The E2 reaction is a type of elimination reaction that follows a one-step, bimolecular mechanism. It is called “E2” because the rate-determining step depends on two species — the substrate and the base.
Step-by-Step Mechanism
Single Concerted Step
- A strong base abstracts a proton (β-hydrogen) from the carbon next to the one bearing the leaving group.
- At the same time, the leaving group departs with its bonding electrons.
- Simultaneously, a carbon–carbon double bond forms between the α- and β-carbons.
Everything happens in one coordinated step (no intermediates).
Key Features
- Rate Law:
- Rate = k [substrate][base]
- Bimolecular → depends on both the substrate and the base concentration.
- Base Strength:
- Requires a strong base (e.g., hydroxide, alkoxide, tert-butoxide, LDA).
- Substrate:
- Works best with secondary and tertiary alkyl halides.
- Primary halides can undergo E2 only with bulky strong bases.
- Stereochemistry (Anti-Periplanar Requirement):
- The β-hydrogen and the leaving group must be aligned opposite each other in the same plane.
- This requirement makes the E2 mechanism stereospecific, often giving a defined E (trans) or Z (cis) alkene.
- Regioselectivity:
- Usually follows Zaitsev’s rule (more substituted alkene favored).
- With bulky bases, the Hofmann product (less substituted alkene) may dominate.
Comparison with E1
- E2: Single step, no carbocation, no rearrangements.
- E1: Two steps, carbocation intermediate, rearrangements possible.
In summary:
The E2 mechanism is a concerted, single-step elimination where a base removes a β-hydrogen as the leaving group departs, producing an alkene. It requires a strong base, a good leaving group, and an anti-periplanar geometry for proper orbital overlap.