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