Hydrolysis of Esters (Oxford AQA International A Level Chemistry)

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 

Acid hydrolysis of esters

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)

Alkaline hydrolysis of esters

Table showing differences in hydrolysis of esters

Making Soap

• 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

Biodiesel

• Biodiesel is made from renewable vegetable oils rather than non- sustainable petrochemicals

• Natural triglyceride oils are converted to esters of methanol, which makes them less viscous

• Biodiesel made from rapeseed oil for example is produced by transesterification

  • The triglyceride is converted, or transesterified, into the less viscous fatty acid methyl ester (FAME) using methanol

  • Acids and alkalis can both be used to catalyse the reaction

    • An acid works by protonating the carbonyl group

    • An alkali works by deprotonating the alcohol / methanol

    • However, it is more common to use an alkaline catalyst such as NaOH / KOH

  • The transesterification is reversible, so an excess of methanol is used to drive the equilibrium to the right

  • Under optimum conditions this process can produce a very successful yield of 98%

Transesterification forming methyl esters