Acid & Alkaline Hydrolysis of Esters (Edexcel IAL Chemistry)

Spec point — 15.15

Acid & Alkaline Hydrolysis of Esters

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 ester directly reacts with the alkali (NaOH) to produce to form a** carboxylate salt** and alcohol in an irreversible step

• 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:

Answer 1: Ethanoic acid and ethanol

CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH

Answer 2: Sodium propanoate and methanol

CH₃CH₂COOCH₃ + NaOH → CH₃CH₂COONa + CH₃OH