12.30: Oxidations of Aldehydes and Ketones to Carboxylic Acids
Oxidation of aldehydes and ketones results in the formation of carboxylic acids. Aldehydes, bearing hydrogen next to the carbonyl group, are easily oxidized compared to ketones. This is because an aldehydic proton can easily be abstracted during oxidation.
Aldehydes readily undergo oxidation in strong oxidizing agents such as potassium permanganate and chromic acid. The oxidation can also be carried out using mild oxidizing agents such as silver oxide. In fact, aldehydes can be easily oxidized by atmospheric oxygen. Though stored in airtight containers to delay slow autoxidation, some aldehyde samples are contaminated with traces of carboxylic acid.
Ketones are more resistant and undergo oxidation in the presence of strong oxidizing agents at higher temperatures. The reaction is not as helpful because a mixture of carboxylic acids is obtained as a product.
In the functional-group tests, aldehydes can be differentiated from ketones and other oxidizable functional groups using Tollens’ reagent. Tollens’ reagent is a mixture of silver nitrate in aqueous ammonia to give a diaminosilver(I) ion. It is a weak oxidizing agent and can selectively oxidize aldehydes to carboxylic acid in the presence of other oxidizable functional groups. Simultaneously, the silver ion is reduced to metallic silver. The silver metal is precipitated onto the surface of the reaction vessel to give a mirror-like effect. Hence, this test is popularly known as the silver mirror test or Tollens’ test.