What happens when Aniline (C6H5NH2) is diazotized followed by warming in water?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Phenol is produced
Diazotisation turns an aromatic primary amine into a diazonium salt with nitrous acid (from \( \ce{NaNO2 + HCl} \)) in the cold. Aniline gives benzenediazonium chloride:
\( \displaystyle \ce{C6H5NH2 + NaNO2 + 2HCl} \xrightarrow{273-278\ \mathrm{K}} \ce{C6H5N2^+Cl^- + NaCl + 2H2O} \)
When the solution is warmed with water, the diazonium salt is hydrolysed: the –N2+ group leaves as nitrogen gas and an –OH group takes its place, giving phenol:
\( \displaystyle \ce{C6H5N2^+Cl^- + H2O} \xrightarrow{\text{warm}} \ce{C6H5OH + N2 + HCl} \)
Why the other options are incorrect
Option (1) Benzene is produced — Incorrect. Replacing the diazo group by H needs a mild reducing agent such as hypophosphorous acid or ethanol, not water.
Option (3) Benzene diazonium chloride is produced — Incorrect. This salt is only the intermediate of the cold diazotisation step; on warming with water it is hydrolysed to phenol.
Option (4) Aminophenol is produced — Incorrect. The nitrogen of the original –NH2 group is lost as \( \ce{N2} \) gas, so no amino group remains in the product.
NCERT (2026-27 edition):
7.4.2 Preparation of Phenols – “3. From diazonium salts
A diazonium salt is formed by treating an aromatic primary amine with nitrous acid (NaNO2 + HCl) at 273-278 K. Diazonium salts are hydrolysed to phenols by warming with water or by treating with dilute acids (Unit 9, Class XII).”