Question

Arrange the following in decreasing ease of acid-catalysed esterification:

Answer 1

During acid-catalysed esterification, the nucleophilic attack of the alcohol $$(R'OH)$$ molecule occurs on the protonated carbonyl group of the acid $$(ArCOOH)$$ to give the tetrahedral intermediate as shown below:
Evidently, as the number of methyl groups at the o-position of the benzene ring increases, the carbonyl group becomes more and more sterically hindered. As a result, the rate of nucleophilic attack by ROH on the carbonyl group decreases and hence the rate of esterification decreases accordingly. Thus, the rate of esterification decreases in the order : benzoic acid $$> 2, 4$$-dimenthylbenzoic acid $$> 2, 4, 6 - $$ trimethyl benzoic acid.