Question

Write four differences between SN$^2$ and SN$^1$ reaction.

Hint:

SN1 = "1" stands for Unimolecular, but it has 2 steps. SN2 = "2" stands for Bimolecular, but it has 1 step.

Answer 1

1. Steps: SN$^1$ is a two-step reaction (formation of carbocation, then nucleophile attack). SN$^2$ is a single-step concerted reaction (bond breaking and making happen simultaneously).
2. Kinetics: SN$^1$ is first order (Unimolecular), Rate $\propto$ [Alkyl Halide]. SN$^2$ is second order (Bimolecular), Rate $\propto$ [Alkyl Halide][Nucleophile].
3. Stereochemistry: SN$^1$ leads to racemization (retention + inversion). SN$^2$ leads to complete inversion of configuration (Walden inversion).
4. Reactivity Order: For SN$^1$: $3^\circ>2^\circ>1^\circ$ (due to carbocation stability). For SN$^2$: $1^\circ>2^\circ>3^\circ$ (due to steric hindrance).