E2 反应:立体化学和区域化学

The Zaitsev product is the more substituted alkene formed as the major product in E2 eliminations, while the Hofmann product is the less substituted alkene. When 2-bromo-2-methyl-butane reacts with a strong base like sodium ethoxide, both products form through different pathways. The Zaitsev product predominates because its transition state has greater double bond character, requiring less energy and proceeding faster.

Strong bases like sodium ethoxide favor the Zaitsev product due to lower transition state energy. However, sterically hindered bases like potassium tert-butoxide create crowding around the transition state leading to the Zaitsev product, making the reaction regioselective for the less substituted Hofmann product instead.

E2 eliminations preferentially occur via the anti-coplanar transition state because the base and leaving group are farther apart, and the filled carbon-hydrogen σ orbital and empty carbon-halogen σ* antibonding orbital are fully parallel. This geometry allows maximum orbital overlap to form the π bond with lower energy than the syn-coplanar alternative.

The filled carbon-hydrogen σ bonding orbital and the empty carbon-halogen σ* antibonding orbital must lie in the same plane and overlap to form a π bond. Both anti-coplanar and syn-coplanar conformations fulfill this coplanarity requirement, though anti-coplanar is energetically favored because it positions the orbitals parallel for optimal overlap.

Alkyl halides with two β hydrogens undergo stereoselective elimination, forming the more stable E-alkene as the major product. However, substrates with only one β hydrogen give a single stereospecific isomer—either E-alkene or Z-alkene depending on substrate stereochemistry—regardless of relative alkene stability.

The Zaitsev product is more substituted, making its transition state have greater double bond character and higher stability. Since the transition state resembles the product, the pathway to the more substituted alkene requires less activation energy and proceeds faster, making the Zaitsev product the thermodynamically favored major product.

While E2 eliminations preferentially occur via the lower energy anti-coplanar transition state, some rigid molecules can proceed through syn-coplanar conformations where the hydrogen and halide are eclipsed. Both conformations satisfy the coplanarity requirement for orbital overlap, though anti-coplanar remains energetically favorable in most cases.