Chapter 17: Aldehydes and Ketones. Nucleophilic Addition to C=O

The Baeyer-Villager Reaction

Reaction type:  Oxidation-reduction via Nucleophilic addition

Summary

• Ketones, RCOR', are oxidised by peracids (or hydrogen peroxide) to give esters, RCO₂R'.
• Aldehydes, RCHO, are oxidised under the same conditions to give carboxylic acids, RCO₂H.
• For ketones, it can be viewed as the insertion of O into one of the C-C bonds adjacent to the carbonyl.
• For non-symmetrical systems, there is a selectivity issue to be aware of, and experimental evidence shows that it is usually the more highly substituted alkyl group that migrates and becomes attached to the inserted O atom. : i.e. (more likely) H > 3^o R > 2^o R, Ar > 1^o R > -CH₃ (less likely)
• This is described as the migratory aptitude of the group and relates to the ability of the group to stablise +ve charge (nucleophilicity).

In this example the primary ethyl group migrates in preference to the methyl group so we end up with an ethyl ester.

• Cyclic ketones give cyclic esters which are also known as lactones.

lactone

In this example the O ends up on the more substituted side.

Study Tip: It's easy to make an avoidable mistake here, especially with the cyclic systems and loose a C atom. Count C atoms! In the example above, the 5-membered ring becomes a 6-membered ring because an O atom has been added to the cyclic system.

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 C=O π bond going to the positive O.
Step 3: Electrons from the O come back (this reforms the π 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.

© Dr. Ian Hunt, Department of Chemistry