SN1 vs SN2 Reactions: Key Differences, Mechanisms & Examples
SN1 and SN2 are two competing nucleophilic substitution pathways. SN1 is a two-step, unimolecular process forming a carbocation, while SN2 is a one-step, bimolecular backside attack that inverts stereochemistry.
Students often jumble them because both replace a leaving group, yet their kinetics, solvents, and stereochemistry differ sharply—like confusing a single-track race with a relay. Misjudging which operates can derail synthesis grades and lab yields.
Key Differences
SN1 favors tertiary substrates, polar protic solvents, and produces racemic mixtures. SN2 prefers primary substrates, polar aprotic solvents, and gives clean inversion. SN1 rate = k[substrate]; SN2 rate = k[substrate][nucleophile].
Which One Should You Choose?
Need fast inversion and no rearrangement? Run SN2. Facing a bulky, stable carbocation? Switch to SN1. Match solvent and substrate to the mechanism your product demands.
Examples and Daily Life
T-butyl bromide in water → SN1 alcohol mix. Methyl iodide with CN⁻ in DMSO → SN2 nitrile, clean and quick. Knowing the pathway keeps aspirin synthesis on track.
Why does SN1 give racemates?
The planar carbocation lets nucleophiles attack either face equally.
Can SN2 occur at tertiary carbons?
Rarely; steric bulk blocks the backside attack.