a,b-unsaturated carbonyl compounds

The main aldol condensation involves reaction between two aldehydes or ketones of the same structure. But, the procedure can be modified so that the enolate can react with another aldehyde of different structure. The requirement is that the other aldehyde has to be more reactive than the first and it contains no a-hydrogens. Formaldehyde, CH2C=O, and benzaldehyde, PhCH=O, both meet these requirements and
are useful in this procedure called the crossed-aldol condensation. All three of the ?- hydrogens in acetaldehyde can react in a crossed-aldol condensation with formaldehyde


Carbanions are units that contain a negative charge on a carbon atom. The negative charge gives good nucleophilic properties to the unit that can be used in the formation of new carbon carbon bonds. Carbanions thus act as nucleophiles in substitution reactions, in carbonyl addition and substitution reactions, and in 1,4- addition (Michael) reactions.

An aldehyde or ketone that has a hydrogen next to the carbonyl group, an ?-hydrogen, can form an enolate in basic solution, and the enolate can react by nucleophilic addition at the carbonyl group of another molecule. This process is a very important synthetic procedure and is known as the Aldol Condensation. The final product from aliphatic aldehydes or ketones contains both a carbonyl and an alcohol group. The product is called an aldol.

Aromatic ketones bearing ?-hydrogens give aldol reaction products readily, but in this case the aldol product spontaneously loses water to form the unsaturated ketone. When benzaldehyde is used in the crossed-aldol condensation the final product is the unsaturated aldehyde or ketone. Conjugation of the double bond with the aromatic ring is the reason for the spontaneous dehydration.