SN2 reactions are organic chemistry reactions in which an ion called the nucleophile attacks the center of another molecule and replaces one of the ions, called the leaving group. These types of reactions are called nucleophilic substitution reactions, and there are two types, SN1 and SN2. SN1 reactions occur in two steps, but SN2 reactions occur in a single step.
The single step for SN2 reactions occurs because the nucleophile, or incoming atom, is strong enough to essentially steal the molecular bonds from the leaving group. The SN2 reaction is most commonly seen during reactions on carbon atoms that have four single bonds attached to other atoms, one of which is an electronegative, stable ion. This stable ion is frequently one of the halides, and can leave the molecule when it is forced out by the incoming nucleophile.
During an SN2 reaction, the incoming ion attacks from the opposite side of the carbon atom from the leaving group. The molecule undergoes a brief transition state when both the nucleophile and leaving group are partially attached to the carbon atom. This only occurs for a fraction of a second, and then the leaving group detaches from the central carbon atom, and the electron bonds transfer to the incoming ions.