In an S_N2 reaction, the reaction rate depends on both the type of nucleophile and the substrate. A hindered tertiary alkyl halide is practically inert to the S_N2 mechanism despite using a strong nucleophile.

However, Sir Christopher Ingold and Edward D. Hughes, who studied the kinetics of various nucleophilic substitution reactions, noticed that a tertiary alkyl halide does undergo a nucleophilic substitution reaction in the presence of a weak nucleophile. While studying the substitution reactions involving a tertiary halide, they observed that the rate of product formation remained unchanged even upon varying the nucleophile concentration while keeping the substrate concentration constant. Thus, neither the concentration nor the nucleophile's reactivity influenced the substitution rate of a tertiary alkyl halide.

The product generation was independent of the nucleophile’s nature. Instead, the reaction rate was solely dependent on the concentration of the substrate. It was determined that the nucleophilic substitution reactions of tertiary alkyl halides are first-order concerning the substrate while zeroth-order for the nucleophile and thus, first-order overall.

As the reaction rate is independent of the nucleophile concentration, the nucleophile does not participate in the rate-determining step. The molecularity of such reactions is said to be unimolecular. Hence, reactions following this mechanism are classified as Substitution, Nucleophilic, 1^st order, or in short as an S_N1 reaction.