alleryThe KOLBE reaction converts phenol (in the form of sodium phenoxide) into salicylic acid. The conditions and key reagent are
- $\mathrm{CO_2}$ under pressure
- $\mathrm{CHCl_3}$ + NaOH
- $\mathrm{Br_2}$ in $\mathrm{CS_2}$
- $\mathrm{H_2SO_4}$ at $443\,\text{K}$
Show answer and solution
Correct answer: A. $\mathrm{CO_2}$ under pressure
1. Your syllabus: KOLBE reaction (also Kolbe-Schmitt) introduces a carboxylic acid group onto the phenoxide.
2. PROCEDURE: sodium phenoxide is heated with $\mathrm{CO_2}$ under PRESSURE (~125°C, ~4 atm), then ACIDIFIED.
3. MECHANISM: the phenoxide ion (activated nucleophile, with negative charge delocalised to ortho/para) attacks CO$_2$ at the ortho position. After workup with acid, the product is ORTHO-hydroxybenzoic acid (SALICYLIC ACID).
4. PRODUCT: salicylic acid is the precursor of ASPIRIN (acetylsalicylic acid) — historically important industrial chemistry.
5. Option B describes the REIMER-TIEMANN reaction (gives aldehyde, not acid). Option C is mild bromination (gives bromophenol). Option D is dehydration of alcohol, unrelated.
_Source: syllabus Chemistry Part 2, Ch 7, §7.7 (Kolbe reaction), p. 18–19._