10.12: Oxidation of Alcohols
In this lesson, the oxidation of alcohols is discussed in depth. The various reagents used for oxidation of primary and secondary alcohols are detailed, and their mechanism of action is provided.
The process of oxidation in a chemical reaction is observed in any of the three forms:
- (i) loss of one or more electrons,
- (ii) loss of hydrogen,
- (iii) addition of oxygen.
Oxidation is the opposite process of reduction, and hence, as carbonyls are reduced to alcohols, alcohols are oxidized to carbonyls. However, the oxidation of alcohols to carbonyls is dictated by the number of hydrogens present on the α-carbon linked to the hydroxyl group in the starting alcohol. Accordingly, while primary alcohols can be oxidized to aldehydes and further carboxylic acids, the secondary alcohols can only be oxidized to their corresponding ketones. Since there are no α-protons, tertiary alcohols cannot be oxidized. During the oxidation, there is a corresponding increase in the oxidation state of the central species.
Reagents and Mechanism
A popular reagent is the Jones reagent, a chromium trioxide solution in aqueous sulfuric acid in the presence of acetone. The reaction proceeds via an intermediate chromate ester and a subsequent E2 pathway to produce the carbonyl species. However, while the Jones oxidation ends at a ketone for secondary alcohol, the oxidation is repeated for primary alcohol resulting in a carboxylic acid. The other popular reagent used for oxidation is potassium permanganate. Similar to the Jones reagent, potassium permanganate is also a strong oxidizing agent converting the primary alcohol to a carboxylic acid. Hence, when an aldehyde is desired, a selective reagent like pyridinium chlorochromate or PCC should be used.
One major drawback of using these reagents is that they involve the higher oxidation states of chromium that are toxic. Accordingly, greener alternatives like Swern oxidation and Dess-martin oxidation have been designed. They employ reagents such as oxalyl chloride, dimethyl sulfoxide (DMSO), triethylamine, and dichloromethane, which are relatively non-toxic to convert the primary alcohols into aldehydes and secondary alcohols into ketones. In their mechanism, the Swern oxidation advances via an alkyl-sulfonium compound, while the Dess–Martin oxidation proceeds via a periodinane intermediate.