Overview of SN1, SN2, E1, and E2 Reactions

Oct 25, 2024

Lecture Notes on SN1, SN2, E1, and E2 Reactions

Introduction

  • Discussion of SN1, SN2, E1, and E2 reactions and their mechanisms.
  • Focus on how substrate structure (methyl, primary, secondary, tertiary) and nucleophile/base influence the reaction pathway.

SN2 Reactions

  • Methyl Substrate: Always proceeds via SN2, regardless of solvent or base used.
  • Primary Substrate: Generally SN2, unless a bulky base is used (E2 dominant).
  • SN2 reactions favor non-sterically hindered substrates.

E2 Reactions

  • Primary Substrate with Bulky Base: Results in E2 reaction.
  • Secondary Alkyl Halide: Can be both SN2 and E2. With strong bases, E2 is typically major.
  • Bulky or sterically hindered secondary substrates favor E2.

SN1 and E1 Reactions

  • Tertiary Substrate: No SN2 due to steric hindrance; SN1 and E1 are favored.
  • Protic Solvents: Favor SN1 and E1, especially for tertiary.
  • Heat favors E1 over SN1.

Reaction Examples

  • 2-bromobutane with sodium cyanide in acetone: Secondary alkyl halide, proceeds by SN2 due to good nucleophile.
  • Tert-butyl chloride with water: SN1 and E1 products formed due to tertiary substrate and protic solvent.

Factors Affecting Mechanisms

  • Substrate Steric Hindrance: Hinders SN2, favors E2 or rearrangement in SN1.
  • Nucleophile Strength: Strong nucleophiles favor SN2.
  • Solvent Type: Aprotic solvents favor SN2, protic solvents favor SN1/E1.

Detailed Reaction Mechanisms

  • Example: 2-bromo-3-methylbutane with sodium methoxide in methanol: E2 is major product due to strong base; SN2 is minor.

  • Comparison of Alkyl Halides with Different Leaving Groups: Fluorides favor Hoffman products due to poor leaving group nature.

Special Cases

  • Sterically Hindered Primary Substrates: Can undergo SN1 with methyl shifts to avoid unstable carbocations.

Conclusion

  • General Guidelines: Always consider steric effects, nucleophile/base strength, and solvent type when predicting reaction pathways.
  • Exceptions and Variability: Acknowledge that guidelines have exceptions and each reaction must be considered individually.