SN1 vs SN2
MCAT glossary · MCAT
Definition
SN1: unimolecular, carbocation intermediate, racemization, polar protic solvent, 3° substrate; SN2: bimolecular, backside attack, inversion, polar aprotic solvent, 1° substrate.
SN1 vs SN2 in plain English
SN1 and SN2 are two ways one atom or group on a molecule gets swapped out for another (nucleophilic substitution). In SN2, the incoming attacker hits from the back and boots the old group out in a single smooth motion, like pushing a door so it flips inside-out — this needs open, uncrowded carbons (1°, meaning few other groups attached) and 'inversion' means the molecule's 3D shape flips. In SN1, the old group leaves FIRST, creating a temporary positively charged carbon (carbocation) that the attacker then hits from either side — so you get a mix of both mirror-image shapes (racemization), and it favors crowded carbons (3°) that can stabilize that charge. The quick contrast: SN2 is one crowded-step door-flip that needs elbow room, while SN1 is a two-step 'leave-then-attack' that needs a carbon able to hold a charge — the amount of crowding around the carbon largely decides which path a molecule takes.
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