SN2 Mechanism and Kinetics

S_N2 Substitution Nucleophilic - Bimolecular

The SN2 mechanism is a single-step, concerted reaction that follows the rate law:

1. Single-Step Mechanism (Bimolecular Nucleophilic Substitution):

   • The nucleophile directly attacks the substrate from the opposite side (Back side attack) of the leaving group. This backside attack leads to the simultaneous breaking of the bond to the leaving group and the formation of the bond to the nucleophile.
   • Since this is a concerted process, the reaction proceeds in a single step without forming intermediates.
   • Rate = k[R-LG][Nu⁻]
   • The rate depends on both the concentration of the substrate ([R-LG]) and the nucleophile ([Nu⁻]).

Key Characteristics of SN2 Mechanism:

• Nucleophile:

  • A strong nucleophile is required to attack the substrate effectively, as it directly affects the reaction rate.
  • Nucleophiles with a negative charge or lone pairs (e.g., OH⁻, CN⁻, or I⁻) are typically more effective.
  • Nucleophile sterics are important.

• Solvent:

  • SN2 reactions are favored in polar aprotic solvents (e.g., DMSO, acetone, DMF). These solvents stabilize the nucleophile less, allowing it to remain highly reactive.

• Stereochemistry:

  • The reaction results in complete inversion of configuration (Walden Inversion) at the reactive carbon center, making it stereospecific.

• Substrate:

  • Sterically accessible substrates (e.g., methyl or primary carbons) react faster. Secondary carbons are slower, and tertiary carbons are essentially unreactive in SN2 due to steric hindrance.

• Rate Law:
  Rate = k[R-LG][Nu⁻]
  The reaction rate depends on both the nucleophile and the substrate, making it bimolecular.

Why backside attack?

1. LG is in the way for frontside attack
2. Backside has the required empty antibonding orbital to accept electrons from Nu (Nucleophile)