SN2 Inversion of Configuration

SN2 reactions cause inversion of configuration at the carbon being substituted. The nucleophile attacks from the back side, while the leaving group departs at the same time. This happens through a transition state, where the carbon changes from tetrahedral to trigonal bipyramidal and then back to tetrahedral.

Recall that an S_N2 reaction follows a concerted mechanism, where the nucleophilic attack and the departure of the leaving group occur simultaneously. 

High electron density around the leaving group blocks the frontside of the substrate, which forces the nucleophile to initiate a backside attack.

As the HOMO of the nucleo...