Methods of Preparation of Carboxylic Acids

Carboxylic acids can be reached from several different starting points, and the choice of method in practice usually depends on which starting material is conveniently on hand.

Oxidation of primary alcohols and aldehydes. A primary alcohol, oxidised all the way with a sufficiently strong oxidant such as hot acidified or alkaline $\text{KMnO}_4$ or $\text{K}_2\text{Cr}_2\text{O}_7$, passes first through the aldehyde stage (§8.3) and then continues, under these vigorous conditions, straight through to the carboxylic acid -- unlike the milder, selective reagents (PCC, or hot copper dehydrogenation) used in §8.3 specifically to stop the oxidation at the aldehyde, these stronger classical oxidants do not stop and simply carry the reaction to completion. An aldehyde that has already been isolated can likewise be oxidised directly to the corresponding acid by the same reagents, or, more selectively, by Tollens' or Fehling's reagent as covered in §8.11 (though these are used analytically rather than preparatively, given the small scale and cost of silver/copper reagents).

Side-chain oxidation of an alkylbenzene. An alkyl group attached to a benzene ring, regardless of how long that alkyl chain is, is oxidised by hot alkaline $\text{KMnO}_4$ (followed by acidification) straight through to a single carboxyl group directly on the ring, with every other carbon of the original side chain lost as $\text{CO}_2$ -- so toluene, ethylbenzene, and propylbenzene alike all give exactly the same product, benzoic acid, $\text{C}_6\text{H}_5\text{COOH}$, on this treatment. This is a distinctive and useful method precisely because it is indifferent to the exact length of the starting side chain, provided at least one benzylic hydrogen is present to be attacked.

Hydrolysis of a nitrile. A nitrile, $\text{RCN}$, is hydrolysed by heating with aqueous acid (or, alternatively, aqueous base followed by acidification) to give the carboxylic acid with the same number of carbons, via an amide intermediate: $\text{RCN} \rightarrow \text{RCONH}_2 \rightarrow \text{RCOOH}$. Since a nitrile is itself typically made by $\text{S}_\text{N}2$ displacement of a halide by cyanide, this route effectively extends a carbon chain by one carbon on the way to the acid, but, being an $\text{S}_\text{N}2$ process at the nitrile-forming step, it works poorly (or not at all) when the starting halide is tertiary, due to steric hindrance to backside attack. …