E2 Regio and Stereochemistry

E2: Regioselectivity and Stereochemistry

I. Regioselectivity: Zaitsev vs. Hoffmann

The E2 mechanism can lead to different alkene products depending on base size and structure:

• Zaitsev's Rule: The major product is the more substituted alkene. This pathway is favored by small, unhindered bases like EtO⁻ or OH⁻.
• Hofmann Product: The less substituted alkene becomes the major product when a bulky base like (CH₃)₃CO⁻ (tert-butoxide) is used. Steric hindrance limits access to the more substituted β-hydrogens.

Placeholder: [Chemical reaction: Example showing Zaitsev and Hofmann products with different bases]

II. Stereospecificity: Anti-periplanar Geometry

The E2 reaction requires that the β-hydrogen and the leaving group are in an anti-periplanar arrangement (180° dihedral angle):

• This spatial orientation allows for optimal orbital overlap during the concerted elimination step.
• In cyclic systems (e.g., cyclohexanes), elimination can occur only when both the β-hydrogen and the leaving group occupy axial positions in a suitable chair conformation.
• The stereochemical outcome—E vs. Z isomer—depends on the specific geometry of the anti-periplanar arrangement.

Placeholder: [Chemical illustration: Newman projection or chair conformation showing anti-periplanar geometry and E/Z isomer impact]