Recall that the rate of an S_N2 reaction depends on the concentration of both the nucleophile and the substrate. While increased nucleophile basicity increases the rate of S_N2 reactions, the increased steric hindrance of the substrate decreases the reaction rate.

This explains why sterically hindered tertiary halides, with bulky alkyl groups, cannot undergo substitution by the S_N2 mechanism, despite the presence of strong nucleophiles.

However, when Sir Christopher Ingold and Edward D. Hughes studied the kinetics of various substitution reactions in aqueous solutions, they noticed that a tertiary halide, like tert-butyl chloride, underwent an alternate substitution mechanism to give tert-butyl alcohol.

In order to investigate the mechanism, the reaction was first performed at a neutral pH with a 10⁻⁷ M hydroxide ion concentration with water as the predominant nucleophile, and later, in a 0.05 M hydroxide solution, where the stronger hydroxide nucleophile was present in surplus amounts.

The results indicated that irrespective of the concentration and nature of the nucleophiles, the rate of product generation stayed constant, suggesting that the nucleophile was not involved in the rate-determining step.

Instead, the reaction rate linearly depended on the concentration of the substrate, indicating that only the substrate participates in the rate-determining step.

As only one chemical entity was involved, the molecularity of this step is said to be unimolecular. Therefore, the substitution reaction is first-order for the alkyl halide, zeroth-order for the nucleophile, and first-order overall.

In essence, reactions that follow this mechanism are classified as Substitution, Nucleophilic, 1^st order, or in short as S_N1 reactions.