An organic compound A, molecular formula C6H6O, gives a violet colour with neutral FeCl3 solution. A on treatment with CO2 and NaOH at 400 K under pressure gives B, which on acidification gives a compound C. Compound C reacts with acetyl chloride to give D, which is a popular painkiller.
In the reaction sequence given in the passage, which reaction converts compound (A) into compound (B)?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Kolbe's reaction
(A), \( \ce{C6H6O} \), is phenol. The reaction of phenol with \( \ce{CO2} \) and NaOH at 410 K under pressure is Kolbe’s reaction: NaOH gives the phenoxide ion, which takes up \( \ce{CO2} \) at the ortho position, and (B) is sodium salicylate:
\( \displaystyle \ce{C6H5OH + NaOH -> C6H5ONa + H2O} \)
\( \displaystyle \ce{C6H5ONa + CO2} \xrightarrow[\text{pressure}]{410\,\mathrm{K}} \ce{HOC6H4COONa} \)
Why the other options are incorrect
Option (1) Williamson synthesis — Incorrect. It is a method of preparing ethers: an alkyl halide reacts with a sodium alkoxide. No \( \ce{CO2} \) is used and no acid is formed.
Option (3) Reimer-Tiemann reaction — Incorrect. Phenol is treated with chloroform and NaOH, and a –CHO group enters the ortho position to give salicylaldehyde, not a –COOH group.
Option (4) Wurtz reaction — Incorrect. Alkyl halides react with sodium in dry ether to give hydrocarbons with double the number of carbon atoms; phenol and \( \ce{CO2} \) play no part.
NCERT (2026-27 edition):
2. Kolbe’s reaction – “Phenoxide ion generated by treating phenol with sodium hydroxide is even more reactive than phenol towards electrophilic aromatic substitution. Hence, it undergoes electrophilic substitution with carbon dioxide, a weak electrophile. Ortho hydroxybenzoic acid is formed as the main reaction product.”