Reaction Kinetics
The rate of an SN2 reaction is second order, as the rate-determining step depends on the nucleophile concentration, as well as the concentration of substrate, .
- r = k
This is a key difference between the SN1 and SN2 mechanisms. In the SN1 reaction the nucleophile attacks after the rate-limiting step is over, whereas in SN2 the nucleophile forces off the leaving group in the limiting step. In other words, the rate of SN1 reactions depend only on the concentration of the substrate while the SN2 reaction rate depends on the concentration of both the substrate and nucleophile. In cases where both mechanisms are possible (for example at a secondary carbon centre), the mechanism depends on solvent, temperature, concentration of the nucleophile or on the leaving group.
SN2 reactions are generally favored in primary alkyl halides or secondary alkyl halides with an aprotic solvent. They occur at a negligible rate in tertiary alkyl halides due to steric hindrance.
It is important to understand that SN2 and SN1 are two extremes of a sliding scale of reactions, it is possible to find many reactions which exhibit both SN2 and SN1 character in their mechanisms. For instance, it is possible to get a contact ion pairs formed from an alkyl halide in which the ions are not fully separated. When these undergo substitution the stereochemistry will be inverted (as in SN2) for many of the reacting molecules but a few may show retention of configuration. Sn2 reactions are more common than Sn1 reactions.
Read more about this topic: SN2 Reaction
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