The Jones Oxidation allows a relatively inexpensive conversion of secondary alcohols to ketones and of most primary alcohols to carboxylic acids. The oxidation of primary allylic and benzylic alcohols gives aldehydes. Jones described for the first time a conveniently and safe procedure for a chromium (VI)-based oxidation, that paved the way for some further developments such as Collins Reaction and pyridinium dichromate, which also enabled the oxidation of primary alcohols to aldehydes.
Mechanism of the Jones Oxidation
The Jones Reagent is a mixture of chromic trioxide or sodium dichromate in diluted sulfuric acid, which forms chromic acid in situ.
The alcohol and chromic acid form a chromate ester that either reacts intramolecularly or intermolecularly in the presence of a base (water) to yield the corresponding carbonyl compound:
Aldehydes that can form hydrates in the presence of water are further oxidized to carboxylic acids:
Some alcohols such as benzylic and allylic alcohols give aldehydes that do not form hydrates in significant amounts; these can therefore be selectively oxidized with unmodified Jones Reagent to yield aldehydes. Although the reagent is very acidic, the substrate in acetone is essentially titrated with the oxidant solution and only very acid-sensitive groups are incompatible. For example esters, even tert-butyl esters, remain unchanged. The concentration of sulfuric acid can be decreased to minimize side reactions, although the oxidation power increases too.
Disproportionations and single electron transfers lead to chromium (V) acid and stable Cr(III) hydroxide. The chromium (V) acid promotes a two-electron oxidation of an alcohol and becomes Cr(III). Any residues of toxic Cr(V) and Cr(VI) compounds can be destroyed by the addition of an excess of 2-propanol once the intended reaction is complete. The formation of Cr(III) is indicated by a color change to green.
For the synthesis of aldehydes, the Collins Reaction or use of more modern - although more expensive - chromium (VI) reagents such as PCC and PDC can be an appropriate choice. If toxic chromium compounds must be avoided, there are some greener alternative procedures available. For example, molecular oxygen can be used in the presence of palladium as catalyst: synthesis of aldehydes, synthesis of ketones, synthesis of carboxylic acids.
Some newer protocols are available in which a catalytic amount of CrO3 in aqueous solution is used in tandem with a strong stoichiometric oxidant, which is able to reoxidize Cr(IV) but does not interfere with the organic compounds. These are also interesting for the development of greener methods, and examples can be found in the recent literature section.
A full review of chromium-based reagents can be found in the book written by Tojo and Fernández (Oxidation of Alcohols to Aldehydes and Ketones, Springer Berlin, 2006, 1-97.).
A novel chromium trioxide catalyzed oxidation of primary alcohols to the carboxylic acids
M. Zhao, J. Li, Z. Song, R. Desmond, D. M. Tschaen, E. J. J. Grabowski, P. J. Reider, Tetrahedron Lett., 1998, 39, 5323-5326.