Nucleophilic substitution reactions of alkyl halides can proceed via an SN1 or an SN2 mechanism. While in SN2 reactions, the nucleophile attacks the substrate simultaneously as the leaving group departs, in SN1 reactions, the substrate first dissociates to give the carbocation intermediate. Various factors such as the structure of the substrate, the strength of the nucleophile, and the nature of the solvent promote one mechanism over the other.
With increased substitution on the alkyl halide, steric hindrance increases, and more stable carbocations are formed. Thus, with increased alkyl substitution, SN1 reactions are favored over SN2 reactions.
According to the kinetic studies of the rate-limiting step, the nature and concentration of nucleophiles affect only SN2 reaction rates. Thus, strong nucleophiles speed up SN2 reactions, while weak nucleophiles slow down SN2 reactions. As nucleophiles do not participate in the rate-determining step of an SN1 reaction, neither strong nor weak nucleophiles affect the reaction rate.
In SN2 reactions, polar protic solvents cage the nucleophiles through hydrogen bonds, delaying their approach towards the substrate. Polar aprotic solvents, on the contrary, destabilize the nucleophiles, thereby decreasing the activation energy and increasing the reaction rate. In SN1 reactions, polar protic solvents facilitate the departure of the leaving group by stabilizing the ions through solvation.
