Oxidation of Aldehydes

Summary

• Aldehydes, RCHO, can be oxidized to carboxylic acids, RCO₂H.
• Ketones are not oxidized under these conditions as they lack the critical H for the elimination to occur (see mechanism below).
• The reactive species in the oxidation is the hydrate formed when the aldehyde reacts with the water.
• Typical reagents are aqueous Cr (VI) species:

• Oxidation of Alcohols

OXIDATION OF ALDEHYDES
Part 1: Formation of the hydrate (mechanism) occurs first.
Part 2: Now we essentially have an alcohol which reacts with the chromium species to form a chromate ester.
Part 3: A base (here a water molecule) abstracts a proton from the chromate ester, the C=O forms and a Cr species leaves. This is really an E2 elimination reaction. Note the importance of the original aldehyde H... if its' missing, no oxidation can occur.

The Baeyer-Villager Reaction

Reaction type:  Oxidation -reduction via Nucleophilic addition

Summary

• Ketones, RCOR', can be oxidized by peracids to give esters, RCO₂R'.
• It can be viewed as the insertion of O into one of the C-C bonds adjacent to the carbonyl.
• Cyclic ketones give cyclic esters which are also known as lactones.

lactone

• In non-symmetrical cases, it is the more highly substituted alkyl group that migrates : i.e. 3^o R > 2^o R > 1^o R > -CH3

In this example the primary ethyl group migrates in preference to the methyl group

Related Reactions

• Epoxidation of Alkenes

THE BAEYER-VILLAGER REACTION
Step 1: An acid/base reaction. Protonation of the carbonyl activates it while creating a more reactive nucleophile, the percarboxylate.
Step 2: Now the nucleophilic O attacks the carbonyl C with the electrons from the p bond going to the positive O.
Step 3: Electrons from the O come back (this reforms the p bond of the C=O) and we migrate the C-C electrons to form a new C-O bond displacing the carboxylate as a leaving group.
Step 4: Finally an acid/base reaction reveals the C=O and therefore the ester product.