Oxidation of Alcohols
Oxidation of Alcohols to Aldehydes or Ketones: Mechanism
Principally, two possibilities have to be considered for the mechanism - oxygenation and dehydrogenation.
Oxygenation implies introduction of oxygen, i.e. all H atoms bound to the C atom of the OH group (α-hydrogens) are successively oxygenated to hydroxyl groups. According to the Erlenmeyer rule, two OH groups attached to one C-atom (hydrates of aldehydes or ketones) are not stable and lose water.
On the other hand, the hydrate of an aldehyde, which is in equilibrium with aldehyde and water, can be further oxidized to carboxylic acid. Therefore, the reaction has to be carried out in non-aqueous media if the oxidation is to be stopped at the aldehyde level.
During dehydrogenation, hydrogen is removed from the alcohol. Often, primary alcohols can be converted into aldehydes by hydrogen acceptors (e.g. palladium) in the absence of oxygen.
During the commonly used oxidation with either chromic acid or chromates in acidic solution, the alcohol is added to Cr(VI) in a nucleophilic addition to form a chromic acid ester. In the second step, the α-hydrogen of the alcohol is presumably transferred to the chromate residue via a cyclic transition state whereby the metal assumes an oxidation level of +IV:
Disproportionation of the generated chromium(IV) species gives Cr(III) or Cr(V) derivatives or it can be reduced further, i.e. it acts as a reducing agent. In the end, Cr(VI) is completely reduced to Cr(III).