6.11:
SN1 Reaction: Kinetics
Recall that the rate of an SN2 reaction depends on the concentration of both the nucleophile and the substrate. While increased nucleophile basicity increases the rate of SN2 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 SN2 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, 1st order, or in short as SN1 reactions.
6.11:
SN1 Reaction: Kinetics
In an SN2 reaction, the reaction rate depends on both the type of nucleophile and the substrate. A hindered tertiary alkyl halide is practically inert to the SN2 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, 1st order, or in short as an SN1 reaction.