12.23:
Oxidations of Aldehydes and Ketones to Carboxylic Acids
Since aldehydes contain an easily abstractable proton, they are more readily oxidized than ketones.
In the case of aldehydes, both weak and strong oxidizing agents can convert them into corresponding carboxylic acids. When the oxidant is chromic acid, a hydrate intermediate is observed.
Aldehydes readily react with atmospheric oxygen. So, stored aldehyde samples undergo autoxidation and often contain traces of acid contaminants.
On the other hand, ketones are more resistant and oxidize only in the presence of strong oxidizing agents at higher temperatures.
For example, nitric acid converts cyclohexanone—via its enol form—to adipic acid, which is used to synthesize polymers.
In functional-group analysis, Tollens reagent—a mild oxidizing agent comprising diaminosilver(I) ions—can selectively oxidize an aldehyde in the presence of other oxidizable groups.
As the aldehyde gets oxidized, silver reduces to its metallic form, which precipitates to give a distinct mirror-like deposit on the surface of the glassware.
12.23:
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.
