Sn2 Stereochemistry

Bimolecular nucleophilic substitution proceeds in one concerted step in which the incoming nucleophile attacks the carbon from the face exactly opposite the leaving group. As the new bond forms and the carbon-bromine bond breaks, the three other groups on the carbon flip through to the far side, much like an umbrella turning inside out in a strong wind. This Walden inversion converts an enantiomerically pure substrate into a product of inverted configuration at the stereocentre. Complete retention is wrong because backside attack necessarily reverses the spatial arrangement. An equal mixture of both configurations describes racemisation, which is characteristic of carbocation SN1 chemistry, not a clean SN2 process. The loss-of-stereocentre option is incorrect because the carbon still bears four different groups after cyanide replaces bromine, so it remains chiral. Historically this inversion was proved by Walden through a cycle of reactions that returned a compound to its mirror-image configuration, giving direct experimental evidence for backside attack. Because the rate of an SN2 reaction depends on both the substrate and the nucleophile concentration, the reaction is also second order overall, which is the kinetic counterpart of the stereochemical inversion. This matches the NCERT description of SN2 stereochemistry. Sanity check: a single inversion of one stereocentre gives one specific opposite configuration, not a racemate, consistent with concerted backside attack.