The SN2 reaction is a one step reaction the bonding of the nucleophile and the seperation of the leaving group takes place in one single step.
SN2 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.
Unlike the SN1 reaction, SN2 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 SN1 reaction, however, with the key difference being that in the SN1 reaction the leaving group comes off by itself whereas in SN2 the nucleophile forces off the leaving group.
Most of the SN reactions procede via both mechanisms. Wich of the two possibilitys is favoured dependes on solvent, temperature, concentration of the nucleophile or most on the leaving group.