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. But in a SN2 reaction, 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 center), the mechanism depends on solvent, temperature, concentration of the nucleophile or on the leaving group.
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