In organic chemistry, the Michael reaction or Michael 1,4 addition is a reaction between a Michael donor (an enolate or other nucleophile) and a Michael acceptor (usually an α,β-unsaturated carbonyl) to produce a Michael adduct by creating a carbon-carbon bond at the acceptor's β-carbon. It belongs to the larger class of conjugate additions and is widely used for the mild formation of carbon–carbon bonds.

The Michael addition is an important atom-economical method for diastereoselective and enantioselective C–C bond formation, and many asymmetric variants exist

In this general Michael addition scheme, either or both of R and R' on the nucleophile (the Michael donor) represent electron-withdrawing substituents such as acyl, cyano, nitro, or sulfone groups, which make the adjacent methylene hydrogen acidic enough to form a carbanion when reacted with the base, B:. For the alkene (the Michael acceptor), the R" substituent is usually a carbonyl, which makes the compound an α,β-unsaturated carbonyl compound (either an enone or an enal), or R" may be any electron withdrawing group.

As originally defined by Arthur Michael, the reaction is the addition of an enolate of a ketone or aldehyde to an α,β-unsaturated carbonyl compound at the β carbon. The current definition of the Michael reaction has broadened to include nucleophiles other than enolates. Some examples of nucleophiles include doubly stabilized carbon nucleophiles such as beta-ketoesters, malonates, and beta-cyanoesters. The resulting product contains a highly useful 1,5-dioxygenated pattern. Non-carbon nucleophiles such as water, alcohols, amines, and enamines can also react with an α,β-unsaturated carbonyl in a 1,4-addition.

Some authors have broadened the definition of the Michael addition to essentially refer to any 1,4-addition reaction of α,β-unsaturated carbonyl compounds. Others, however, insist that such a usage is an abuse of terminology, and limit the Michael addition to the formation of carbon–carbon bonds through the addition of carbon nucleophiles. The terms oxa-Michael reaction and aza-Michael reaction have been used to refer to the 1,4-addition of oxygen and nitrogen nucleophiles, respectively. The Michael reaction has also been associated with 1,6-addition reactions.

In the reaction mechanism, there is 1 as the nucleophile:

Deprotonation of 1 by a base leads to carbanion 2, stabilized by its electron-withdrawing groups. Structures 2a to 2c are three resonance structures that can be drawn for this species, two of which have enolate ions. This nucleophile reacts with the electrophilic alkene 3 to form 4 in a conjugate addition reaction. Finally, enolate 4 abstracts a proton from protonated base (or solvent) to produce 5.

The reaction is dominated by orbital, rather than electrostatic, considerations. The HOMO of stabilized enolates has a large coefficient on the central carbon atom while the LUMO of many alpha, beta unsaturated carbonyl compounds has a large coefficient on the beta carbon. Thus, both reactants can be considered soft. These polarized frontier orbitals are of similar energy, and react efficiently to form a new carbon–carbon bond.