Hydrolysis of an ester uses water to split it back into a carboxylic acid (or its salt) and an alcohol. It can be carried out with an acid or with an alkali.1. Acid hydrolysis
Heat the ester under reflux with a dilute acid, such as dilute sulfuric acid or hydrochloric acid. The acid is a catalyst. This is the reverse of esterification, so the reaction is reversible and does not go to completion.
CH3COOCH2CH3 + H2O ⇌ CH3COOH + CH3CH2OH
2. Alkaline hydrolysis
Heat the ester under reflux with dilute sodium hydroxide solution. The products are the sodium salt of the carboxylic acid and the alcohol.
CH3COOCH2CH3 + NaOH → CH3COONa + CH3CH2OH
This reaction is not reversible, because the carboxylate ion formed does not react with the alcohol. So alkaline hydrolysis goes to completion and gives a better yield. The alcohol can be distilled off. To obtain the free carboxylic acid, the carboxylate salt is then acidified with a strong acid such as dilute hydrochloric acid.Alkaline hydrolysis of fats and oils (esters of propane-1,2,3-triol) is used to make soaps, which are sodium salts of long-chain carboxylic acids.Worked example: methyl propanoate, CH3CH2COOCH3, is heated under reflux with NaOH(aq). The acid part gives sodium propanoate, CH3CH2COONa, and the alcohol part gives methanol, CH3OH. The ratio is 1 : 1, so 0.0500 mol of ester needs 0.0500 mol of NaOH.
Heat the ester under reflux with a dilute acid, such as dilute sulfuric acid or hydrochloric acid. The acid is a catalyst. This is the reverse of esterification, so the reaction is reversible and does not go to completion.
2. Alkaline hydrolysis
Heat the ester under reflux with dilute sodium hydroxide solution. The products are the sodium salt of the carboxylic acid and the alcohol.
This reaction is not reversible, because the carboxylate ion formed does not react with the alcohol. So alkaline hydrolysis goes to completion and gives a better yield. The alcohol can be distilled off. To obtain the free carboxylic acid, the carboxylate salt is then acidified with a strong acid such as dilute hydrochloric acid.Alkaline hydrolysis of fats and oils (esters of propane-1,2,3-triol) is used to make soaps, which are sodium salts of long-chain carboxylic acids.Worked example: methyl propanoate, CH3CH2COOCH3, is heated under reflux with NaOH(aq). The acid part gives sodium propanoate, CH3CH2COONa, and the alcohol part gives methanol, CH3OH. The ratio is 1 : 1, so 0.0500 mol of ester needs 0.0500 mol of NaOH.