SN2 reaction

S_N2 is a kind of nucleophilic substitution. It involves two molecules at the slow, rate-determining step of the reaction.

The S_N2 reaction is a one step reaction the bonding of the nucleophile and the separation of the leaving group takes place in one single step.

In the S_N2 reaction, the addition of OH^- and the elimination of Br^- take place simultaneously.

S_N2 occurs where the backside of the central carbon atom is easily accessible to the nucleophile. The rate is second order, depending on the nucleophile concentration as well as the substrate.

rate = k[RX][OH^-]

The nucleophile enters on the opposite side of the carbon to the leaving group, so an optically active substrate has its structure inverted. This inversion is called the Walden inversion.

Unlike the S_N1 reaction, S_N2 reactions do not involve any carbocation intermediate, it involves an transitionstate in which the central carbon atome hase five bonds. 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 comes off by itself whereas in S_N2 the nucleophile forces off the leaving group.

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