E1 vs E2 Elimination Reactions
E1 vs E2
Two mechanisms for elimination reactions that remove HX from a substrate to form alkenes. E1 is a two-step process via carbocation, while E2 is a concerted one-step mechanism.
Comparison Table
| Feature | E1 | E2 |
|---|---|---|
| Mechanism | Two-step via carbocation | One-step concerted |
| Rate Law | Rate = k[substrate] | Rate = k[substrate][base] |
| Base Strength | Weak base okay | Strong base required |
| Substrate Preference | Tertiary > Secondary | All substrates (3 > 2 > 1) |
| Stereochemistry | Not stereospecific | Anti-periplanar required |
| Geometry Requirement | None | H and LG must be anti |
| Competing Reaction | SN1 competition | SN2 competition |
| Zaitsev Product | Usually forms | Usually forms (unless bulky base) |
Key Differences
- →E1 forms a carbocation intermediate; E2 is a single concerted step
- →E2 requires the H and leaving group to be anti-periplanar
- →E1 can occur with weak bases; E2 requires strong bases
- →E1 competes with SN1; E2 competes with SN2
- →E2 is stereospecific; E1 is not
When to Use E1
- ✓Tertiary substrates with weak bases
- ✓Polar protic solvents
- ✓When carbocation is stabilized
- ✓High temperatures favor elimination over substitution
When to Use E2
- ✓Strong, bulky bases (t-BuOK, DBU)
- ✓Primary or secondary substrates
- ✓When stereospecific product needed
- ✓Polar aprotic solvents
Common Confusions
- !Forgetting that E2 requires anti-periplanar geometry
- !Not recognizing that heat favors elimination over substitution
- !Confusing Zaitsev (more substituted alkene) with Hofmann product
- !Assuming tertiary always means E1 (strong base still gives E2)
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Common questions about this comparison
Anti-periplanar means the H being removed and the leaving group are on opposite sides and in the same plane (180 degrees apart). This geometry is required for E2 because the electrons from the C-H bond form the new pi bond as the leaving group departs.
The Zaitsev rule states that elimination reactions favor the more substituted (more stable) alkene product. This is because the transition state resembles the product, so more stable products form faster. Exception: bulky bases give the less substituted Hofmann product.