1. Dr. Pandit Khakre
Asst. Prof
Mrs. K.S.K. College, Beed

2. Aldol Condensation
In some cases, the adducts obtained from the Aldol Addition can easily be converted (in situ) to α,β-unsaturated carbonyl compounds, either thermally or under acidic or basic catalysis. The formation of the conjugated system is the driving force for this spontaneous dehydration. Under a variety of protocols, the condensation product can be obtained directly without isolation of the aldol.
The aldol condensation is the second step of the Robinson Annulation.
Mechanism of the Aldol Condensation
For the addition step see Aldol Addition

3. Benzoin Condensation
The Benzoin Condensation is a coupling reaction between two aldehydes that allows the preparation of α-hydroxyketones. The first methods were only suitable for the conversion of aromatic aldehydes.
Mechanism of Benzoin Condensation
Addition of the cyanide ion to create a cyanohydrin effects an umpolung of the normal carbonyl charge affinity, and the electrophilic aldehyde carbon becomes nucleophilic after deprotonation: A thiazolium salt may also be used as the catalyst in this reaction (see Stetter Reaction).
A strong base is now able to deprotonate at the former carbonyl C-atom:

4. A second equivalent of aldehyde reacts with this carbanion; elimination of the catalyst regenerates the carbonyl compound at the end of the reaction:

5. Knoevenagel Condensation Doebner Modification
The condensation of carbon acid compounds with aldehydes to afford α,β-unsaturated compounds.
The Doebner Modification, which is possible in the presence of carboxylic acid groups, includes a pyridine-induced decarboxylation.
Mechanism of the Knoevenagel Condensation
An enol intermediate is formed initially:

6. This enol reacts with the aldehyde, and the resulting aldol undergoes subsequent base-induced elimination:
A reasonable variation of the mechanism, in which piperidine acts as organocatalyst, involves the corresponding iminium intermediate as the acceptor:

7. Mechanism of the Baeyer-Villiger Oxidation
The Baeyer-Villiger Oxidation is the oxidative cleavage of a carbon-carbon bond adjacent to a carbonyl, which converts ketones to esters and cyclic ketones to lactones. The Baeyer-Villiger can be carried out with peracids, such as MCBPA, or with hydrogen peroxide and a Lewis acid.
The regiospecificity of the reaction depends on the relative migratory ability of the substituents attached to the carbonyl. Substituents which are able to stabilize a positive charge migrate more readily, so that the order of preference is: tert. alkyl > cyclohexyl > sec. alkyl > phenyl > prim. alkyl > CH3. In some cases, stereoelectronic or ring strain factors also affect the regiochemical outcome.
Mechanism of the Baeyer-Villiger Oxidation