Consider the following statements about the preparation of phenols.
(A) Chlorobenzene is fused with NaOH at 623 K under high pressure, and the sodium phenoxide so produced is acidified to give phenol.
(B) Benzene is sulphonated with oleum, and the benzenesulphonic acid so formed is converted to sodium phenoxide on heating with molten sodium hydroxide.
(C) A diazonium salt is formed by treating an aromatic tertiary amine with nitrous acid (NaNO2 + HCl) at 273–278 K.
(D) Diazonium salts are hydrolysed to phenols by warming with water or by treating with dilute acids.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A), (B) and (D) only
In the laboratory, phenols are prepared from benzene derivatives. In the first two methods sodium phenoxide is formed first, and the phenoxide is then acidified to give phenol.
(A) Chlorobenzene is fused with NaOH at 623 K under high pressure, and the sodium phenoxide so produced is acidified to give phenol. — True. The chlorine of chlorobenzene is replaced only under these high temperature and pressure conditions:
\( \displaystyle \ce{C6H5Cl + NaOH} \xrightarrow{\text{623 K, high pressure}} \ce{C6H5O^-Na^+} \xrightarrow{\ce{HCl}} \ce{C6H5OH} \)
(B) Benzene is sulphonated with oleum, and the benzenesulphonic acid so formed is converted to sodium phenoxide on heating with molten sodium hydroxide. — True. Acidification of this sodium salt then gives phenol:
\( \displaystyle \ce{C6H6} \xrightarrow{\text{oleum}} \ce{C6H5SO3H} \xrightarrow[\text{(ii) } \ce{H+}]{\text{(i) } \ce{NaOH}} \ce{C6H5OH} \)
(C) A diazonium salt is formed by treating an aromatic tertiary amine with nitrous acid (NaNO2 + HCl) at 273–278 K. — False. The diazonium salt is formed from an aromatic primary amine such as aniline, not from a tertiary amine.
(D) Diazonium salts are hydrolysed to phenols by warming with water or by treating with dilute acids. — True. The diazonium group is replaced by –OH and nitrogen gas is given off:
\( \displaystyle \ce{C6H5N2^+Cl^- + H2O} \xrightarrow{\text{warm}} \ce{C6H5OH + N2 + HCl} \)
NCERT (2026-27 edition):
7.4.2 Preparation of Phenols – “1. From haloarenes
Chlorobenzene is fused with NaOH at 623K and 320 atmospheric pressure. Phenol is obtained by acidification of sodium phenoxide so produced…”
“2. From benzenesulphonic acid
Benzene is sulphonated with oleum and benzene sulphonic acid so formed is converted to sodium phenoxide on heating with molten sodium hydroxide. Acidification of the sodium salt gives phenol.”
“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…”