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Organic Chemistry

19.3 Oxidation of Aldehydes and Ketones

Organic Chemistry19.3 Oxidation of Aldehydes and Ketones

19.3 • Oxidation of Aldehydes and Ketones

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.

An aldehyde reacts with nascent oxygen to form a carboxylic acid, while a ketone reacts with nascent oxygen giving no reaction since hydrogen is not attached to the carbonyl group.

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.

Hexanal reacts with chromium trioxide, hydronium ion, and acetone at zero degree Celsius to produce hexanoic acid (85 percent). Hydrogen in aldehyde group gets replaced by hydroxyl in the product.

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).

Aldehyde undergoes a reversible reaction with water to form a hydrate intermediate, which then reacts with chromium trioxide and hydronium ion to produce a carboxylic acid.

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.

Cyclohexanone reacts first with potassium permanganate, water, sodium hydroxide, then with hydronium to form hexanedioic acid (79 percent), a six-carbon chain with carboxylic acid groups at the terminal positions.
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