SN2 reaction

Ball-and-stick representation of the SN2 reaction of CH3SH with CH3I yielding dimethylsulfonium. Note that the attacking group attacks from the backside of the leaving group

The bimolecular nucleophilic substitution (SN2) is a type of reaction mechanism that is common in organic chemistry. In the SN2 reaction, a strong nucleophile forms a new bond to an sp3-hybridised carbon atom via a backside attack, all while the leaving group detaches from the reaction center in a concerted (i.e. simultaneous) fashion.

The name SN2 refers to the Hughes-Ingold symbol of the mechanism: "SN" indicates that the reaction is a nucleophilic substitution, and "2" that it proceeds via a bimolecular mechanism, which means both the reacting species are involved in the rate-determining step. What distinguishes SN2 from the other major type of nucleophilic substitution, the SN1 reaction, is that the displacement of the leaving group, which is the rate-determining step, is separate from the nucleophilic attack in SN1.

The SN2 reaction can be considered as an organic-chemistry analogue of the associative substitution from the field of inorganic chemistry.