Reactions of Carboxylic Acids (Cambridge International A-Level Chemistry)

Reactions of Carboxylic Acids to Produce Acyl Chlorides

• 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

Using ethanoic acid to form ethanoyl chloride

Ethanoic acid can be used to produce ethanoyl chloride with different by-products depending on the reagent used

Further Oxidation of 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

Oxidation of ethanedioic acid

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