The E1 mechanism is a unimolecular elimination that occurs in two steps. In the slow first step the leaving group departs to form a carbenium ion; in the second step a β-hydrogen (on the carbon adjacent to the cationic carbon) is lost to give an alkene. Because only the substrate is involved in the rate-determining step, the reaction shows first-order kinetics. It shares its first step with the SN1 mechanism and competes with it.

E1 机理是分两步完成的单分子消除。在缓慢的第一步中,离去基团离去生成碳正离子;在第二步中,失去一个 β-氢(位于带正电碳的相邻碳上)生成烯烃。由于只有底物参与决速步,反应呈一级动力学。它与 SN1 机理共有第一步,并与之相互竞争。