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Both pathways replace a leaving group with a nucleophile, yet differ sharply in mechanism and stereochemistry (Figure 7.9).
In the SN2 mechanism, the nucleophile attacks the electrophilic carbon from the side opposite the leaving group, in a single concerted step. This backside attack inverts the configuration of the stereocenter — Walden inversion.
Because bond-making and bond-breaking occur together, the rate depends on the concentration of both the substrate and the nucleophile, giving second-order kinetics. Bulky groups around the carbon slow the reaction dramatically.
The SN1 mechanism, by contrast, proceeds in two steps through a planar carbocation intermediate, so the nucleophile may attack either face and the product is typically racemic.
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An SN2 reaction proceeds in a single concerted step: as the nucleophile bonds to the carbon, the leaving group departs, and the molecule passes through a five-coordinate transition state.
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