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SN1 vs SN2 vs E1 vs E2: Choosing the Mechanism
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Pick the mechanism by three questions: how substituted is the carbon, is the reagent a strong nucleophile or strong base, and what is the solvent.
Comparison
| SN2 | SN1 | E2 | E1 | |
|---|---|---|---|---|
| Rate law | k[RX][Nu] | k[RX] | k[RX][B] | k[RX] |
| Steps | 1 | 2 (carbocation) | 1 | 2 (carbocation) |
| Substrate | Methyl > 1° > 2° | 3° > 2° | 3° > 2° > 1° | 3° > 2° |
| Reagent | Strong nucleophile | Weak nucleophile | Strong base | Weak base |
| Solvent | Polar aprotic | Polar protic | Any | Polar protic |
| Stereochemistry | Inversion | Racemization | Anti-periplanar | Zaitsev product |
Decision rule
- Tertiary halide with strong base: E2. Tertiary with weak nucleophile: SN1 and E1.
- Methyl or primary with a good nucleophile: SN2.
- Secondary: strong nucleophile gives SN2; strong bulky base gives E2.
Notes
Heat favors elimination. Carbocations can rearrange in SN1 and E1. A good leaving group (weak base such as I-, Br-, tosylate) is needed in all four.
Common traps
- SN2 at a tertiary carbon. Steric hindrance blocks backside attack.
- Expecting retention in SN2. It is inversion.
- Forgetting rearrangements in SN1 and E1.
One-minute recap
- SN2: one step, inversion, unhindered carbon.
- SN1/E1: carbocation, first order.
- E2: strong base, anti-periplanar.
Check yourself: 5-question mini-quiz
1. What stereochemical outcome does SN2 give?
Inversion of configuration.
2. Which mechanisms have a carbocation intermediate?
SN1 and E1.
3. What is the rate law for E2?
Rate = k[substrate][base].
4. Which solvent type favors SN2?
Polar aprotic.
5. A tertiary halide reacts with a strong base. What mechanism?
E2.
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