SN2 → Complete Inversion: Every molecule undergoes backside attack. If starting with pure (R), product is pure (S). ee = 100% inversion.

SN1 → Racemization (mostly): Planar carbocation attacked from both faces equally → 50:50 R and S → racemic mixture. In practice, slight excess inversion (~5-20%) due to ion-pair shielding by departing LG.

Neighboring group participation (retention): When an internal nucleophile (like a neighboring -OH, -SR, or -Br on an adjacent carbon) assists ionization → cyclic intermediate → external Nu opens from backside of the cyclic intermediate → net RETENTION of configuration (double inversion = retention).