In SN2 reactions, what happens to the stereochemistry at a chiral center?

• A. Retention of configuration
• B. Racemization
• C. Inversion of configuration
• D. No change

Answer: (C)

SN2 reactions occur in one concerted step where the nucleophile attacks from the side opposite the leaving group. As the new bond forms and the old bond breaks, the carbon’s geometry flips, giving inversion of the stereochemistry. So a chiral center that started as one enantiomer becomes the opposite enantiomer in the product. This is known as Walden inversion. The backside approach is driven by orbital alignment into the C–LG σ* antibonding orbital, which forces the configuration to switch. If the substrate is racemic, each reacting center inverts to its opposite configuration, but SN1 would instead lead to racemization because it goes through a planar carbocation intermediate.