The S_N2 reaction is a type of nucleophilic substitution. It involves two molecules in the slow, rate-determining step of the reaction, which leads to the name: Substitution, Nucleophilic, bimolecular, or S_N2.

The S_N2 reaction is a one step reaction whereby the addition of the nucleophile and the elimination of the leaving group take place in a single step.

In an example of the S_N2 reaction, the addition of OH^-, the nucleophile, to an alkyl bromide results in an alcohol, with bromide ejected as the leaving group. The breaking of the alkyl carbon-bromine bond and the formation of the new carbon-oxygen bond occur simultaneously to form a transition state in which the carbon under nucleophilic attack is pentavalent, and approximately sp² hybridised. The nucleophile enters on the opposite side of the carbon to the leaving group, and if the substrate under nucleophilic attack is chiral, leads to an inversion of stereochemistry. This inversion is called the Walden inversion.

S_N2 attack occurs if the backside route of attack is not sterically hindered by substituents on the substrate. If there is steric crowding on the substrate near the leaving group, the S_N1 reaction can compete with the S_N2. The rate of an S_N2 reaction is second order, as the rate-determining step depends on the nucleophile concentration as well as the concentration of substrate.

rate = k[RX][OH^-]

Unlike the S_N1 reaction, the S_N2 reaction does not involve a carbocation intermediate; rather, it involves a transition state in which the central carbon atom has five bonds, and where only partial charges exist. The reaction is very similar to the S_N1 reaction, however, with the key difference being that in the S_N1 reaction the leaving group dissociates by itself and thusly forms a carbocation, whereas in S_N2 the nucleophile forces off the leaving group.

Most of the S_N reactions proceed via both mechanisms. Which of the two possibilities is favoured dependes on solvent, temperature, concentration of the nucleophile or most importantly on the leaving group.