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SN2 - Second-order Nucleophilic Substitution

Summary: SN2 Reaction

Primary and secondary alkyl halides react with nucleophiles in an SN2 mechanism.

  • SN2 reactions are second-order reactions, as their reaction rate depends on both the substrate's and the nucleophile's concentration.
  • The nucleophile attacks the substrate from the opposite side of the leaving group, which is called a back-side attack. The absolute configuration at the reaction center of the substrate is inverted during an SN2 reaction. This inversion is known as the Walden inversion.
  • The higher the reaction center's number of substituents (not hydrogen) is, the lower the reaction rate of the SN2 reaction is, as well. The SN2 reaction is then superseded by the SN1 reaction or elimination more and more.
  • Substituents in the position adjacent to the reaction center reduce the reaction rate.
  • The more effective the nucleophile is, the higher the reaction rate is. Soft bases that do not contain bulky substituents are usually good nucleophiles.
  • Weak bases are often good leaving groups.
  • Protic solvents reduce the reaction rate, as they shield the nuceophile by solvation.
  • Polar aprotic solvents do not reduce the reaction rate.
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