SN1 vs SN2 Reactions: What's the Difference?

Both are substitution reactions, where one group attached to a carbon gets replaced by another. In SN2 the swap happens in one step, with the new group pushing in as the old one is pushed out. In SN1 it takes two steps, and how crowded the carbon is decides which route runs.

SN2 first: the attacking group (the nucleophile) strikes the carbon from the side directly opposite the leaving group and bonds while pushing it out — one continuous step that flips the carbon's other three groups to the opposite side, inverting the molecule's 3D arrangement. Because that step is a collision between two molecules, the speed depends on the concentration of both — the "2," bimolecular — and the backside strike needs a clear path, so SN2 runs on uncrowded carbons. SN1 takes over on crowded carbons, where the path is blocked but neighboring groups stabilize positive charge: the leaving group departs first, leaving a carbocation, and the nucleophile then attaches to this flat intermediate from either side, producing a mix of 3D arrangements. The slow step involves one molecule only, so speed depends on the substrate alone — the "1," unimolecular. The exam shortcut: open carbon → SN2, crowded carbon → SN1.

SN2 is a stop shot in pool: the cue ball needs a clear straight line, comes in fast from the opposite side, and in one collision knocks the target ball out while stopping in its exact spot. SN1 is street parking, where the parked car pulls out on its own, the spot sits empty, and the nearest waiting car slides in from either direction. Access to the carbon decides which one runs.