Benzenediazonium chloride is obtained by treating aniline with nitrous acid (NaNO2 + HCl) at 273–278 K. How is benzenediazonium chloride converted into phenol?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → By warming it with water
Diazonium salts are hydrolysed to phenols by warming with water (or by treating with dilute acids). The –N2+ group is replaced by –OH and nitrogen gas is given off:
\( \displaystyle \ce{C6H5NH2} \xrightarrow[\text{273-278 K}]{\ce{NaNO2 + HCl}} \ce{C6H5N2+Cl-} \xrightarrow[\text{warm}]{\ce{H2O}} \ce{C6H5OH + N2 + HCl} \)
Why the other options are incorrect
Option (2) By heating it with zinc dust — Incorrect. Zinc dust acts on phenol itself and removes its –OH group, giving benzene. It is not used to make phenol.
Option (3) By fusing it with NaOH at 623 K and 320 atm — Incorrect. These are the conditions for making phenol from chlorobenzene: the haloarene is fused with NaOH to give sodium phenoxide, which is then acidified.
Option (4) By treating it with chloroform and aqueous NaOH — Incorrect. Chloroform and NaOH act on phenol in the Reimer-Tiemann reaction and introduce a –CHO group; they do not form phenol.
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 …”