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The SN2 reaction is a one-step bimolecular nucleophilic substitution occurring via a single transition state with inversion of stereochemistry. Second-order kinetics depend on both substrate and nucleophile concentrations. Factors favoring SN2 include primary carbon centers, polar aprotic solvents, strong nucleophiles, and good leaving groups.
You already know from the basic SN2 reaction and Walden inversion that the nucleophile attacks the electrophilic carbon from the back side, pushing out the leaving group in a single concerted step with complete inversion of stereochemistry. This topic zooms in on why the reaction behaves this way kinetically and what structural factors make it faster or slower.
The rate law is the defining fingerprint: rate = k[substrate][nucleophile]. Both species appear in the rate expression because both are present in the single transition state — that is what "bimolecular" means. Double the nucleophile concentration and the rate doubles. Double the substrate concentration and the rate doubles again. This second-order kinetics distinguishes SN2 from SN1, where only the substrate appears in the rate law. The practical consequence is immediate: if you want a faster SN2 reaction, increasing nucleophile concentration works, whereas it would have no effect on an SN1 reaction.
Substrate structure is the most powerful factor. The nucleophile must physically reach the electrophilic carbon, so anything that blocks the back side slows the reaction dramatically. Methyl substrates (CH₃-LG) are fastest because there are only hydrogen atoms flanking the carbon — essentially no steric obstruction. Primary substrates are nearly as good. Secondary substrates are much slower because two carbon-containing groups partially block approach. Tertiary substrates are essentially unreactive by SN2 — three bulky groups create a wall the nucleophile cannot penetrate. Think of it like trying to thread a needle: methyl is an open doorway, primary is a normal door, secondary is a narrow gap, and tertiary is a locked wall.
The remaining three factors fine-tune reactivity. A strong nucleophile (one with high nucleophilicity — recall that this is a kinetic property distinct from basicity) accelerates the reaction because it appears in the rate law. A good leaving group stabilizes the developing negative charge in the transition state; the better it departs, the lower the activation energy. And solvent choice matters enormously: polar aprotic solvents like DMSO and acetone do not solvate anions through hydrogen bonding, leaving the nucleophile's electron pair fully available for back-side attack. Switching from a protic solvent like methanol to an aprotic solvent like DMSO can increase SN2 rates by factors of a million. These four factors — substrate, nucleophile, leaving group, and solvent — form a checklist for predicting when an SN2 pathway will dominate over competing mechanisms.