Ester Hydrolysis (AQA A-Level Chemistry): Revision Notes

7.3.4 Ester Hydrolysis

Ester Hydrolysis

Ester hydrolysis is the process of breaking down esters into alcohols and carboxylic acids (or their salts) using water in the presence of an acid or an alkali. Hydrolysis of esters can occur under acidic or alkaline conditions, each leading to slightly different products and with different reaction properties.

Overview of Ester Hydrolysis

• Types of Hydrolysis:
  • Acidic Hydrolysis: Uses a dilute acid catalyst and produces a reversible reaction.
  • Alkaline Hydrolysis: Uses a dilute alkali (like $NaOH$) and results in an irreversible reaction.

Acid Hydrolysis of Esters

• Conditions:
  • The ester is heated under reflux with a dilute acid (such as dilute hydrochloric acid, $HCl$, or dilute sulfuric acid, $H₂SO₄$).
  • The reaction with pure water alone is too slow, so an acid catalyst is used to speed up the process.
• Reaction Mechanism:
  • The acid catalyses the addition of water to the ester, resulting in the formation of an alcohol and a carboxylic acid.
  • For example, the hydrolysis of ethyl ethanoate ($CH₃COOCH₂CH₃$) with dilute acid

$${CH₃COOCH₂CH₃} + {H₂O} \rightarrow {CH₃COOH} + {CH₃CH₂OH}$$

Products: Ethanoic acid and ethanol.

• Reversibility:
  • This reaction is reversible. To increase the yield of the hydrolysis products (alcohol and carboxylic acid), an excess of water is added, shifting the equilibrium towards product formation.
• Comparison with Esterification:
  • This process is the reverse of esterification (where a carboxylic acid and an alcohol combine to form an ester in the presence of a concentrated acid).
  • In esterification, removing water favours the formation of the ester, whereas in hydrolysis, adding water shifts the reaction towards the formation of the acid and alcohol.

Alkaline Hydrolysis of Esters (Saponification)

• Conditions:
  • The ester is heated under reflux with a dilute alkali, typically sodium hydroxide ($NaOH$).
  • This reaction is commonly referred to as saponification because it is the basis for soap-making.
• Reaction Mechanism:
  • Instead of reacting with water, the ester reacts with hydroxide ions (OH⁻) from the alkali.
  • This breaks the ester bond, forming an alcohol and a carboxylate salt (the salt of the carboxylic acid).
  • Example: Hydrolysis of ethyl ethanoate with sodium hydroxide:

$${CH₃COOCH₂CH₃} + {NaOH} \rightarrow\ {CH₃COO⁻Na⁺} + {CH₃CH₂OH}$$

Products: Sodium ethanoate (a carboxylate salt) and ethanol.

• Irreversibility:
  • Alkaline hydrolysis is irreversible because the carboxylate salt formed is not prone to recombine with the alcohol to reform the ester. This makes it advantageous in industrial processes, where a complete reaction is desired.
• Separation of Products:
  • The alcohol can be easily separated by distillation.
  • If the carboxylic acid (rather than its salt) is needed, the solution can be acidified by adding a strong acid (like HCl). The acid displaces the carboxylate salt:

$${RCOO⁻Na⁺} + \text{H⁺} \rightarrow \ {RCOOH} + {Na⁺}$$

The resulting carboxylic acid can then be distilled off.

Hydrolysis of Complex Esters and Soap Production

• Hydrolysis of Triglycerides:
  • Large esters, such as triglycerides (fats and oils), can be hydrolyzed in a similar manner. Triglycerides are esters of glycerol (propane-1,2,3-triol) with three fatty acid chains.
  • When triglycerides undergo alkaline hydrolysis with $NaOH$, they form glycerol (an alcohol) and fatty acid salts (carboxylate salts of long-chain fatty acids).
• Saponification Process:
  • The salts formed in this process, such as sodium stearate, are the main components of soap. These salts have both hydrophilic (water-attracting) and hydrophobic (oil-attracting) properties, which enable them to effectively clean by dissolving oils in water.
  • Products of Saponification:
  • Glycerol (useful in cosmetics and as a humectant) and sodium salts of fatty acids (which form soap).