Conjugated ketones and aldehydes can undergo a reaction which is analogous to the 1,4-addition reaction of dienes, in which a nucleophile adds to the terminal carbon of the double bond. As in dienes, resonance throughout the conjugated system places cationic character on both carbons #1 and 3.

Addition of Amines: Amines add to conjugated ketones and aldehydes to give the conjugate addition product, almost exclusively.

Addition of HCN: HCN, or preferably (CH₃CH₂)2AlCN, will also add to conjugated ketones and aldehydes to exclusively give the conjugate addition product.

Addition of Alkyl and Aryl Groups: Alkyl, aryl and vinyl groups can be added to conjugated ketones using the appropriate dialkylcopperlithium reagent.

The utility of conjugate addition reactions in organic synthesis is demonstrated by the reaction sequence shown below: