Which of the following transformation will not give phenol ?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Decarboxylation of benzoic acid followed by hydrolysis
Phenol is made by putting an –OH group directly on the benzene ring. The usual laboratory and industrial routes start from chlorobenzene, benzenesulphonic acid, a diazonium salt or cumene.
Decarboxylation of benzoic acid (heating its sodium salt with soda lime) removes the –COOH group as \( \ce{CO2} \) and leaves benzene, not phenol:
\( \displaystyle \ce{C6H5COONa + NaOH} \xrightarrow{\ce{CaO},\ \text{heat}} \ce{C6H6 + Na2CO3} \)
Benzene has no group that water can replace, so “hydrolysis” afterwards gives nothing. So this transformation does not give phenol.
Option (1) Chlorobenzene is fused with NaOH at 623 K and 320 atmospheric pressure followed by acidification. — Gives phenol. This is the Dow process; sodium phenoxide is formed and acidification releases phenol.
\( \displaystyle \ce{C6H5Cl + NaOH} \xrightarrow{623\ \mathrm{K},\ 320\ \mathrm{atm}} \ce{C6H5ONa} \xrightarrow{\ce{HCl}} \ce{C6H5OH} \)
Option (2) Sulphonation of benzene followed by treatment with NaOH and subsequent acidification. — Gives phenol. Benzenesulphonic acid fused with NaOH gives sodium phenoxide, and acid gives phenol.
\( \displaystyle \ce{C6H6} \xrightarrow{\text{oleum}} \ce{C6H5SO3H} \xrightarrow[\text{(ii) }\ce{H^+}]{\text{(i) }\ce{NaOH}} \ce{C6H5OH} \)
Option (3) Treating benzenediazonium chloride with warm water. — Gives phenol. The diazonium group is replaced by –OH and nitrogen gas escapes.
\( \displaystyle \ce{C6H5N2^+Cl^- + H2O} \xrightarrow{\text{warm}} \ce{C6H5OH + N2 + HCl} \)
Option (4) Decarboxylation of benzoic acid followed by hydrolysis — Does not give phenol. Decarboxylation gives benzene, as shown above.