The SN2 reaction has one of the simplest mechanisms of all that we study, but is one of the most important reactions synthetically because of the large variety of possible nucleophiles and leaving groups.
Remember that the nucleophile must attack from the rear, and that inversion of configuration occurs.
- Rear attack is required because this is a "filled-empty" interaction between a filled orbital on the nucleophile and the empty s* orbital of the C-X bond, which has its greatest extension at the rear of the reacting carbon.
In aqueous and other protic media, the best nucleophiles are the weakest bases, such as I-, because they are only weakly solvated. In dipolar aprotic solvents like DMSO, in which anions are not solvated, nucleophilicity and basicity are the same thing.
Remember also that SN2 never occurs on a tertiary substrate. It is particularly facile at benzylic and allylic positions.