Aldehydes are easily oxidized to yield carboxylic acids, but ketones are generally inert toward oxidation. The difference is a consequence of structure: aldehydes have a –CHO proton that can be abstracted during oxidation, but ketones do not.

Many oxidizing agents, including alkaline KMnO₄ and hot HNO₃, convert aldehydes into carboxylic acids. The oxidation occurs rapidly at room temperature and generally works well.

Aldehyde oxidations occur through intermediate 1,1-diols, or hydrates, which are formed by a reversible nucleophilic addition of water to the carbonyl group. Even though it’s formed to only a small extent at equilibrium, the hydrate reacts like any typical primary or secondary alcohol and is oxidized to a carbonyl compound (Section 17.7).

Ketones are inert to most oxidizing agents but undergo a slow cleavage reaction of the C–C bond next to the carbonyl group when treated with hot alkaline KMnO₄. The reaction is not often used, however, and is mentioned here only for completeness.