Preparing carboxylic acids

Primary alcohols are oxidised to carboxylic acids by KMnO₄ in neutral, acidic or alkaline solution, or by K₂Cr₂O₇ or CrO₃ in acid (Jones reagent). Aldehydes give acids even with mild oxidants.

Alkylbenzenes are oxidised vigorously by chromic acid or by acidic or alkaline KMnO₄. The whole side chain, however long, ends up as –COOH; primary and secondary alkyl groups react, but a tertiary group is untouched. Suitably substituted alkenes also give acids.

Nitriles are hydrolysed with H⁺ or OH⁻ first to amides and then to acids; milder conditions stop the reaction at the amide.

Grignard reagents react with carbon dioxide (dry ice) to give carboxylate salts, and mineral acid then releases the acid.

Because nitriles and Grignard reagents are made from alkyl halides, these two routes turn an alkyl halide into an acid with one carbon more (ascending the series).

Acyl chlorides hydrolyse with water, and faster with aqueous base, which gives carboxylate ions that acid converts to the acid. Anhydrides hydrolyse with water.

Esters give the acid directly by acidic hydrolysis; basic hydrolysis gives the carboxylate, which is then acidified.

Worked: 4-methylacetophenone → benzene-1,4-dicarboxylic acid and ethylbenzene → benzoic acid by KMnO₄ side-chain oxidation; cyclohexene → hexane-1,6-dioic acid by KMnO₄/H⁺; bromobenzene → benzoic acid via Mg (Grignard), CO₂, then H₃O⁺.