In SN2 reactions, what configuration change typically occurs?

In SN2 reactions, the mechanism involves a backside attack by the nucleophile on the electrophile, which leads to the inversion of configuration at the carbon center being attacked. This means that if the original configuration of the substrate is R, it will convert to S upon undergoing the SN2 reaction. Conversely, if the substrate starts in an S configuration, it will invert to an R configuration.

The inversion of configuration is a fundamental characteristic of SN2 mechanisms, and it is often referred to as "Walden inversion." This results from the nature of the nucleophilic attack, which occurs on the side opposite to that of the leaving group, effectively flipping the spatial arrangement of the substituents around the central carbon atom.

This concept is crucial in understanding stereochemistry in reaction mechanisms, particularly for chiral centers in organic chemistry. In essence, during an SN2 reaction, the stereochemical outcome is always a complete inversion of the configuration.