The SN1 mechanism is a unimolecular nucleophilic substitution that takes place in two steps. In the slow first step the leaving group departs to form a carbenium ion; in the fast second step the nucleophile attacks that cation. Because only the substrate is involved in the rate-determining step (the slow first step), the rate depends on the substrate alone, giving first-order kinetics. It is favoured by substrates that form stable carbenium ions, such as tertiary ones, in contrast to the SN2 mechanism.

SN1 机理是分两步完成的单分子亲核取代。在缓慢的第一步中,离去基团离去生成碳正离子;在快速的第二步中,亲核试剂进攻该阳离子。由于只有底物参与决速步(即缓慢的第一步),反应速率仅取决于底物,呈一级动力学。它有利于能生成稳定碳正离子的底物,如叔底物,与 SN2 机理形成对照。