SN2 reaction

Picture the nucleophile sneaking up on the carbon from the back door, directly opposite the leaving group. As it pushes in, the leaving group is shoved out the front — and the whole thing happens in one smooth, continuous motion, with no pause in between. That single-step, concerted dance is the SN2 reaction. The "2" means two molecules (the substrate and the nucleophile) are both involved in the slow, rate-determining step.

Here is the mechanism in plain steps. The nucleophile approaches the carbon from the side opposite the leaving group (backside attack). At the high point of the reaction — the transition state — the carbon is momentarily bonded to five things at once: the incoming nucleophile, the departing leaving group, and three other groups splayed out flat in a plane. Then the leaving group breaks away as the new bond fully forms. Because the attack comes from behind, the three remaining groups flip inside out, like an umbrella caught in a gust of wind — this is the Walden inversion of configuration. Because both partners meet in that single step, the rate depends on both concentrations: rate = k[substrate][nucleophile], second-order kinetics.

The SN2 pathway is exquisitely sensitive to crowding around the carbon. A methyl or primary carbon, with little blocking it, reacts fast; a secondary carbon is sluggish; and a tertiary carbon — surrounded by three bulky groups — is effectively blocked, because the nucleophile simply cannot reach the back door. So the SN2 speed order is methyl > primary > secondary >> tertiary. SN2 is favored by good (strong, often negatively charged) nucleophiles and polar aprotic solvents, and it is the chemist's reliable tool for making a bond with clean, predictable inversion of stereochemistry.