Differentiate between the $S_N2$ and $S_N1$ mechanisms of alkyl halides.

(N/A) The differences between $S_N2$ and $S_N1$ mechanisms are as follows:
| Feature | $S_N2$ Mechanism | $S_N1$ Mechanism |
| :--- | :--- | :--- |
| $(i)$ | Bimolecular nucleophilic substitution | Unimolecular nucleophilic substitution |
| $(ii)$ | Kinetics: Rate depends on both substrate and nucleophile concentration. | Kinetics: Rate depends only on substrate concentration. |
| $(iii)$ | Steps: Single-step process with one transition state. | Steps: Two-step process with two transition states. |
| $(iv)$ | Intermediate: No carbocation intermediate is formed. | Intermediate: Carbocation intermediate is formed. |
| $(v)$ | Reactivity order: $3^\circ < 2^\circ < 1^\circ < CH_3X$ | Reactivity order: $3^\circ > 2^\circ > 1^\circ > CH_3X$ |
| $(vi)$ | Stereochemistry: Inversion of configuration. | Stereochemistry: Racemization ($50:50$ mixture of inversion and retention). |
| $(vii)$ | Optical activity: Can lead to inversion of configuration. | Optical activity: Racemic mixture formed,optically inactive. |