Lecture on SN2, SN1, E2, E1 Mechanisms

Introduction

• Understanding mechanisms: SN2, SN1, E2, E1
• Focus on haloalkanes and their degree of substitution

Mechanism Determinants

Tertiary Haloalkanes

• SN2 Not Possible: Due to steric hindrance (too much electron density repelling nucleophiles)
• SN1, E1, E2 Possible: Stability from hyperconjugation and stable carbocation formation

Primary Haloalkanes

• SN2 Possible: Little steric hindrance, nucleophile can easily approach
• E2 Possible: Also viable
• SN1, E1 Unlikely: Primary carbocation is unstable, exceptions like allylic systems exist

Secondary Haloalkanes

• Limited information from substrate alone
• Other factors (e.g. steric hindrance, branching) play a role

Nucleophile and Leaving Group Strength

• Nucleophilicity Parallels Basicity: Stronger base = stronger nucleophile
• Common Example: Hydroxide vs. Water
  • Hydroxide: Strong nucleophile, capable of SN2
  • Water: Weaker nucleophile, capable of SN1
• Leaving Groups
  • Halides are good leaving groups; stability due to full octet

Solvent Effects

Polar Aprotic Solvents

• Fluoride as Strongest Nucleophile: Due to small ionic radius and localized charge

Polar Protic Solvents

• Iodide as Best Nucleophile: Larger ions spend less time with solvent, more polarizable

Steric Hindrance

• Impact on Mechanism:
  • Small nucleophiles (e.g., hydroxide) can approach substrates easily
  • Sterically hindered bases (e.g., tert-butoxide) favor E2 over SN2

Temperature Effects

• Substitution vs. Elimination:
  • Cold temperatures favor substitution (SN2)
  • Hot temperatures favor elimination (E2)
• Gibbs Free Energy Equation: ΔG = ΔH - TΔS
  • High temperature increases the entropic favorability of elimination

Example Analysis

Primary Haloalkane with Strong Base

• Likely SN2 or E2: Based on substrate and base strength
• Temperature Influence: Hot favors E2, cold favors SN2

Secondary Haloalkane with Weak Base

• Likely SN1 or E1: Weak nucleophile unable to perform SN2 or E2
• Temperature Influence: Cold favors SN1

Tertiary Haloalkane with Strong Base

• E2 Favored: Strong nucleophile, steric hindrance prevents SN2

Tertiary Haloalkane with Weak Base

• Likely E1: Weak base can't perform SN2 or E2, temperature favors elimination

Conclusion

• Analyze substrate, nucleophile strength, temperature to determine mechanism
• Systematic elimination narrows down possible reactions

• Tip: Always consider steric hindrance, nucleophile/basicity strength, and temperature contexts when predicting mechanisms.
• Call to Action: Subscribe and reach out with questions for further clarification.