How do I decide whether a reaction will go by SN1 or SN2?
Look at the substrate first.
Look at the substrate first. A tertiary (3°) halide can't be attacked from behind (too crowded) but forms a stable carbocation, so it goes SN1 — two steps, racemised product, helped by polar protic solvents. A primary (1°) halide has no stable carbocation but is open to attack, so a strong nucleophile does SN2 — one step, backside attack, inverted product. Secondary sits in between and depends on the nucleophile and solvent.
Key point
3° + weak nucleophile/protic solvent → SN1; 1° + strong nucleophile → SN2.
Common mistake
predicting the mechanism from the nucleophile alone, ignoring the substrate.
Memory tip
crowded carbon (3°) → SN1 (carbocation); open carbon (1°) → SN2 (backside).
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