Give short notes on the following :
(a) Kolbe’s reaction
(b) Riemer Tiemann reaction
(c) Coupling reaction

Explanation

(a) Kolbe's reaction:
In this reaction, phenol is first converted into sodium phenoxide which is more reactive than phenol towards electrophilic substitution reaction with `CO_(2)`. Treatment of sodium phenoxide with `CO_(2)` at 400 K, 4-7 bar pressure followed by acid hydrolysis gives salicylic acid.

(ii) Riemer- Tiemann reaction:
On treating phenol with `CHCl_(3)//NaOH`, a-CHO groupis introduced at ortho position. This reaction proceeds through the formation of substituted benzal chloride intermediate.