Preparation of Carboxylic Acids

Carboxylic acids can be made in two main ways.1. Oxidation of primary alcohols and aldehydes
The oxidising agent is acidified potassium dichromate(VI), K₂Cr₂O₇ with dilute sulfuric acid. To oxidise a primary alcohol all the way to the acid, use an excess of the oxidising agent and heat under reflux, so that the aldehyde formed first cannot escape before it is oxidised further. The acid is then separated by distillation. The colour changes from orange to green as Cr₂O₇²⁻ is reduced to Cr³⁺.Using [O] for the oxidising agent:
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
CH₃CHO + [O] → CH₃COOH2. Hydrolysis of nitriles
A nitrile is heated under reflux with a dilute acid, such as hydrochloric acid. The C≡N group is converted into COOH and the nitrogen ends up as ammonium ions:
CH₃CH₂CN + 2H₂O + HCl → CH₃CH₂COOH + NH₄ClThe nitrile carbon becomes the COOH carbon, so the acid has the same number of carbon atoms as the nitrile. This route follows on from reactions that make nitriles, such as the addition of HCN to carbonyl compounds, which add a carbon atom to the chain.Worked example: which alcohol is oxidised to butanoic acid, CH₃CH₂CH₂COOH? The alcohol must have the same carbon skeleton with the OH group on the end carbon: butan-1-ol, CH₃CH₂CH₂CH₂OH. Butan-2-ol is a secondary alcohol and would give a ketone instead.