Esters (CIE A Level Chemistry)

• Esters are compounds with an -COOR functional group and are characterised by their sweet and fruity smells
• They are prepared from the condensation reaction between a carboxylic acid and alcohol with concentrated H₂SO₄ as catalyst
  • This is also called esterification

• The first part of the ester’s name comes from the alcohol and the second part of the name comes from the carboxylic acid
  • E.g. Propanol and ethanoic acid will give the ester propyl ethanoate

Esters are formed from the condensation reaction between carboxylic acids and alcohols

• Esters can be hydrolysed to reform the carboxylic acid and alcohol by either dilute acid or dilute alkali and heat
• When an ester is heated under reflux with dilute acid (eg. sulfuric acid) an equilibrium mixture is established as the reaction is reversible

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

• However, heating the ester under reflux with dilute alkali (eg. sodium hydroxide) is an irreversible reaction as the ester is fully hydrolysed
• This results in the formation of a sodium carboxylate salt which needs 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