medium · MCAT chem-phys

Why does the rate of S_N2 reactions decrease as the alkyl substrate changes from 1^circ to 2^circ to 3^circ?

1. The nucleophile becomes progressively more easily solvated overall.
2. The carbocation intermediate formed along the pathway becomes less stable.
3. The carbon to leaving-group bond becomes increasingly more tightly bound and shorter.
4. Increasing steric hindrance destabilizes the pentacoordinate transition state.

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