6.12: SN1 Reaction: Mechanism
Kinetic studies of ionization of a tertiary halide in a protic solvent suggest that only the substrate participates in the rate-determining step (slow step). The nucleophile is involved only after the slowest step. The SN1 reaction takes place in a multiple-step mechanism.
Firstly, the haloalkane ionizes to generate a carbocation intermediate and a halide ion. This heterolytic cleavage is highly endothermic with large activation energy. The ionization of the substrate, facilitated by a polar protic solvent, is the slowest of all steps, making it the rate-determining step of an SN1 reaction. The ions formed are stabilized through solvation. In the second step, the reactive carbocation intermediate behaves as a strong electrophile and is attacked by the nucleophilic solvent molecule that quickly donates an electron pair to generate an oxonium ion. This process is exothermic. In the third step, the solvent abstracts a proton from the oxonium ion to yield the final nucleophilic substituted product.
Thus, the SN1 reaction consists of two core steps for substitution and an additional step of proton loss. The mechanism further suggests that several factors such as the stability of the carbocation, the nature of the leaving group, and the nature of the solvent used, favor the SN1 mechanism.