SN1, SN2, E1, E2 Reactions

SN2 Reactions

• Methyl Substrates:
  • Example: Methyl bromide
  • Always proceeds via SN2 mechanism
  • Independent of solvent or base
• Primary Substrates:
  • Example: Ethyl bromide
  • Proceed via SN2 unless a bulky base is used
    • Bulky base results in E2 mechanism
• Strong Unhindered Bases:
  • Example: Hydroxide, methoxide, ethoxide
  • Favor SN2 unless the substrate is sterically hindered

E2 Reactions

• Bulky Bases:
  • Example: Tert-butoxide
  • Prefer E2, especially with primary or secondary sterically hindered substrates
• Substrate Steric Hindrance:
  • Hindered substrates favor E2 over SN2

SN1 and E1 Reactions

• Protic Solvents:
  • Examples: Water, methanol, ethanol
  • Favor SN1 and E1 over SN2
• Tertiary Alkyl Halides:
  • Do not proceed via SN2 due to steric hindrance
  • Favor SN1 and E1
  • Heat can enhance E1 over SN1

Factors Influencing Reaction Mechanism

• Solvent Type:
  • Aprotic solvents favor SN2
  • Protic solvents favor SN1/E1
• Base Strength:
  • Strong bases can lead to E2
  • Bulky bases favor E2
• Nucleophile Quality:
  • Good nucleophiles (negatively charged) can drive SN2

Example Reactions

Example 1: 2-Bromo Butane + Sodium Cyanide in Acetone

• Mechanism: SN2
• Product: Inversion of configuration (R to S isomer)

Example 2: Tert-Butyl Chloride + Water

• Mechanism: SN1/E1
• Products:
  • SN1: Alcohol formation
  • E1: Alkene formation

Example 3: 2-Bromo 3-Methylbutane + Sodium Methoxide

• Mechanism: E2 (major), SN2 (minor)
• Products:
  • E2: Zaitsev product (more stable alkene)
  • SN2: Minor substitution product

Special Cases

• Methyl Bromide + Tert-Butoxide:
  • Only SN2 occurs (100% yield), cannot form double bond
• 2-Bromo Pentane + Methoxide:
  • SN2 predominates with unhindered base
• 2-Fluoro Pentane + Methoxide:
  • Hoffmann product due to fluoride being a poor leaving group
• Sterically Hindered Primary Halides:
  • Favor E2 with either bulky base or sterically hindered substrate

Reaction Mechanism Considerations

• Secondary Alkyl Halides:
  • Can proceed via SN1 or SN2 depending on steric factors and solvent
• Sterically Hindered Alkyl Halides:
  • Often lead to E2 or SN1, not SN2

Conclusion

• Reaction pathways in SN1, SN2, E1, and E2 are influenced by substrate type, solvent, nucleophile/base strength, and steric hindrance.
• Understanding these factors is crucial for predicting the major mechanism and product in organic reactions.