SN1 vs SN2 vs E1 vs E2: The Decision Framework

Key structures for this topic — drawn live.

Step 1 — Classify the substrate

The carbon bearing the leaving group sets the boundaries of what's possible, because carbocation stability and steric access differ sharply by substitution class:

• Methyl and primary (1°): Carbocations are too unstable to form, so S_N1 and E1 are ruled out. These undergo S_N2. The only elimination they'll do is E2, and only when treated with a strong, bulky base.
• Tertiary (3°): Too crowded for backside attack, so S_N2 is ruled out. With weak nucleophiles/bases they go S_N1/E1 (through a stable 3° carbocation); with a strong base they go E2.
• Secondary (2°): The ambiguous case — capable of all four mechanisms. Here the reagent makes the decision.

Step 2 — Characterize the reagent

Rank your reagent on two independent axes: how good a nucleophile it is (wants to attack carbon) versus how good a base it is (wants to grab a proton), and whether it is bulky or small.

• Strong nucleophile, weak base (e.g., I^–, Br^–, RS^–, N₃^–, CN^–): drives S_N2.
• Strong, bulky base (e.g., tert-butoxide, LDA, DBU): drives E2 — too big to attack carbon, so it removes a proton instead. Bulky bases also favor the less-substituted (Hofmann) alkene.
• Strong, small base that is also a good nucleophile (e.g., hydroxide, methoxide, ethoxide): can do either S_N2 or E2. On 1° it leans S_N2; on 2°/3° and with heat it leans E2.
• Weak nucleophile/base (neutral molecules like H₂O, ROH, or the leaving group's conjugate): can only manage S_N1/E1, and only on substrates that form decent carbocations (2°/3°).

Step 3 — Weigh the conditions

Solvent and temperature nudge the outcome:

• Polar protic solvents (water, alcohols) stabilize carbocations and ions, favoring S_N1/E1.
• Polar aprotic solvents (acetone, DMSO, DMF, acetonitrile) leave nucleophiles "naked" and reactive, boosting S_N2.
• Heat favors elimination. Elimination increases the number of molecules (more entropy) and is entropically favored, so raising the temperature tips substitution/elimination competitions toward E1 or E2.

The decision table

Put the three steps together:

• Methyl / 1° + strong nucleophile (weak base): S_N2.
• 1° + strong bulky base: E2.
• 2° + strong nucleophile (weak base): S_N2.
• 2° + strong base (small or bulky): E2.
• 2° or 3° + weak nucleophile/base, polar protic, heat: S_N1 and E1 together (mixture).
• 3° + strong base: E2.
• 3° + weak nucleophile/base: S_N1/E1.

Common traps

A few reminders that catch students out: S_N2 inverts the stereocenter (Walden inversion) and is second-order, while S_N1 goes through a planar cation and gives racemization plus possible carbocation rearrangements — watch for hydride and methyl shifts in S_N1/E1. E2 needs an anti-periplanar H and leaving group and usually gives the more-substituted (Zaitsev) alkene, unless a bulky base steers it to Hofmann. And remember that S_N1 and E1 almost always occur together because they share the same rate-determining carbocation-forming step — you just get a product mixture whose ratio shifts toward elimination as you add heat.