Preparation of Carboxylic Acids (Edexcel A Level Chemistry): Revision Note

Preparation of Carboxylic Acids

• Carboxylic acids are compounds with a -COOH functional group

• They can be prepared by a series of different reactions

Oxidation of primary alcohols & aldehydes

• Carboxylic acids can be formed from the oxidation of primary alcohols and aldehydes by either acidified K₂Cr₂O₇ or acidified KMnO₄ and reflux

• The oxidising agents themselves get reduced causing the solutions to change colour

  • In K₂Cr₂O₇ the orange dichromate ions (Cr₂O₇^2-) are reduced to green Cr³⁺ ions

  • In KMnO₄ the purple manganate(VII) ions (MnO₄^-) are reduced to colourless Mn²⁺ ions

Oxidation of primary alcohols (1) and aldehydes (2) gives carboxylic acids

Examples: (1) ethanol, CH₃CH₂OH, is oxidised by [O] to ethanal, CH₃CHO, and then to ethanoic acid, CH₃COOH; (2) propanal, CH₃CH₂CHO, is oxidised by [O] to propanoic acid, CH₃CH₂COOH. [O] = acidified K₂Cr₂O₇ or acidified KMnO₄.

Hydrolysis of nitriles

• Carboxylic acids can also be prepared from the hydrolysis of nitriles using either dilute acid (e.g. hydrochloric acid) or dilute alkali followed by acidification, and heating under reflux

• Hydrolysis by dilute acid results in the formation of a carboxylic acid and ammonium salt, e.g.:

CH₃CH₂CN + HCl + 2H₂O → CH₃CH₂COOH + NH₄Cl

• Hydrolysis by dilute alkali results in the formation of a sodium carboxylate salt and ammonia, e.g.:

  • Acidification is required to change the carboxylate ion into a carboxylic acid

CH₃CH₂CN + NaOH + H₂O → CH₃CH₂COO^-Na⁺ + NH₃

Then acidification gives CH₃CH₂COOH

• The -CN group at the end of the hydrocarbon chain is converted to a -COOH group

Hydrolysis of nitriles by either dilute acid (1) or dilute alkali and acidification (2) will form a carboxylic acid