Esters — common mistakes

A common exam question asks why alkaline hydrolysis is irreversible. The carboxylic acid formed initially is immediately neutralised by the excess alkali to form a carboxylate salt. This salt is stable and does not react with the alcohol to reform the ester, thus driving the reaction to completion.

Yes, esterification is a reversible reaction. It reaches a state of dynamic equilibrium. To maximise the yield of the ester, we use a dehydrating agent (like concentrated sulfuric acid) to remove water, which shifts the equilibrium to the product side according to Le Chatelier's principle.

Heating under reflux allows the reaction to be carried out at the boiling point of the solvent for an extended period without losing volatile reactants or products. The vapours are condensed and returned to the reaction flask, ensuring the reaction can proceed at a high temperature to increase the rate, without boiling dry.

In acid hydrolysis, the products are the original carboxylic acid and alcohol. In alkaline hydrolysis, the products are the salt of the carboxylic acid (a carboxylate) and the alcohol. This is because the carboxylic acid formed reacts with the alkali present.

While it is possible, the esterification of tertiary alcohols with carboxylic acids is very difficult and slow due to steric hindrance around the hydroxyl group. They are more likely to undergo an elimination reaction (dehydration) in the presence of hot, concentrated acid.