In SN2 reactions at a stereocenter, what is the resulting stereochemical outcome?

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

Answer: (C)

In SN2 reactions at a stereocenter, the nucleophile attacks from the side opposite the leaving group. This backside attack happens in a single concerted step as the new bond forms while the old bond breaks, causing the four substituents around the carbon to flip their arrangement. The result is inversion of configuration, often called Walden inversion, because the stereochemistry is inverted relative to the starting material.

This contrasts with SN1, where the leaving group leaves first to give a planar carbocation, and the nucleophile can attack from either face, leading to racemization. Retention would require a mechanism that preserves the original arrangement, which SN2 does not. So the stereochemical outcome of SN2 at a stereocenter is inversion.