The major product of Kolbe's reaction is used for the synthesis of a precursor substrate in the preparation of aspirin. The electrophile used in Kolbe's reaction is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Carbon dioxide
In Kolbe’s reaction, sodium phenoxide reacts with carbon dioxide, and acidification gives salicylic acid (2-hydroxybenzoic acid):
\( \displaystyle \ce{C6H5OH + NaOH -> C6H5O^-Na^+ + H2O} \)
\( \displaystyle \ce{C6H5O^-Na^+} \xrightarrow[\text{(ii) }\ce{H^+}]{\text{(i) }\ce{CO2}} \text{2-hydroxybenzoic acid} \)
Salicylic acid is the precursor of aspirin: its –OH group is acetylated with acetic anhydride.
\( \displaystyle \text{salicylic acid} + \ce{(CH3CO)2O} \xrightarrow{\ce{H^+}} \text{aspirin} + \ce{CH3COOH} \)
So the electrophile in Kolbe’s reaction is carbon dioxide.
Why the other options are incorrect
Option (2) Carbon monoxide — Incorrect. CO (with HCl and anhydrous \( \ce{AlCl3} \)) is used in the Gattermann–Koch reaction to make benzaldehyde from benzene; it plays no part in Kolbe’s reaction.
Option (3) Acetic anhydride — Incorrect. It is the acetylating agent in the next step, salicylic acid to aspirin, not the electrophile of Kolbe’s reaction.
Option (4) Phenol — Incorrect. Phenol (as phenoxide) is the substrate that is attacked; it supplies the electron-rich ring, not the electrophile.
NCERT (2026-27 edition):
Kolbe’s Reaction
