Nucleophilic substitution reactions of alkyl halides can proceed via an S_N1 or an S_N2 mechanism. While in S_N2 reactions, the nucleophile attacks the substrate simultaneously as the leaving group departs, in S_N1 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, S_N1 reactions are favored over S_N2 reactions.

According to the kinetic studies of the rate-limiting step, the nature and concentration of nucleophiles affect only S_N2 reaction rates. Thus, strong nucleophiles speed up S_N2 reactions, while weak nucleophiles slow down S_N2 reactions. As nucleophiles do not participate in the rate-determining step of an S_N1 reaction, neither strong nor weak nucleophiles affect the reaction rate.

In S_N2 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 S_N1 reactions, polar protic solvents facilitate the departure of the leaving group by stabilizing the ions through solvation.