: 200
SN2 → Complete Walden inversion: Backside attack at 180 degrees inverts configuration (R → S or S → R). Like an umbrella flipping. 100% inversion gives 100% optical purity of inverted product.
SN1 → Racemization: Planar sp2 carbocation attacked from both faces → 50:50 R:S theoretically. In practice, slight excess inversion due to ion-pair effect (leaving group shields one face briefly). An SN1 product with 80% optical purity means: 90% major enantiomer + 10% minor.
Mixed mechanisms: If optical purity is between 0% (complete racemization) and 100% (complete inversion), a mixture of SN1 and SN2 is occurring.
E2 stereochemistry: Anti-periplanar requirement determines which diastereomer of the alkene product forms. The geometry of H and X in the starting material dictates cis/trans outcome. In cyclohexanes, only trans-diaxial H and X can undergo E2.