SN1 vs SN2: The Decision Tree Students Use on Exams

Stop guessing SN1 vs SN2. Use a decision tree based on substrate, nucleophile, solvent, and leaving group. Includes energy diagrams, stereochemistry, and practice problems.

• Fastest decision: Start with substrate (1°/2°/3°) → then solvent → then nucleophile/base.
• SN1 signature: 3° (or resonance-stabilized) + polar protic + weak nucleophile
• SN2 signature: 1°/methyl + strong nucleophile + polar aprotic
• Stereochemistry: SN2 = inversion; SN1 = racemization (often partial)
• Rearrangements: Possible in SN1, never in SN2
• Exam type: Choose the mechanism
• Exam type: Rate / competition
• Exam type: Stereochemistry