Illustrated Glossary of Organic Chemistry

Conjugate addition (1,4-addition): Addition to the β carbon of an α, β-unsaturated molecule such as an enone, enal, or 1,3-diene.

This Michael addition reaction is a nucleophilic conjugate addition reaction because lithium dimethyl cuprate (a nucleophile; in red) adds to the β carbon of an enone (an α, β-unsaturated ketone). The reaction can also be called a 1,4-addition because the nucleophile added to position 4 of the α, β-unsaturated system.


1,4-addition product

1,2-addition product
Electrophilic addition of Br2 to cyclopentadiene gives two products: 3,5-dibromocyclopent-1-ene (from 1,4-addition) and 3,4-dibromocyclopent-1-ene (from 1,2-addition).