Esters can be hydrolysed in acid or alkaline conditions to form alcohols…

The idea

Both routes cut the ester at the same place, between the carbonyl carbon and the single-bonded oxygen, but they end in different places. Heating under reflux with dilute sulfuric acid is reversible: water attacks the δ⁺ carbonyl carbon, and you reach an equilibrium containing the alcohol, the carboxylic acid and unreacted ester, so a large excess of water is used to push it over. Heating under reflux with aqueous sodium hydroxide goes to completion, because the product is the carboxylate ion, which carries a negative charge and is not attacked by the alcohol, so the reverse reaction cannot happen. The alcohol is the same either way; the acid is obtained from the salt afterwards by adding a strong acid such as dilute hydrochloric acid.

What earns the marks

• Naming acid hydrolysis as reversible and alkaline hydrolysis as going to completion.
• The alcohol and the carboxylic acid as the products of acid hydrolysis.
• The alcohol and the carboxylate salt as the products of alkaline hydrolysis.
• Naming the reaction type shown in a given diagram as alkaline or base hydrolysis.
• Stating that heating under reflux is needed in both cases.

Common mistakes and how to put them right

• Giving the free carboxylic acid as the product of hydrolysis with sodium hydroxide.

• Writing the alcohol as a sodium salt rather than the carboxylate.

• Omitting reflux, so the volatile product would escape before the reaction finished.

• Saying acid hydrolysis goes to completion because an acid catalyst speeds it up.

Exam tips

• If the question gives NaOH(aq), name the product as the sodium carboxylate and say how you would recover the acid.
• Cut the ester mentally at the C-O single bond: the OH from water joins the carbonyl carbon and the H joins the oxygen.