Oxidation using chromic acid

A common method for oxidizing secondary alcohols to ketones uses chromic acid (H₂CrO₄) as the oxidizing agent.  Chromic acid, also known as Jones reagent, is prepared by adding chromium trioxide (CrO₃) to aqueous sulfuric acid.  The Jones oxidation also uses acetone as a co-solvent in the reaction to prevent over-oxidation of the organic product.

A mechanism for the chromic acid oxidation of a ketone is shown below.

Note that the chromium reagent has lost two bonds to oxygen in this reaction, and thus has been reduced (it must have been reduced – it is the oxidizing agent!).

Ketones are not oxidized by chromic acid, so the reaction stops at the ketone stage.  In contrast, primary alcohols are oxidized by chromic acid first to aldehydes, then straight on to carboxylic acids.

It is actually the hydrate form of the aldehyde that is oxidized:

One of the hydroxyl groups of the hydrate attacks chromic acid, and the reaction proceeds essentially as shown for the oxidation of a secondary alcohol.

Under some conditions, chromic acid will even oxidize a carbon in the benzylic position to a carboxylic acid (notice that a carbon-carbon bond is broken in this transformation). 

The related chromium(VI) compound pyridinium chlorochromate (PCC) is also useful for oxidizing primary alcohols to aldehydes. Further oxidation of the aldehyde to the carboxylic acid stage does not occur, because the reaction is carried out in anhydrous (water-free) organic solvents such as dichloromethane, and therefore the hydrate form of the aldehyde is not able to form.

Pyridinium chlorochromate is generated by combining chromium trioxide, hydrochloric acid, and pyridine.

The PCC oxidation conditions can both also be used to oxidize secondary alcohols to ketones.