SN2+stereochemistry

When an SN2 reaction occurs on a chiral carbon, there is an "inversion" of the stereo center. Recall the SN2 mechanism:



The effect is similar to when an umbrella gets blown inside out in a wind gust.

As the nucleophile bonds, it creates a transition state resembling a dsp 3 hybridization (triangular bipyramid), which eventually forces X, Y, and Z to the opposite side of the molecule.

Here is an example: