Oxidation
In aldehydes and ketones, the formal oxidation state of the carbonyl carbon is +1 and +2, respectively. In carboxylic acids and their derivatives it is +3, which means that an aldehyde or ketone can still be oxidized, with the appropriate reagent, to a carboxylic acid or a derivative.
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Oxidation of aldehydes
The oxidation of aldehydes is very easy and can be achieved with very mild oxidants such as silver oxide (Ag2O).
Silver oxide is a very mild oxidant that is selective for aldehydes.
The other functions of the molecule are not oxidized by Ag2O.
Of course, stronger oxidants (KMnO4, K2Cr2O7), which do oxidize other functions, also oxidize aldehydes to carboxylic acids.
If a complex molecule contains an aldehyde that must not be oxidized, it has to be protected first.
Oxidation of ketones
The oxidation of ketones necessarily involves breaking a C–C bond. Vigorous oxidation (KMnO4, K2Cr2O7) gives two carboxylic acids; mild oxidation (Baeyer-Villiger oxidation) gives an ester, which on hydrolysis gives a carboxylic acid and an alcohol.
If the ketone is not cyclic, its oxidation gives a mixture of two molecules, which can be a drawback from a preparative point of view. The oxidation of a ketone contained in a ring does not have this drawback.
Baeyer-Villiger oxidation
Peroxycarboxylic acids can oxidize ketones by inserting their "extra" oxygen between the carbonyl carbon and one of the two α carbons.
The reaction is named after Adolf von Baeyer and Victor Villiger, who described it in 1899.
The mild oxidation of unsymmetrical ketones raises the question of which side the oxygen is inserted on.
The reaction is usually highly regioselective: the oxygen is inserted on the side of the more substituted carbon.
The insertion of the oxygen (red) involves the migration of one of the alkyl groups: the more substituted one migrates preferentially.
