Here’s a clear explanation of the differences between E1 and E2 reactions in organic chemistry:
1. Mechanism
- E1 (Unimolecular Elimination): The reaction occurs in two steps: first, the leaving group leaves, forming a carbocation intermediate, and then a proton is removed to form the double bond.
- E2 (Bimolecular Elimination): The reaction occurs in a single concerted step: the base removes a proton at the same time the leaving group leaves, forming the double bond.
2. Rate of Reaction
- E1: Depends only on the concentration of the substrate (first-order reaction).
- E2: Depends on the concentration of both the substrate and the base (second-order reaction).
3. Substrate Preference
- E1: Favours tertiary > secondary > primary (carbocation stability is key).
- E2: Favours tertiary > secondary > primary as well, but steric hindrance can affect base attack.
4. Base Strength
- E1: Requires a weak base.
- E2: Requires a strong base.
5. Stereochemistry
- E1: Double bond formation may lead to a mixture of isomers (less control over stereochemistry).
- E2: Often shows anti-periplanar geometry, giving more control over the double bond formation.
6. Reaction Conditions
- E1: Favoured by polar protic solvents (like water, alcohols).
- E2: Favoured by polar aprotic solvents (like DMSO, acetone) and strong bases.
In short:
- E1: Two-step elimination, forms carbocation, depends on substrate, weak base, slower.
- E2: One-step elimination, no carbocation, depends on substrate and base, strong base, faster.