Lecture Notes: SN1 vs SN2 Reactions

Importance of SN1 and SN2 Mechanisms

• Predicting whether SN1 or SN2 will predominate helps:
  • Determine product stereochemistry
  • Predict possible rearrangements
• Predominance indicates which mechanism occurs more quickly, leading to major products.

Reaction Rate Comparison

• SN1 Reaction
  • Rate-determining step is unimolecular (involves only the substrate).
  • Rate unaffected by nucleophile structure.
• SN2 Reaction
  • Rate-determining step is bimolecular (involves substrate and nucleophile).
  • Nucleophile structure affects the rate.

Effect of Nucleophile on Rate

• SN1 Reaction
  • Nucleophile has no effect on the rate.
• SN2 Reaction
  • Strong nucleophile: speeds up the reaction, promotes SN2.
  • Weak nucleophile: slows down the reaction, favoring SN1 if the substrate permits.

Substrate Influence on Reaction Rate

• Methyl and Primary Substrates
  • Poor for SN1 due to unstable carbocation formation.
  • Good for SN2 due to minimal steric hindrance.
• Tertiary Substrates
  • Poor for SN2 due to steric hindrance.
  • Good for SN1 due to stable carbocation formation.
• Secondary Substrates
  • Can proceed via SN1 or SN2 based on other factors like solvent and nucleophile.

Solvent Effects on Reaction Rates

• SN1 Mechanism
  • Benefits from polar protic solvents due to stabilization of charged intermediates.
• SN2 Mechanism
  • Benefits from polar aprotic solvents due to stronger nucleophile and better dissolution of reactants.

Summary

• Understanding the structure of the nucleophile, substrate, and solvent allows prediction of whether the SN1 or SN2 mechanism will predominate.
• These predictions help determine the structure, stereochemistry, and potential rearrangements of the reaction products.