Overview
This lecture covers the E2 (bimolecular elimination) reaction mechanism, focusing on its characteristics, requirements, and factors that influence its outcome.
E2 Reaction Mechanism
- E2 stands for bimolecular elimination, meaning the rate depends on both the substrate and base concentrations.
- The E2 reaction involves a single concerted step where the base removes a proton while the leaving group departs.
- A strong base is essential for E2 to occur efficiently.
- E2 prefers anti-periplanar geometry, with the hydrogen and leaving group on opposite sides.
- E2 reactions are favored by high substrate concentration and strong, bulky bases.
Stereochemistry of E2
- The anti-periplanar transition state leads to specific stereochemistry in the alkene product.
- For cyclic compounds, the leaving group and hydrogen must be axial to achieve anti-periplanar arrangement.
- E2 can control the E/Z configuration of the resulting alkene depending on the available hydrogens.
Regioselectivity (Zaitsev vs. Hofmann)
- The major alkene product is usually the most substituted (Zaitsev rule).
- Bulky bases favor the less substituted (Hofmann) product due to steric hindrance.
Competing Reactions
- E2 competes with SN2 (bimolecular nucleophilic substitution) when good nucleophiles are also strong bases.
- Poor leaving groups or weak bases decrease E2 reaction efficiency.
Key Terms & Definitions
- E2 Mechanism — Bimolecular elimination reaction with a concerted single-step mechanism.
- Anti-periplanar — Geometry where the proton and leaving group are directly opposite each other in the transition state.
- Zaitsev Rule — Predicts the most substituted alkene is the major product.
- Hofmann Product — Less substituted alkene, favored by bulky bases.
Action Items / Next Steps
- Review examples of E2 mechanisms in the textbook.
- Practice drawing anti-periplanar conformations and predicting major products for assigned problems.