Micheal Addition

1,2 and 1,4 Addition

The Aldol condensation results in the formation of α,β-unsaturated aldehydes.  These functional groups have two electrophilic carbons as seen from the following resonance structures.

Thus a nucleophile has two places it can react.  In 1,2 addition the nucleophile adds to the carbonyl carbon, while in 1,4 addition the nucleophile adds to the β carbon. 

The competition between 1,2 and 1,4-addition depends on many factors.  One of the most important being the nature of the nucleophile.  If the nucleophile is a strong base, such as Grignard reagents, both the 1,2 and 1,4 reactions are irreversible and therefore are under kinetic control.  The barrier to 1,2-addition is lower than that for 1,4-addition (1,2 reactions are faster than 1,4) therefore the 1,2-products predominate.

If the nucleophile is weakly basic, such as alcohols or amines, then the 1,2 addition is usually reversible and it is under thermodynamic control.  Therefore the more stable resonance stabilized enolate intermediate of 1,4-addition predominates and therefore the 1,4-product.

Nucleophiles that add 1,4-addition

Weakly basic nucleophiles

Water, alcohols, Enolates, 1º and 2º amines, Gilman reagents, cyanide and Grignard reagents with Cu modifiers.

Nucleophiles that add 1,2-addition

Strongly basic nucleophiles

NaBH₄, LiAlH₄, Organolitiums and Grignards.

Micheal Addition

The Micheal addition is the 1,4-addition (sometimes called conjugate addition) of an enolate to an α,β-unsaturated aldehyde or ketone.

Can you propose a mechanism for this?  Google it up and see what you can find.