Loading VerChem
Preparing your chemistry tools...
EliminationIntroductory
E2 Elimination
Also known as: Bimolecular Elimination, Anti-Periplanar Elimination
General Equation
B⁻ + H-C-C-LG → C=C + BH + LG⁻
Description
A concerted one-step mechanism where the base abstracts a proton anti-periplanar to the leaving group. The C-H and C-LG bonds break simultaneously while the C=C double bond forms. Follows Zaitsev's rule: the more substituted alkene is the major product.
Reaction Scheme
Reactants:
- • R₂CH-CR₂-LG
Products:
- • R₂C=CR₂ + HB + LG⁻
Reagents:
- • Strong base (NaOEt, t-BuOK, NaOH)
Conditions:
- • Heat, polar solvent
Mechanism (1 steps)
1
The base abstracts a β-hydrogen that is anti-periplanar (180°) to the leaving group. Simultaneously, the C-H bond breaks, the C=C π bond forms, and the C-LG bond breaks. All in one concerted step.
Arrow: curved
Anti-periplanar geometry required (H and LG on opposite sides)
Key Points to Remember
- ✓Rate = k[substrate][base] — second order
- ✓Requires anti-periplanar geometry (H and LG at 180°)
- ✓Zaitsev's rule: more substituted alkene is major product
- ✓Hofmann's rule: with bulky bases (t-BuOK), less substituted alkene forms
- ✓Strong, non-nucleophilic bases favor E2 over SN2
- ✓Trans alkenes preferred from acyclic substrates
- ✓In cyclohexanes, H and LG must both be axial (trans-diaxial)
Examples
Reactant:
2-Bromopentane
Reagent:
NaOEt/EtOH, heat
Product:
2-Pentene (major) + 1-Pentene (minor)
Yield: 85%
Reactant:
2-Bromo-2-methylbutane
Reagent:
t-BuOK/t-BuOH, heat
Product:
2-Methyl-1-butene (major with t-BuOK)
Yield: 80%
Synthetic Utility
Primary method for forming alkenes from alkyl halides or tosylates. Predictable regiochemistry via Zaitsev or Hofmann rules.
Limitations
- ⚠Competes with SN2 (especially with primary substrates)
- ⚠Anti-periplanar requirement limits some cyclohexane substrates
- ⚠Regiochemistry mixtures with unsymmetrical substrates
#elimination#alkene formation#Zaitsev#anti-periplanar#base