In one laboratory method, phenol is prepared from benzene through benzenesulphonic acid. Which treatment converts benzenesulphonic acid into sodium phenoxide in this method?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Heating with molten sodium hydroxide
The method has three steps. Benzene is first sulphonated with oleum. The benzenesulphonic acid formed is then heated with molten sodium hydroxide, which gives sodium phenoxide. Finally, acidification of the sodium salt gives phenol.
\( \displaystyle \ce{C6H6} \xrightarrow{\text{oleum}} \ce{C6H5SO3H} \xrightarrow[\text{(ii) } \ce{H+}]{\text{(i) NaOH (molten)}} \ce{C6H5OH} \)
Why the other options are incorrect
Option (1) Heating with oleum — Incorrect. Oleum is used in the first step, to sulphonate benzene into benzenesulphonic acid. It does not turn the acid into sodium phenoxide.
Option (2) Oxidation with air — Incorrect. Air oxidises cumene to cumene hydroperoxide in the manufacture of phenol from cumene; it plays no part in the sulphonic acid method.
Option (3) Warming with water — Incorrect. Warming with water converts a diazonium salt into phenol; it is the last step of the method that starts from an aromatic primary amine.
NCERT (2026-27 edition):
7.4.2 Preparation of Phenols – “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.”