Carboxylic Acids (AQA A Level Chemistry): Revision Note

Carboxylic Acids

Carboxylic acids

• Carboxylic acid is the name given to the family of compounds that contain the carboxyl functional group, -COOH

• The general formula of a carboxylic acid is C_nH_2n+1COOH which can be shortened to just RCOOH

  • (In some countries, this family is also called alkanoic acid)

• The nomenclature of carboxylic acid follows the pattern alkan + oic acid, e.g. propanoic acid

• There is no need to use numbers in the name as the carboxyl group, COOH, is always on the number 1 carbon atom

Carboxylic Acids Examples Table

Weak Acids

• Carboxylic acids with fewer than six carbon atoms per molecule are water-soluble

  • This is because water molecules can hydrogen-bond with the functional group

• In aqueous solution they are only slightly ionised, to give low concentrations of hydronium ions and alkanoate ions (often called carboxylate ions)

Carboxylic acids are weak acids that do not fully dissociate in water, the position of the equilibrium lies to the left

• This partial ionisation in solution means that carboxylic acids are weak acids

• This means that the position of the equilibrium lies to the left and that the concentration of H⁺ is much smaller than the concentration of the carboxylic acid

• However, the concentration of hydrogen ions is sufficient to react with an aqueous solution of sodium carbonate or sodium hydrogen carbonate to produce carbon dioxide

• These reactions are a useful test for the possible presence of a carboxylic acid:

  • Sodium carbonate: 2RCOOH + Na₂CO₃ → 2RCOO^-Na⁺ + CO₂ + H₂O

  • Ionic equation with carbonates: 2RCOOH + CO₃^2- → 2RCOO^- + CO₂ + H₂O

  • Sodium hydrogen carbonate: RCOOH + NaHCO₃ → RCOO^-Na⁺ + CO₂ + H₂O

  • Ionic equation with hydrogen carbonates: RCOOH + HCO₃^- → RCOO^- + CO₂ + H₂O

Preparation of esters

• Esters are formed from the condensation reaction between an alcohol and carboxylic acid in the presence of a concentrated strong acid catalyst e.g. sulfuric or hydrochloric acid

• This reaction is often referred to as esterification

• A condensation reaction involves the elimination of a small molecule not always water

  • Esterification is one example of a condensation reaction as water is eliminated from the acid and alcohol reacting together