Conjugate addition

A) Alkenes adjacent to a carbonyl are conjugated and are therefore electrophilic.

B) These species are called a,b unsaturated carbonyl compounds.

C) a,b unsaturated carbonyl compounds are conjugated, in that the pi electrons of the C=C and C=O bonds can delocalize over all four atoms. This lends some degree of extra stabilization to these species, because pi electrons prefer to delocalize.

D) Nucleophiles can, however, react at the b carbon atom in a process called conjugate addition.

E) Conjugate addition is favorable because the intermediate formed is a resonance stabilized enolate, thus relatively low energy.
F) A key reason that conjugate addition occurs is that the two pi bonding orbitals, the one from the C=C bond and the one from the C=O bond, overlap in space, so electron density is shared between them. Shown above are the two pi bonding orbitals showing how they overlap. Next chapter we will talk a great deal about overlapping pi orbitals and how pi elecctrons prefer to be delocalized over such systems so you can think of the C=C and C=O bonds are really belonging to the same four-atom functional group.