Esterification of carboxylic acids with alcohols is a nucleophilic acyl substitution.
In such reactions the nucleophile first adds to a carbonyl group to give a tetrahedral intermediate which then readily loses the leaving group to give the substitution products. The mechanism involves the following steps.
Step 1: Protonation of the carbonyl group:
In the presence of mineral acids (H2SO4 or HCl), the carbonyl group of the carboxylic acid accepts a proton to form protonated carboxylic acid.
Step 2: Nucleophilic attack of the alcohol molecule. As a result of protonation, the carbonyl carbon becomes electrophilic (i.e. more electropositive) and hence readily undergoes nucleophilic attack by lone pairs of electrons on the oxygen atom of the alcohol molecule to form a tetrahedral intermediate (II).
Step 3: Loss of a molecule of water and a proton. The tetrahedral intermediate (II) undergoes a proton transfer to give another tetrahedral intermediate (III). During this proton transfer, the OH group gets converted to – OH2 group which being a good leaving group is lost as a neutral water molecule. The protonation ester (IV), thus formed, finally loses a proton to give the ester (V).
