Producing Benzoic Acid (Cambridge (CIE) A Level Chemistry): Revision Note

Production of Benzoic Acid

• Benzoic acids are the simplest aromatic carboxylic acids with the molecular formula of C₆H₅COOH

• Benzoic acids and their derivatives are often used as reagents in the synthesis of esters

• The compounds can be produced from the oxidation of alkylbenzenes

Oxidation of alkylbenzenes

• The alkyl side-chain in alkylbenzenes, such as methylbenzene, can be oxidised to a carboxylic acid

• The alkylbenzene is heated under reflux with a solution of hot alkaline KMnO₄ (this is the oxidising agent)

  • The purple colour of the Mn⁷⁺ ions disappears as they are reduced to Mn⁴⁺ ions

  • A brown precipitate of MnO₂ is formed

• The mixture is then acidified with dilute acid (such as hydrochloric acid) to protonate the organic product form and produce a benzoic acid

Making benzoic acid from methylbenzene

Alkylbenzenes such as methylbenzene undergo oxidation to form benzoic acid