The E2 mechanism is a bimolecular elimination that occurs in a single concerted step. A base removes a β-hydrogen (a hydrogen on the carbon next to the leaving group) at the same moment as the leaving group departs, and a new carbon–carbon double bond (C=C) forms between the two carbons. Because both base and substrate appear in the rate-determining step, the reaction shows second-order kinetics. It is the elimination counterpart of the SN2 mechanism.

E2 机理是一步协同完成的双分子消除。碱在离去基团离去的同时夺取一个 β-氢(连有离去基团的碳的相邻碳上的氢),并在这两个碳之间形成一个新的碳碳双键(C=C)。由于碱和底物都出现在决速步中,反应呈二级动力学。它是 SN2 机理在消除反应中的对应。