SN1 vs SN2: How to Tell Which Mechanism You're Looking At These four letters cause more confusion than almost anything else in organic chemistry — not because the mechanisms are hard individually, but because students never learn a reliable way to decide which one they're looking at under exam pressure. Here's a clean way to tell them apart, every time. The basics first SN1 = Substitution, Nucleophilic, Unimolecular (rate depends on only 1 species) SN2 = Substitution, Nucleophilic, Bimolecular (rate depends on 2 species) That's literally what the numbers mean — not "1 step" and "2 steps," which is the most common misconception. It's about how many molecules are involved in the rate-determining step. SN2: the one-step story SN2 happens in a single, smooth motion — the nucleophile attacks from the opposite side of the leaving group at the exact same time the leaving group departs. No intermediate forms. Rate = k[substrate][nucleo...
Free FBISE Board Chemistry notes, MCQs, past papers and interactive tools for Class 9-12, plus IGCSE, O Level, AS & A Level students.