Elimination reactions are grouped into different types based on how the atoms or groups leave and how the reaction proceeds mechanistically.
1. By Mechanism
(a) E1 Reaction (Unimolecular Elimination)
- Two-step process.
- First, the leaving group departs, forming a carbocation.
- Then, a base removes a proton from a neighboring carbon to form the double bond.
- Common with tertiary alcohols or alkyl halides under weak base and polar protic solvent conditions.
(b) E2 Reaction (Bimolecular Elimination)
- One-step, concerted process.
- A strong base removes a proton while the leaving group leaves at the same time.
- Requires an anti-periplanar geometry (hydrogen and leaving group opposite each other).
- Common with strong bases and primary or secondary alkyl halides.
(c) E1cB Reaction (Conjugate Base Elimination)
- Occurs when a hydrogen is unusually acidic (next to electron-withdrawing groups).
- First, a base removes this hydrogen to form a carbanion.
- Then the leaving group departs, forming the double bond.
- Example: Elimination in β-keto esters.
2. By the Type of Groups Eliminated
(a) Dehydrohalogenation
- Removal of a hydrogen atom and a halogen atom from adjacent carbons of an alkyl halide.
- Produces an alkene.
(b) Dehydration
- Removal of water (–OH and –H) from an alcohol.
- Produces an alkene.
(c) Dehydrogenation
- Removal of two hydrogens from adjacent carbons.
- Produces alkenes or alkynes, often using metal catalysts.
(d) Other Special Eliminations
- Hofmann Elimination: Converts quaternary ammonium salts into alkenes (usually gives the less substituted alkene).
- Cope Elimination: Elimination from amine oxides, producing alkenes.
- Chugaev Elimination: Involves xanthate esters, yielding alkenes upon heating.
3. Based on Orientation of Products
- Zaitsev Elimination: Major product is the more substituted alkene (more stable).
- Hofmann Elimination: Major product is the less substituted alkene (favored with bulky bases or certain leaving groups).
In summary:
Elimination reactions can be classified by mechanism (E1, E2, E1cB), by the groups eliminated (dehydrohalogenation, dehydration, dehydrogenation, special eliminations), and by product orientation (Zaitsev vs. Hofmann).