SN2 Introduction

SN2 Reactions: Bimolecular Nucleophilic Substitution

I. Definition

An SN2 reaction is a bimolecular nucleophilic substitution, meaning the rate of the reaction depends on the concentration of both the nucleophile and the substrate.

II. Key Characteristics

  • One-step, concerted mechanism – The bond to the leaving group breaks as the bond to the nucleophile forms.
  • No intermediate – The reaction proceeds through a single transition state.
  • Rate law: Rate = k[substrate][nucleophile]
  • Inversion of configuration – Known as Walden inversion, the reaction inverts the stereochemistry at the electrophilic carbon center.
  • Favored by:
    • Methyl & primary substrates (less hindered)
    • Polar aprotic solvents (e.g., acetone, DMSO)
    • Strong nucleophiles (e.g., I−, HS−, CN−)