6.17: E2 Reaction: Stereochemistry and Regiochemistry
Elimination reactions of alkyl halides can yield one or more alkenes depending on the specific regiochemical and stereochemical considerations. While the regiochemistry of the reaction governs the location of the double bond in the product, the stereochemical requirements often influence the geometry.
When a substrate with two different β hydrogens undergoes an E2 elimination, the presence of a strong base can yield two regioisomeric alkenes. The more-substituted alkene is the major product and is called the Zaitsev product. The less-substituted alkene is called the Hofmann product.
The predominance of the Zaitsev product is a reflection of the relative stabilities of the transition states for the possible products. Since the transition state has a significant double bond character, the presence of substituents increases its stability. Thus, the transition state yielding the more substituted alkene requires less energy and proceeds faster to give the Zaitsev product. However, in the presence of a sterically hindered base like potassium tert-butoxide, the transition state leading to the Zaitsev product can be highly crowded; in such cases, the E2 reactions can become regioselective for the less substituted Hoffman product.
In E2 reactions, the filled carbon–hydrogen σ orbital and the empty carbon–halogen σ* antibonding orbital must lie on the same plane to enable the formation of the π bond. Two conformations fulfill this requirement: a) the hydrogen and halide are anti-coplanar and staggered, and b) they are syn-coplanar and eclipsed. E2 eliminations preferentially occur via a lower energy anti-coplanar transition state where the base and leaving group are far apart, and the two orbitals are fully parallel, permitting maximum overlap. However, E2 reactions of some rigid molecules can proceed via a syn-coplanar transition state.
The stereochemistry of E2 reactions depends on the number of β hydrogens. Alkyl halides with two β hydrogens undergo stereoselective elimination to yield the more stable E-alkene as the major product. However, an alkyl halide with only one β hydrogen gives a stereospecific isomer, even if it is the Z-alkene.