Elimination Reactions

Elimination reactions are a type of organic reaction where two atoms or group of atoms are eliminated from a molecule, resulting in the formation of a new bond (double or triple bond). These reactions are the opposite of addition reactions, where atoms are added to a molecule. Generally, elimination reactions occurs in two steps: departure of the leaving group (LG) with its electron pair and removal of proton by base leaving behind electrons to form a new bond.

Two atoms or group of atoms may be eliminated from the same carbon atom or different carbon atoms. Most comonly a nucleophile from the α carbon and a proton from the β carbon are eliminated. Hence, the reaction is known as 1,2 or αβ-elimination or simply β-elimination.

The presence of at least one hydrogen at the β-carbon is necessary for elimination. The driving force for elimination are
1. stability of the olefin formed and
2. the relief from steric strain due to crowding in the substrate.

Elimination reaction is of four types. They are-

1. Unimolecular Elimination Reaction (E₁ Reaction)
2. Bimolecular Elimination Reaction (E₂ Reaction)
3. Unimolecular Conjugate Base Elimination Reaction (E₁cB Reaction)
4. Internal/Intramolecular Elimination (E_i) Reaction