A SN2 reaction on a chiral carbon (with a single stereocentre) results in

1. syllabus describes the SN2 mechanism's stereochemical outcome. 2. MECHANISM: the nucleophile attacks the chiral carbon from the BACKSIDE (opposite side to the leaving group). 3. As the new bond forms and the leaving group departs simultaneously, the three remaining substituents on the carbon pass through a planar transition state then 'flip' to the opposite side — like an umbrella turning inside-out in the wind. 4. Net effect: the configuration around the chiral centre is INVERTED. This is called WALDEN INVERSION. 5. So if you start with $R$ configuration, the product is $S$ (and vice versa) — provided no other stereocentres change. 6. Option A applies to certain retention mechanisms (not SN2). Option B is the SN1 outcome (carbocation has lost stereochemistry). Option D contradicts the deterministic SN2 outcome. _Source: syllabus Chemistry Part 2, Ch 6, §6.5.1 (SN2 — Walden inversion), p. 11–12._