Lecture: Comparing the SN1 and SN2 Reactions

Introduction to Nucleophilic Substitution Reactions

• In nucleophilic substitution reactions, a carbon-leaving group (CLG) bond is broken, and a carbon-nucleophile (CNu) bond is formed.
• Two main pathways: SN1 and SN2.
• Key difference: timing of bond-forming and bond-breaking steps.

SN1 Mechanism

• SN1 = Substitution Nucleophilic Unimolecular.
• Occurs in two steps:
  1. Slow, rate-determining step: Breaking of CLG bond to form a carbocation intermediate.
  2. Fast step: Nucleophile adds to carbocation to form product.
• Often a third step involving deprotonation when neutral nucleophiles are used.
• Carbocation stability is crucial; tertiary carbocations are more stable than secondary, which are more stable than primary.

SN2 Mechanism

• SN2 = Substitution Nucleophilic Bimolecular.
• Occurs in a single, concerted step: Backside attack by nucleophile, passing through a five-membered transition state.
• Results in inversion of configuration at the carbon.
• Steric hindrance is the big barrier; reaction is faster with methyl and primary alkyl halides.

Comparing Key Differences Between SN1 and SN2

• Rate Laws
  • SN1: Unimolecular, depends on substrate concentration.
  • SN2: Bimolecular, depends on both nucleophile and substrate concentrations.
• Stereochemistry
  • SN1: Can result in a mixture of retention and inversion.
  • SN2: Results in inversion of configuration.
• Relative Reaction Rates
  • SN1: Tertiary > Secondary > Primary.
  • SN2: Primary > Secondary > Tertiary.
• Solvent Effects
  • SN1 is favored by polar protic solvents.
  • SN2 is favored by polar aprotic solvents.

Mechanisms in Detail

• Reaction Coordinate Diagrams
  • SN1: Two peaks with a carbocation intermediate valley.
  • SN2: Single peak without intermediates.
• Role of Solvents and Nucleophiles
  • SN1: Solvent often acts as nucleophile due to concentration.
  • SN2: Strong, charged nucleophiles are more effective; polar aprotic solvents are preferred.

Practical Applications

• Choosing Between SN1/SN2
  • Consideration of substrate structure and solvent.
  • SN1 favored by stable carbocations, SN2 by low steric hindrance.

Analogies

• Cats and Comfy Chair
  • SN1: Leaving group leaves, then nucleophile attacks.
  • SN2: Nucleophile attacks as the leaving group departs.

Conclusion

• Understanding the mechanisms helps in predicting reaction outcomes and conditions.
• Further reading and quizzes are available for practice and deeper understanding.