Walden inversion

Hold an umbrella upright in a strong gust and it can flip inside out, the ribs snapping from one side to the other. A carbon undergoing an SN2 reaction does the very same thing. As the nucleophile pushes in from the back, the three other groups on the carbon swing through the plane and end up pointing the opposite way. This flip — a change in the spatial arrangement at the carbon — is the Walden inversion, named after Paul Walden, who first demonstrated such a configurational switch in 1896.

Concretely: before the reaction, the carbon is a tetrahedron with the leaving group on one vertex. As the SN2 transition state forms, the carbon flattens momentarily into a trigonal plane (the three spectator groups splayed out like a propeller, the nucleophile coming in on one face and the leaving group on the other). As the leaving group departs and the new bond completes, the carbon re-pyramidalizes — but with everything turned through, so the molecule is now the mirror-image arrangement at that center. If the carbon was a stereocenter, an (R) configuration becomes (S), and vice versa.

This is the diagnostic fingerprint of SN2. Run a substitution on a single, pure enantiomer and check the product's configuration: clean inversion means SN2; a scrambled 50:50 mixture (racemization) means SN1. The Walden inversion is also a practical tool — chemists exploit it to deliberately flip the stereochemistry of a center, for example in synthesizing a specific enantiomer of a drug. Note the careful wording: inversion is about spatial arrangement, and whether the (R)/(S) label actually changes depends on the priorities of the groups, not just on the flip itself.