Oxidations of Aldehydes and Ketones to Carboxylic Acids

Aldehydes contain an easily abstractable proton adjacent to the carbonyl group, making them more readily oxidized than ketones. This hydrogen is readily removed during oxidation, allowing both weak and strong oxidizing agents to convert aldehydes into carboxylic acids. Ketones lack this reactive proton and are therefore more resistant to oxidation.

Stored aldehyde samples readily undergo autoxidation when exposed to atmospheric oxygen, even in airtight containers. This slow oxidation process produces traces of carboxylic acid contaminants that accumulate over time. Although storage in airtight containers delays this autoxidation, complete prevention is difficult, and samples often contain measurable acid impurities.

When chromic acid acts as the oxidizing agent, a hydrate intermediate forms during aldehyde oxidation. This intermediate facilitates the conversion of the aldehyde to a carboxylic acid. The hydrate formation is a key mechanistic step that distinguishes chromic acid oxidation from other oxidation pathways.

Tollens reagent, containing diaminosilver(I) ions, selectively oxidizes aldehydes in the presence of other oxidizable groups. As the aldehyde oxidizes to a carboxylic acid, silver ions reduce to metallic silver, which precipitates onto the glassware surface as a distinctive mirror-like deposit. This reaction is called the silver mirror test or Tollens test.

Ketones require strong oxidizing agents at elevated temperatures to undergo oxidation. For example, nitric acid converts cyclohexanone via its enol form to adipic acid, a dicarboxylic acid used in polymer synthesis. However, ketone oxidation typically yields a mixture of carboxylic acids rather than a single product.

Tollens reagent is a mild oxidizing agent that selectively oxidizes aldehydes without reacting with other oxidizable functional groups present in the sample. This selectivity allows chemists to differentiate aldehydes from ketones and other reducible compounds in functional-group analysis, making it ideal for identifying aldehydes in complex organic molecules.

Ketone oxidation produces carboxylic acids with industrial applications. Adipic acid, formed from cyclohexanone oxidation, is a key monomer in polymer synthesis and materials science. Although ketone oxidation generates mixed carboxylic acid products, these intermediates remain valuable in organic synthesis and industrial manufacturing.