Carboxylic Acids (OCR A Level Chemistry A): Revision Note

Properties of 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

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)

  

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

Reactions of carboxylic acids

• Carboxylic acids can form salts with metals, alkalis and carbonates

• In the reaction with metals, a salt and hydrogen gas are produced

  • For example, in reaction with magnesium:

2CH₃COOH (aq) + Mg (s) → (CH₃COO)₂Mg (aq) + H₂ (g)

• In the reaction with metal oxides, a metal salt and water are produced

  • For example in reaction with magnesium the salt magnesium ethanoate is formed:

2CH₃COOH (aq) + MgO (s) → (CH₃COO)₂Mg (aq) + H₂O (l) 

• In the reaction with alkalis a salt and water are formed in a neutralisation reaction

  • For example in reaction with potassium hydroxide the salt potassium ethanoate is formed:

CH₃COOH (aq) + KOH (aq) → CH₃COOK (aq) + H₂O (l)

• In the reaction with carbonates a metal salt, water and carbon dioxide gas are produced

  • For example in reaction with potassium carbonate the salt potassium ethanoate is formed:

2CH₃COOH (aq) + K₂CO₃ (s) → 2CH₃COOK (aq) + H₂O (l) + CO₂ (g)