Oxidation
The carboxyl carbon of carboxylic acids and their derivatives is in the +3 formal oxidation state, the highest possible in an organic functional group.
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Carboxylic acids can therefore be obtained by oxidation of almost any other functional group, provided the appropriate oxidant and reaction pathway exist.
Oxidation of alkanes
Alkanes are difficult to oxidize selectively. Nevertheless, the benzylic position is easily oxidized.
Strong oxidation of alkylbenzenes gives benzoic acids. Oxidation thus occurs at the benzylic position.
Oxidation of alkenes and alkynes
Alkenes can be oxidized to carboxylic acids, with cleavage of the C=C double bond. Alkynes can too, but the reaction is not very useful.
Oxidation of alcohols
Primary alcohols are the only ones that can be oxidized to carboxylic acids without C–C bond cleavage. Secondary and tertiary alcohols can be oxidized under very harsh conditions, but with C–C bond cleavage, which makes the reaction useless.
Oxidation of aldehydes
Aldehyde oxidation is fairly easy and can be achieved with mild oxidants such as silver oxide (Ag2O).
Oxidation of ketones
Ketone oxidation necessarily involves C–C bond cleavage. If it is strong (KMnO4, K2Cr2O7), two carboxylic acids are obtained; if it is mild (Baeyer-Villiger oxidation), an ester is produced that can be hydrolysed to an alcohol and a carboxylic acid (in a cyclic ketone, such as the one in the figure, both end up in the same molecule: a hydroxy acid).
