10.11 : アルコールの酸化

As with the reduction of carbonyls to alcohols, the outcome of the oxidation of alcohols to carbonyls depends on the number of alpha protons present in the starting alcohol. Here, the oxidation state of the alpha carbon linked to the hydroxyl group is increased.

A primary alcohol has two hydrogen atoms attached to the alpha carbon atom. Hence, the molecule can be oxidized twice: first to an aldehyde, and then to a carboxylic acid. Here, the reaction is aided by Jones reagent: a chromium trioxide solution in aqueous sulfuric acid in the presence of acetone.

The mechanism of the reaction has two steps. In the first step, the alcohol and chromic acid react to form a chromate ester. Subsequently, the chromate ion leaves via an E2 pathway to form the carbon−oxygen pi bond.

The aldehyde is then hydrated and the process is repeated to generate a carboxylic acid. Similarly, the oxidation of a primary alcohol with potassium permanganate ultimately yields a carboxylic acid.

In either case, it is difficult to isolate the intermediate aldehyde. Achieving this requires a more selective reagent like pyridinium chlorochromate, or PCC.

Both Jones reagent and PCC also transform secondary alcohols into ketones. However, as chromium(VI) compounds are highly toxic, oxalyl chloride, DMSO, triethylamine, and dichloromethane are greener alternatives to convert primary alcohols into aldehydes and secondary alcohols into ketones.

Swern oxidation employs oxalyl chloride and DMSO to create a reactive chlorosulfonium species, which first reacts with the alcohol to form an alkylsulfonium compound. In step two, deprotonation with an organic base like triethylamine leads to the oxidized product.

In Dess–Martin oxidation, the alcohol reacts with the hypervalent Dess–Martin periodinane, or DMP, in dichloromethane. This proceeds via a periodinane intermediate to form the corresponding aldehyde or ketone.

However, none of these reagents can oxidize tertiary alcohols, which have no alpha hydrogen atom.