Aldol Addition and Condensation Reactions

In aldol addition reactions, a carbon-carbon bond is formed between the carbonyl carbon of one aldehyde molecule and the a-carbon of another aldehyde molecule. The product is a (i-hydroxy aldehyde. After the nucleophilic addition of the enolate ion to the carbonyl group, removal of water of the (i-hydroxy aldehyde can occur resulting in an a,p-xmsaturated aldehyde. Study Sample reactions 22-11 and 22-12. 

This a- carbon loses one proton to the base and forms the enolate ion 

The addition product upon heating will undergo elimination (dehydration) to form the a,P-unsaturated aldehyde. 

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The general mechanistic features of the aldol addition and condensation reactions of aldehydes and ketones were discussed in Section 7.7 of Part A, where these general mechanisms can be reviewed. That mechanistic discussion pertains to reactions occurring in hydroxylic solvents and under thermodynamic control. These conditions are useful for the preparation of aldehyde dimers (aldols) and certain a,(3-unsaturated aldehydes and ketones. For example, the mixed condensation of aromatic aldehydes with aliphatic aldehydes and ketones is often done under these conditions. The conjugation in the (3-aryl enones provides a driving force for the elimination step. 

Aldol addition and condensation reactions involving two different carbonyl compounds are called mixed aldol reactions. For these reactions to be useful as a method for synthesis, there must be some basis for controlling which carbonyl component serves as the electrophile and which acts as the enolate precursor. One of the most general mixed aldol condensations involves the use of aromatic aldehydes with alkyl ketones or aldehydes. Aromatic aldehydes are incapable of enolization and cannot function as the nucleophilic component. Furthermore, dehydration is especially favorable because the resulting enone is conjugated with the aromatic ring. 

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