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SN1 - First-order Nucleophilic Substitution

Summary: SN1 Reactions

SN1 reactions proceed via an intermediate carbenium ion (or carbocation). The reaction rate depends only on the concentration of the substrate and not on the nucleophile's concentration. It is mainly controlled through the variation of the following four parameters: substrate structure, leaving group, nucleophile, and solvent.

  • Substrate structure: Substrates that form a carbenium ion (or carbocation) that is as stable as possible are basically more suitable. Tertiary, allylic, and benzylic alkyl halides therefore react particularly well in SN1 reactions.
  • Leaving group: Good leaving groups (stable anions) increase the reaction rate by lowering the transition state's energy and, thus, by lowering the activation energy.
  • Nucleophile: The nucleophile does not influence the reaction rate of SN1 reactions, as it does not participate in the rate-determining step. However, it should not be too basic. Otherwise, the competing elimination reaction would exceed the substitution.
  • Solvent: Polar, protic solvents stabilize the polar transition state and the ionic intermediate stage of SN1 reactions through solvation. As a result, the activation energy decreases, while the reaction rate increases.
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