18.2 Esters

An ester forms when an alcohol and carboxylic acid react in a condensation reaction. Concentrated H₂SO₄ catalyses the reversible process and water is eliminated.

Name the acid-derived alkyl and alcohol-derived alkyl parts, then balance the equation. Heating and removal of water can improve the ester yield, but equilibrium remains relevant.

Ethanoic acid + ethanol ⇌ ethyl ethanoate + water. The ester has the linkage CH₃COOCH₂CH₃ and a characteristic fruity smell.

The catalyst is not incorporated into the ester, and esterification is not the same as hydrolysis in reverse conditions.

Dilute acid hydrolysis of an ester is reversible and gives a carboxylic acid plus an alcohol. Dilute alkali hydrolysis gives a carboxylate salt plus an alcohol and is effectively driven to completion.

Heat supplies energy and the reagent determines whether the carboxyl group remains protonated. Acidification after alkaline hydrolysis converts the carboxylate into the free acid.

CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH in acid; with NaOH, products are CH₃COONa + CH₃CH₂OH, followed by acidification if ethanoic acid is required.

Do not write identical products for acid and alkaline hydrolysis, and do not forget reversibility in the acid route.