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Elucidation of Reaction Mechanisms - Investigation of Stereochemistry

Differentiation between SN1 and SN2 Mechanisms

Due to the concerted mechanism with a back-side attack of the nucleophile on the saturated sp3 carbon atom, in the bimolecular nucleophilic substitution (SN2 reaction), an inversion of configuration (Walden inversion) takes place. However, the planar carbenium ion, which appears as an intermediate in the SN1 mechanism, displays no preferred side for the attack by the nucleophile. As a result, a pure SN1 reaction leads to a complete racemization. In investigating the mechanisms of substitutions at asymmetric sp3 carbons, the actual mechanism (SN1 or SN2) of the respective substitution can be determined by analysing the stereochemistry of the product:

  • Complete racemization is the ultimate proof of the occurence of an SN1 mechanism.
  • The occurence of an SN2 mechanism is proved by complete inversion.
SN2 and SN1 mechanism.

Nu = entering nucleophile, NuF = leaving nucleofuge, R1 to R3 = any substituents.

In practice, merely an partial racemization is often observed. This is the result of an SN1 reaction with a noticeable portion of an SN2 mechanism in which the nucleofuge and the carbenium ion are largely, though not completely, dissociated into "free" (solvated) ions before the attack by the nucleophile takes place. Consequently, in contrast to a pure SN1 mechanism, one side of the carbenium ion is, therefore, more easily accessible to the nucleophile than the other side is. Certainly, the occurence of merely partial racemization is important information on the reaction mechanism, as well.

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