1. From primary alcohols and aldehydes
Primary alcohols are readily oxidised to carboxylic acids with common oxidising agents such as potassium permanganate (KMnO4) in neutral, acidic or alkaline media.
The acid is first obtained as its potassium salt, which on treatment with mineral acid gives the carboxylic acid.
Oxidation with K2Cr2O7 or CrO3 in acidic media often gives some amount of esters. Hence the oxidation by KMnO4 is preferred.Readily available aldehydes can also be used to obtain carboxylic acids, which may be oxidised even by milder oxidising agents as we have seen earlier.
2. From alkyl benzenes and alkenes
Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzenes with chromic acid or acidic or alkaline KMnO4. The entire side chain, irrespective of its length, is oxidised to the carboxyl group.
However if the benzylic carbon is tertiary, oxidation does not occur. Suitably substituted alkenes are also oxidised to carboxylic acids with these oxidising agents.
3. From nitriles
Hydrolysis of nitriles with aqueous acid or alkali give carboxylic acids.
4. From Grignard's reagent
Grignards reagents react with carbon dioxide to form salts of carboxylic acids which give carboxylic acids on acidification with mineral acids.
As Grignard's reagents and nitriles are prepared from alkyl halides, the above methods are useful for converting an alkyl halide into a carboxylic acid having one carbon atom more than that present in the alkyl halide (ascending the series).