AQA A Level Chemistry

Revision Notes

7.3.4 Ester Hydrolysis

Ester Hydrolysis

Hydrolysis of Esters – Acid

  • The reverse of the esterification reaction is called hydrolysis
  • Ester hydrolysis is a useful reaction for creating biodegradable plastics
  • Esters can be hydrolysed to reform the carboxylic acid and alcohol or salts of carboxylic acids by using either dilute acid (e.g. sulfuric acid) or alkali (e.g. sodium hydroxide) and heat
  • When an ester is heated under reflux with acid an equilibrium mixture is established, meaning that the hydrolysis reaction is not complete 

Ester hydrolysis by dilute acid is a reversible reaction forming carboxylic acid and alcohol

Hydrolysis of Esters – Alkaline

  • However, heating the ester under reflux with dilute alkali (e.g. sodium hydroxide) is an irreversible reaction as the ester is fully hydrolysed and the reaction goes to completion
  • The carboxylic acid produced reacts with excess alkali to form a carboxylate salt and alcohol
  • The sodium carboxylate salt requires further acidification to turn into a carboxylic acid
    • The sodium carboxylate (-COO) ion needs to get protonated by an acid (such as HCl) to form the carboxylic acid (-COOH)

Ester hydrolysis by dilute alkali is an irreversible reaction forming a sodium carboxylate salt and alcohol

Table showing differences in hydrolysis of esters

 

Worked Example

Name the products and write equations for the following hydrolysis reaction:

  1. Ethyl ethanoate with hot dilute sulfuric acid solution
  2. Methyl propanoate by hot sodium hydroxide solution

Answer:

  1. Ethanoic acid and ethanol

CH3COOCH2CH3 + H2O ⇌ CH3COOH + CH3CH2OH

2. Sodium propanoate and methanol

CH3CH2COOCH3 + NaOH → CH3CH2COONa + CH3OH

Making Soap

Soaps

  • Vegetable oils and animal fats can be hydrolysed in alkaline conditions with aqueous sodium hydroxide to form soaps
  • The process is also called saponification
  • Soaps are carboxylate salts of long-chain carboxylic acids, known as fatty acids
  • When triglycerides / fats are hydrolysed in hot alkaline conditions, the product is a mixture containing glycerol (propane-1,2,3-triol) and the salts of the fatty acids, soaps
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