Which compound is formed when $p\text{-nitroaniline}$ is treated with ${\text{NaNO}}_2/\text{HCl}$ at $0-5^{\circ }\text{C}$, followed by reaction with ${\text{C}}_6{\text{H}}_5\text{OH}$ in $\text{NaOH}$?

Solution

1. p-Nitroaniline reacts with ${\text{NaNO}}_2/\text{HCl}$ to form 4-nitrobenzenediazonium chloride.
2. The diazonium cation acts as an electrophile in an azo coupling reaction.
3. Phenol in basic medium exists as a phenoxide ion, which is highly activated for electrophilic aromatic substitution.
4. Coupling occurs at the para position of the phenol to yield 4-nitro-4'-hydroxyazobenzene.
5. Hence the answer is (C).