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…
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.
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Q1: Why can't tertiary alkyl halides undergo SN2 reactions?
Tertiary alkyl halides have bulky alkyl groups that create significant steric hindrance around the carbon bearing the leaving group. This steric obstruction prevents the nucleophile from accessing the substrate for the backside attack required in SN2 mechanisms, making them practically inert to SN2 substitution despite the presence of strong nucleophiles.
Q2: What does it mean when an SN1 reaction is first-order for the substrate?
A first-order dependence on substrate concentration means the reaction rate increases linearly with substrate concentration. When researchers doubled the substrate concentration, the reaction rate doubled proportionally. This indicates that only one substrate molecule participates in the rate-determining step, making the overall reaction first-order.
Q3: How did Ingold and Hughes determine that the nucleophile doesn't participate in the rate-determining step?
They performed substitution reactions with tertiary halides under two conditions: one with low hydroxide concentration (10⁻7 M) and another with high hydroxide concentration (0.05 M). The product formation rate remained constant regardless of nucleophile concentration or strength, proving the nucleophile was not involved in the rate-determining step.
Q4: What is the molecularity of an SN1 reaction's rate-determining step?
The molecularity of the SN1 rate-determining step is unimolecular because only one chemical entity—the substrate—participates in this step. Since only the alkyl halide is involved, the reaction is zeroth-order for the nucleophile and first-order overall, classifying it as an SN1 (Substitution, Nucleophilic, 1st order) reaction.
Q5: How does SN1 kinetics differ from SN2 kinetics?
SN2 reactions depend on both nucleophile and substrate concentrations, making them second-order overall. SN1 reactions depend only on substrate concentration, making them first-order overall. Additionally, SN1 reactions are zeroth-order for the nucleophile, meaning nucleophile concentration and strength do not affect the reaction rate.
Q6: Why do tertiary halides undergo substitution through the SN1 mechanism instead of SN2?
Tertiary halides cannot use the SN2 mechanism due to steric hindrance, but they can undergo SN1 substitution because this mechanism does not require nucleophilic attack on the substrate. Instead, the substrate spontaneously ionizes in the rate-determining step, forming a carbocation intermediate that is subsequently attacked by the nucleophile.
Q7: What experimental evidence showed that SN1 reactions are independent of nucleophile concentration?
When researchers varied the hydroxide ion concentration from 10⁻7 M to 0.05 M while keeping substrate concentration constant, the rate of tert-butyl alcohol formation remained unchanged. This independence of product formation rate from nucleophile concentration and reactivity directly demonstrated that the nucleophile does not participate in the rate-determining step.