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6.11: SN1 Reaction: Kinetics

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Organic Chemistry

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SN1 Reaction: Kinetics

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.


SN1 Reaction Kinetics Nucleophile Substrate Hindered Tertiary Alkyl Halide SN2 Mechanism Strong Nucleophile Weak Nucleophile Sir Christopher Ingold Edward D. Hughes Substitution Reaction Rate Of Product Formation Substrate Concentration Nucleophile Concentration Reactivity Tertiary Alkyl Halide Product Generation First-order Reaction Zeroth-order Reaction Rate-determining Step Molecularity Substitution Nucleophilic 1s

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