SN2 Reaction — Backside Attack & Walden Inversion

Because the nucleophile attacks from the side directly opposite the leaving group, the three other groups on the carbon are pushed through a flat trigonal-bipyramidal transition state and flip to the far side — like an umbrella turning inside out in the wind. This spatial flip is called Walden inversion, and it means a chiral carbon ends up with the opposite configuration.

The SN2 reactivity order is CH₃X > 1° > 2° > 3°. The more alkyl groups attached to the carbon, the more they block the nucleophile's backside approach (steric hindrance). Tertiary halides are essentially unreactive toward SN2 and react by SN1 instead.

SN2 is a single concerted step with a rate that depends on both the substrate and the nucleophile (rate = k[substrate][Nu]), and it gives 100% inversion. SN1 goes through a carbocation intermediate in two steps, its rate depends only on the substrate (rate = k[substrate]), and it gives a racemic mixture. SN2 is favoured by methyl/primary substrates, strong nucleophiles, and polar aprotic solvents.

The rate law is Rate = k[substrate][nucleophile] — second order overall, first order in each reactant. The '2' in SN2 refers to this bimolecularity: both species appear in the rate-determining (and only) step. Doubling either concentration doubles the rate.

Yes. SN2 is part of the Haloalkanes and Haloarenes chapter and is tested almost every year. The most common question patterns are the reactivity order (CH₃ > 1° > 2° > 3°), the stereochemistry (inversion / Walden inversion), the second-order rate law, and distinguishing SN1 from SN2.