The SN2 mechanism is a bimolecular nucleophilic substitution that takes place in a single concerted step. Because both the nucleophile and the substrate appear in the rate-determining step (the only step), the rate depends on the concentrations of both, giving second-order kinetics. The nucleophile attacks the carbon from the side opposite the leaving group, so as the leaving group departs the carbon's configuration is inverted. It is favoured by unhindered (primary) substrates, in contrast to the SN1 mechanism.

SN2 机理是一步协同完成的双分子亲核取代。由于亲核试剂和底物都出现在决速步(也是唯一一步)中,反应速率同时取决于二者的浓度,呈二级动力学。亲核试剂从离去基团的相反一侧进攻碳,因此随着离去基团离去,该碳的构型发生翻转。它有利于位阻小的(伯)底物,与 SN1 机理形成对照。