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EliminationIntroductory
E1 Elimination
Also known as: Unimolecular Elimination
General Equation
R₃C-LG → R₃C⁺ → alkene + H⁺
Description
A two-step mechanism where the leaving group departs first to form a carbocation, followed by deprotonation of a β-hydrogen to form the alkene. Often occurs alongside SN1.
Reaction Scheme
Reactants:
- • R₃C-LG (tertiary substrate)
Products:
- • Alkene + HB + LG⁻
Reagents:
- • Weak base or heat
Conditions:
- • Polar protic solvent, heat
Mechanism (2 steps)
1
Ionization: The leaving group departs to form a carbocation intermediate. This is the rate-determining step (same as SN1).
Arrow: curvedR₃C⁺
Slow step — identical to SN1 first step
2
Deprotonation: A base (often solvent) removes a β-hydrogen adjacent to the carbocation. Electrons from the C-H bond form the new C=C π bond.
Arrow: curved
Fast step — Zaitsev product favored
Key Points to Remember
- ✓Rate = k[substrate] — first order
- ✓Always competes with SN1 (same first step)
- ✓Higher temperatures favor elimination over substitution
- ✓Zaitsev's rule applies (more substituted alkene)
- ✓Carbocation rearrangements possible before elimination
- ✓Follows same substrate requirements as SN1 (tertiary best)
Examples
Reactant:
2-Bromo-2-methylpropane
Reagent:
EtOH, heat
Product:
2-Methylpropene
Yield: 60% (mixed with SN1 product)
Synthetic Utility
Less synthetically useful than E2 due to competing SN1 and rearrangements. More common in biological systems.
Limitations
- ⚠Always mixed with SN1 products
- ⚠Carbocation rearrangements give unexpected products
- ⚠Poor regiocontrol compared to E2
#elimination#carbocation#unimolecular#Zaitsev