Key Definitions
- Nucleophile — an electron-pair donor that attacks a positive (or δ+) centre. E.g. OH⁻, NH₃, CN⁻, H₂O
- Leaving group — departs with the bonding pair. E.g. Cl⁻, Br⁻, I⁻
- Curly arrow — shows the movement of an electron pair from nucleophile to electrophilic carbon
SN1 vs SN2
| SN1 | SN2 | |
|---|---|---|
| Steps | 2 (carbocation intermediate) | 1 (concerted) |
| Rate law | Rate = k[RX] | Rate = k[RX][Nu] |
| Substrate | Tertiary halogenoalkanes | Primary halogenoalkanes |
| Stereochemistry | Racemic mixture | Inversion (Walden inversion) |
Think About It
Why do tertiary halogenoalkanes favour SN1 rather than SN2?
Three bulky alkyl groups around the carbon create steric hindrance — the nucleophile cannot easily attack from behind. Instead, the leaving group departs first to form a stabilised tertiary carbocation, then the nucleophile attacks the planar carbocation.