SN1 vs SN2 vs E1 vs E2: how to tell them apart

Check four things in order: substrate, nucleophile or base, solvent, then heat. That sequence eliminates most of the four pathways before you draw a single arrow.

What each path actually is

• SN2: Concerted substitution. Nucleophile attacks as the leaving group leaves. Rate depends on substrate and nucleophile. Stereochemical hallmark: inversion.
• SN1: Leaving group leaves first; a carbocation forms; nucleophile attacks. Rate depends mainly on the substrate. Racemization is common; rearrangements are possible.
• E2: Concerted elimination. A base takes a β-hydrogen as the leaving group leaves. Needs an anti-periplanar hydrogen. Heat and strong bases help.
• E1: Carbocation, then loss of a β-hydrogen. Shares SN1’s first step. Heat and weak bases in protic solvent push toward E1 over SN1.

Step-by-step decision order

1. Substrate

Methyl and unhindered primary: cross off SN1 and E1 (the carbocation would be miserable). Tertiary: cross off SN2 (backside attack is blocked). Secondary, allylic, and benzylic can go either way — keep reading.

2. Nucleophile vs base

Strong nucleophile / weak base (I⁻, CN⁻, RS⁻): substitution. Strong bulky base (t-BuOK): elimination. Strong base that is also a good nucleophile (HO⁻, RO⁻): primary → SN2, tertiary → E2, secondary → mixture unless heat or bulk says otherwise. Weak (H₂O, ROH): you need a decent carbocation, so SN1/E1 territory.

3. Solvent

Polar aprotic (DMSO, DMF, acetone): SN2. Polar protic (water, methanol, ethanol): SN1/E1.

4. Heat

If the problem shows heat or Δ, shift from substitution toward elimination (SN2 → E2, SN1 → E1).

Useful tips

• SN2 never rearranges. If the carbon skeleton changed, you were in carbocation land (SN1/E1).
• A single stereoisomer with inversion is a gift-wrapped SN2. A racemic mix at the reacting carbon is SN1-like.
• E2 on a cyclohexane needs the leaving group axial, anti to the hydrogen you take.

Common mistakes

• Calling a tertiary alkyl halide SN2 because the nucleophile is strong.
• Forgetting that heat was written on the arrow.
• Mixing up “strong nucleophile” and “strong bulky base.”
• Drawing E2 without checking for a β-hydrogen.

How Organic Chemistry AI helps

When you scan or type a substitution/elimination problem, Organic Chemistry AI names the reaction type — SN1, SN2, E1, E2, and more — and walks the steps with the product highlighted. That is a check against your decision order, not a replacement for learning it. Flashcards cover substitution and elimination, and quizzes practice reaction naming. Those quizzes work offline.

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