7.5.2 Reactions of Carboxylic Acids

• Acyl chlorides are compounds with the functional group -COCl
• They look similar in structure to carboxylic acids but have a Cl atom instead of an -OH group attached to the carbonyl (C=O)
• Acyl chlorides are more reactive than their corresponding carboxylic acids and are therefore often used as starting materials in the production of organic compounds such as esters
• They can be prepared from the reaction of carboxylic acids with:
  • Solid phosphorus(V) chloride (PCl₅)
  • Liquid phosphorus(III) chloride (PCl₃) and heat
  • Liquid sulfur dichloride oxide (SOCl₂)

• For example, the acyl chloride ethanoyl chloride can be formed from ethanoic acid in the above reactions

Production of acyl chlorides from their corresponding carboxylic acids

• Carboxylic acids can be formed from the oxidation of primary alcohols
• The primary alcohols are firstly oxidised to aldehydes and then further oxidised to carboxylic acids
• Some carboxylic acids can get even further oxidised

Methanoic acid

• Methanoic acid is a strong reducing agent and gets further oxidised to carbon dioxide (CO₂)
• The oxidation of methanoic acid can occur by:
  • Warming methanoic acid with mild oxidising agents such as Fehling’s or Tollens’ reagent
    • In a Fehling’s solution, the Cu²⁺ ion is reduced to Cu⁺ ion which precipitates as red Cu₂O
    • With Tollens’ reagent, the Ag⁺ is reduced to Ag

  • Using stronger oxidising agents such as acidified KMnO₄ or acidified K₂Cr₂O₇
    • The purple KMnO₄ solution turns colourless as Mn⁷⁺ ions are reduced to Mn²⁺ ions
    • The orange K₂Cr₂O₇ solution turns green as the Cr⁶⁺ ions are reduced to Cr³⁺ ions

Ethanedioic acid

• Another carboxylic acid that can get further oxidised is ethanedioic acid
• A strong oxidising agent such as warm acidified KMnO₄ is required for the oxidation of ethanedioic acid to carbon dioxide

Ethanedioic acid is a dicarboxylic acid that can get further oxidised to carbon dioxide