Substrate (most important)
Methyl and primary → SN2 wins because back-side attack is unhindered. Tertiary → SN1 wins because the substrate can form a stable carbocation, and SN2 is sterically blocked. Secondary → can go either way; nucleophile and solvent decide.
Nucleophile
Strong, negatively charged nucleophiles (OH⁻, RO⁻, CN⁻, RS⁻) push SN2. Weak, neutral nucleophiles (H₂O, ROH) push SN1. The strong nucleophile attacks before a carbocation can form; the weak one waits and reacts with whatever cation is present.
Solvent
Polar aprotic solvents (DMSO, DMF, acetone) accelerate SN2 by solvating the cation but not the nucleophile — leaving the nucleophile "naked" and reactive. Polar protic solvents (water, alcohols) stabilize carbocations through hydrogen bonding → SN1.
Stereochemistry tells
If the product has inverted stereochemistry at the reacting carbon → SN2. If it's a racemic mixture (≈50/50 R and S) → SN1, because the carbocation is planar and the nucleophile can attack from either face. This is often the cleanest experimental clue.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.