Ester Hydrolysis (AQA A Level Chemistry): Revision Note

Ester Hydrolysis

Hydrolysis of esters under acidic conditions

• Hydrolysis is the reverse of the esterification reaction

• Ester hydrolysis is an important reaction, for example, in the production of biodegradable plastics

• Esters can be hydrolysed to reform a carboxylic acid and an alcohol (or the salt of a carboxylic acid) by heating with either a dilute acid, such as sulfuric acid, or an alkali, such as sodium hydroxide

• When an ester is heated under reflux with dilute acid, an equilibrium mixture is formed

  • This means that the hydrolysis reaction does not go to completion.

Hydrolysis of esters under alkaline conditions

• In contrast, heating an ester under reflux with a dilute alkali, such as sodium hydroxide, is an irreversible reaction.

  • The ester is fully hydrolysed, and the reaction goes to completion

• The carboxylic acid initially formed reacts with the excess alkali to produce a carboxylate salt and an alcohol

• To obtain the carboxylic acid, the sodium carboxylate salt must be acidified.

  • The carboxylate ion (–COO⁻) is protonated by an acid, such as hydrochloric acid (HCl), to form the carboxylic acid (–COOH)

Table showing differences in the hydrolysis of esters

Making Soap

Soaps

• Vegetable oils and animal fats can be hydrolysed in hot alkaline conditions using aqueous sodium hydroxide to form soaps

  • This process is known as saponification

• Soaps are the sodium or potassium salts of long-chain carboxylic acids, which are known as fatty acids

• When triglycerides (fats) are hydrolysed under hot alkaline conditions, the products are glycerol (propane-1,2,3-triol) and the salts of the fatty acids (soaps)