$S_N1$ vs $S_N2$ Mechanisms
| Mechanism | Order & Molecularity | Intermediates | Stereochemistry | Reactivity Order |
|---|---|---|---|---|
| $S_N1$ | 1st Order ($r = k[\text{RX}]$) | Carbocation intermediate (2 steps) | Racemisation | $3^\circ > 2^\circ > 1^\circ > \text{CH}_3\text{X}$ |
| $S_N2$ | 2nd Order ($r = k[\text{RX}][\text{Nu}^-]$) | Concerted transition state (1 step) | Walden Inversion | $\text{CH}_3\text{X} > 1^\circ > 2^\circ > 3^\circ$ |
