SN2 Reaction Lecture Notes

Overview of SN2 Reaction

• SN2 stands for second-order nucleophilic substitution.
• Substitution Reaction: Incoming nucleophile replaces the leaving group on the substrate.
• Nucleophilic: Involves a nucleophile.
• Second-Order: Both nucleophile and substrate are involved in the rate-determining step.

General Reaction Mechanism

• Components:
  • Nucleophile: Electron donor.
  • Substrate (Electrophile): Often an alkyl halide.
  • Leaving Group: Halide, which exits with bonding electrons, becoming negatively charged.
• Reaction Process:
  • Nucleophile attacks the most electrophilic atom of the substrate (carbon attached to halide).
  • Electrophilic carbon has a partial positive charge due to bond dipole with halogen.
  • Formation of a new bond between nucleophile and carbon.
  • Breaking of the bond between carbon and leaving group.

Transition State

• Highest energy point between reactants and products.
• Bond between nucleophile and carbon forms while bond between carbon and leaving group breaks.

Example SN2 Reaction

• Reactants:
  • Nucleophile: Hydroxide ion.
  • Substrate/Electrophile: Iodomethane.
• Products:
  • Methanol (new bond formed).
  • Iodide (leaving group).
• Mechanism:
  • Nucleophile attacks carbon of electrophile.
  • Iodide leaving group is displaced.
  • Transition state involves hydroxide-carbon bond forming and carbon-iodine bond breaking.
• Electron Movement:
  • Nucleophile donates an electron pair to form bond between oxygen and carbon.
  • Electrophile accepts electrons to form new bond.

Kinetics of SN2 Reaction

• One-Step Mechanism: Second order, with first order in each reactant.
• Rate Information:
  • Doubling concentration of either reactant doubles the reaction rate.
  • Indicates rate is first order in each reactant and second order overall.
• Role of SN2: Both reactants are involved in the rate-determining step (only step of the mechanism).