E1 and E2 Elimination Reaction Mechanisms
E₁ & E₂ Reactions
Elimination Reactions
E₁ and E₂ are elimination reactions. These are reactions used in the preparation of alkenes by the dehydrohalogenation of alkyl halides. These reactions involve the removal of a leaving group from a substrate, resulting in the formation of a double bond.
Example:
CH₃–CH₂–Br → CH₂=CH₂ + HBr
There are two types of elimination reactions:
- E₁ reaction
- E₂ reaction
1) E₁ Reaction
E₁ stands for a Unimolecular Elimination reaction. It is unimolecular; therefore, it follows first-order kinetics. The rate depends only on the concentration of the substrate (reactant). It is a reaction that involves a two-step mechanism.
Reactivity order: 3° > 2° > 1°
Mechanism
It is a two-step mechanism in which the first step is the rate-determining step.
Example:
Tertiary butyl bromide → 2-methylpropene
(CH₃)₃C–Br + H₂O Δ → CH₂=C(CH₃)₂ + HBr
Step 1: Formation of Carbocation
It involves the formation of a carbocation intermediate. It is the rate-determining step.
(CH₃)₃C–Br → (CH₃)₃C⁺ + Br⁻
Alkyl halide → Carbocation
Step 2: Deprotonation
It involves the removal of a proton (H⁺) from the carbocation. This results in the formation of an alkene.
(CH₃)₃C⁺ → CH₂=C(CH₃)₂ + H₃O⁺
Kinetics of E₁ Reaction
E₁ reactions follow first-order kinetics because the rate depends on the concentration of the substrate only.
Rate equation:
R = k [alkyl halide]
Where:
Where:
- R = Rate of reaction
- k = Rate constant
Order of Reactivity in E₁
3° > 2° > 1°
In E₁ reactions, the order of reactivity depends on the stability of the carbocation.
2) E₂ Reaction
E₂ stands for a Bimolecular Elimination reaction. It is bimolecular; therefore, it follows second-order kinetics. The rate depends on the concentration of both the substrate and the reagent. It involves a one-step mechanism.
Reactivity order: 1° > 2° > 3°
Mechanism of E₂ Reaction
The reaction is completed in one step. It involves the formation of a transition state.
Example:
Ethyl bromide + KOH → Ethene + H₂O + KBr
CH₃–CH₂–Br + KOH Δ → CH₂=CH₂ + H₂O + KBr
In the transition state (the rate-determining step), both the ethyl bromide and the hydroxide ion are involved.
Kinetics of E₂ Reaction
E₂ follows second-order kinetics. The rate of reaction depends on both the substrate and the reagent.
Rate equation:
R = k [alkyl halide][base]
Where:
- R = Rate of reaction
- k = Rate constant
Order of Reactivity in E₂
Alkyl halides are more reactive towards E₂ reactions due to less steric hindrance.
Order:
1° > 2° > 3°
Factors Affecting E₁ & E₂ Reactions
1) Substrate
For E₁: 3° > 2° > 1° (This is due to the stability of carbocations.)
For E₂: 1° > 2° > 3° (This is due to low steric hindrance.)
2) Leaving Group
The more easily the leaving group is removed from carbon, the faster the elimination reaction.
Order: R–I > R–Br > R–Cl > R–F
3) Nature of Base
For E₁: A weak base is used.
For E₂: A strong base is used.
4) Solvent
For E₁: Polar protic solvents are used.
For E₂: Polar aprotic solvents are used.
5) Temperature
High temperature facilitates both E₁ and E₂ reactions.