Question

Write the mechanism involved in bromination of salicylic acid.

Answer 1

Hint: Salicylic acid is a carboxylic acid derivative of phenol. It works as an oxidizing as well as electrophile. The reaction on salicylic acid with bromine water is an example of oxidation and electrophilic substitution reaction.

Complete step by step solution:

When salicylic acid reacts with bromine water, then it will give Tribromophenol

Mechanism of bromination of salicylic acid: In salicylic acid, hydroxyl group is an electron releasing group which activates the benzene ring and it increases the electron density at ortho and para positions. Carboxylic group deactivates the benzene ring, it decreases the electron density at ortho and para position but the hydroxyl group is a strongly activated group. Salicylic acid is partially activated towards electrophilic aromatic substitution by hydroxyl group and slightly deactivated by carboxyl group. The following steps are given below:

Initially, we have bromination at the carbon bearing the $ - COOH$ group, and this is followed by decarboxylation to give 

o – bromophenol. Phenols, being a highly activated group, undergo bromination at all three available positions in salicylic acid.

Note: The bromination of salicylic acid in aqueous solution is found to be a second order reaction. Salicylic acid also reacts with bromine in the presence of carbon disulphide then it will give the ortho and para product.