Write a note on Kolbe reaction.

Answer: In the Kolbe reaction, sodium phenoxide is heated with carbon dioxide at about 398 K under 4–7 atm pressure. The phenoxide ion, being strongly activated, undergoes electrophilic substitution by the weak electrophile CO₂ at the ortho position, forming sodium salicylate; acidification gives salicylic acid (o-hydroxybenzoic acid).

C₆H₅ONa + CO₂ \(\xrightarrow{398\ \text{K},\ 4\text{–}7\ \text{atm}}\) o-HO–C₆H₄–COONa (sodium salicylate)

o-HO–C₆H₄–COONa + HCl → o-HO–C₆H₄–COOH (salicylic acid) + NaCl

Salicylic acid is used to make aspirin.