Oxidation of Alcohols

Primary alcohols have two hydrogen atoms on the alpha carbon, allowing oxidation to proceed twice: first to an aldehyde, then to a carboxylic acid. Secondary alcohols have only one alpha hydrogen, so oxidation stops at the ketone stage. The number of alpha protons determines the final oxidation product, making primary and secondary alcohols behave differently under oxidizing conditions.

Jones reagent, a chromium trioxide solution in aqueous sulfuric acid with acetone, oxidizes alcohols through a two-step mechanism. First, the alcohol and chromic acid form a chromate ester intermediate. Then, the chromate ion leaves via an E2 pathway, forming the carbon-oxygen pi bond and producing the carbonyl product.

Jones reagent and potassium permanganate oxidize primary alcohols all the way to carboxylic acids, making aldehyde isolation difficult. Pyridinium chlorochromate (PCC) is a more selective reagent that stops oxidation at the aldehyde stage, allowing successful isolation of the intermediate product without further oxidation.

Swern oxidation and Dess-Martin oxidation employ less toxic reagents like oxalyl chloride, DMSO, triethylamine, and dichloromethane. Swern oxidation creates a reactive chlorosulfonium species that forms an alkylsulfonium intermediate before deprotonation yields the oxidized product. Dess-Martin oxidation uses a periodinane intermediate to form aldehydes or ketones.

Tertiary alcohols lack alpha hydrogen atoms attached to the carbon bearing the hydroxyl group. Since oxidation requires alpha protons to facilitate the formation of a carbon-oxygen double bond, tertiary alcohols cannot undergo oxidation to carbonyls under standard conditions. This structural limitation makes them resistant to all common oxidizing reagents.

Swern oxidation uses oxalyl chloride and DMSO to create a reactive chlorosulfonium species. This species reacts with the alcohol to form an alkylsulfonium compound. In the second step, deprotonation with an organic base like triethylamine generates the oxidized aldehyde or ketone product.

Alcohol oxidation and carbonyl reduction are opposite processes. Just as carbonyls are reduced to alcohols, alcohols are oxidized to carbonyls. The outcome of alcohol oxidation depends on the number of alpha protons, mirroring how carbonyl reduction produces different alcohol types based on the carbonyl structure and starting material.