📘 IB Understanding
SN2 is a one-step (concerted) mechanism where the nucleophile attacks the electrophilic carbon at the same time as the leaving group departs. The rate depends on concentrations of both the substrate and nucleophile.
Key Features
| Feature | SN2 Detail |
|---|---|
| Rate law | Rate = k[substrate][nucleophile] (second order) |
| Steps | One step (concerted) |
| Nucleophile attack | Backside attack (180° to leaving group) |
| Stereochemistry | Inversion of configuration (Walden inversion) |
| Preferred substrate | Primary > secondary >> tertiary (steric hindrance) |
The Mechanism
Nu⁻ attacks from the backside (opposite the leaving group)
❗ Inversion of Configuration
Because the nucleophile attacks from the opposite side to the leaving group, the spatial arrangement around the carbon is inverted (like an umbrella flipping inside-out). If you start with an R-enantiomer, you get the S-enantiomer.
📋 Exam Tip
SN2 is favoured by: strong nucleophiles, primary substrates (less steric hindrance), and polar aprotic solvents (e.g. DMSO, acetone). Tertiary substrates cannot undergo SN2 due to too much crowding.