E1 and E2 Elimination
Elimination reactions form alkenes by removing a leaving group and a beta hydrogen.
The two common undergraduate mechanisms are E1 and E2.
E1 overview
Section titled “E1 overview”E1 stands for elimination unimolecular.
The leaving group first leaves to form a carbocation. A base then removes a beta hydrogen to form an alkene.
Because carbocation formation is the slow step, the rate law is:
Rate = k[substrate]
E2 overview
Section titled “E2 overview”E2 stands for elimination bimolecular.
The base removes a beta hydrogen while the leaving group leaves in the same concerted step.
Because the base is involved in the rate-determining step, the rate law is:
Rate = k[substrate][base]
Mechanism comparison
Section titled “Mechanism comparison”| Feature | E1 | E2 |
|---|---|---|
| Steps | Two-step | One-step |
| Intermediate | Carbocation | None |
| Rate law | k[substrate] | k[substrate][base] |
| Base | Weak base can work | Strong base usually required |
| Substrate | Tertiary favoured | Tertiary and secondary common |
| Rearrangement | Possible | Not possible |
| Stereochemical requirement | Flexible | Anti-periplanar H and leaving group |
Regioselectivity
Section titled “Regioselectivity”Many eliminations favour the more substituted alkene, known as the Zaitsev product.
Bulky bases can favour the less substituted Hofmann product because they remove the most accessible beta hydrogen.
Examples of bulky bases include:
- tert-butoxide
- LDA
- DBU in some contexts
E2 stereochemistry
Section titled “E2 stereochemistry”E2 normally requires the beta hydrogen and leaving group to be anti-periplanar.
This matters most in cyclohexane systems:
- the leaving group should be axial
- the beta hydrogen should also be axial and anti to the leaving group
- a ring flip may be needed before elimination can happen
How to choose between E1, E2, SN1 and SN2
Section titled “How to choose between E1, E2, SN1 and SN2”Use these questions in order:
- Is the substrate tertiary and the reagent a strong base? E2 is likely.
- Is the substrate tertiary with weak nucleophile/base in polar protic solvent? SN1/E1 mixture is likely.
- Is the substrate primary with a strong non-bulky nucleophile? SN2 is likely.
- Is the substrate primary with a bulky strong base? E2 is likely.
- Is a carbocation rearrangement possible? Only SN1/E1 can rearrange.
Common exam traps
Section titled “Common exam traps”- Do not draw a carbocation for E2.
- Do not ignore the anti-periplanar requirement.
- Do not assume every strong base gives substitution.
- Check bulky base effects before choosing the major alkene.
- For E1, check rearrangements before drawing the final alkene.
- For cyclohexanes, inspect axial/equatorial positions before predicting E2.
Related pages
Section titled “Related pages”Quick summary
Section titled “Quick summary”E1 is carbocation-based elimination and behaves like SN1 in many condition sets.
E2 is concerted elimination and is driven by strong base, substrate geometry and anti-periplanar alignment.