium ion attacks an electron-rich benzene ring, usually at the para position, and joins the two rings through an –N=N– bond. With phenol:

    \( \displaystyle \ce{C6H5N2^+Cl^- + C6H5OH} \xrightarrow{\ce{OH^-}} \underset{\text{p-hydroxyazobenzene}}{\ce{HO-C6H4-N=N-C6H5}}\ce{ + Cl^- + H2O} \)

Option (1) Aniline — Undergoes azo coupling. The –NH₂ group makes the ring electron-rich; aniline couples to give p-aminoazobenzene.

Option (2) Phenol — Undergoes azo coupling. The –OH group activates the ring; phenol couples at its para position to give p-hydroxyazobenzene.

Option (3) Anisole — Undergoes azo coupling. The –OCH₃ group activates the ring towards electrophilic substitution in the same way as –OH in phenol, so anisole can also couple at the para position.

Option (4) Nitrobenzene — Does not undergo azo coupling. The –NO₂ group is strongly electron withdrawing, so the ring is poor in electrons and the diazonium ion does not attack it.

NCERT (2026-27 edition):
9.9 Chemical Reactions – “… Benzene diazonium chloride reacts with phenol in which the phenol molecule at its para position is coupled with the diazonium salt to form p-hydroxyazobenzene. This type of reaction is known as coupling reaction. Similarly the reaction of diazonium salt with aniline yields p-aminoazobenzene. This is an example of electrophilic substitution reaction.”