In an E2 reaction, which geometric arrangement is required between the leaving group and the β-hydrogen?

• A. Syn-periplanar
• B. Gauche
• C. Anti-periplanar
• D. Anti-coplanar

Answer: (C)

E2 eliminations rely on a single concerted step where the base abstracts a β-hydrogen as the leaving group departs and a double bond begins to form. For this to occur smoothly, the β-hydrogen must be anti to the leaving group along the C–C bond. That anti-periplanar arrangement provides the proper orbital alignment so the σ(C–H) electrons can shift into the developing C=C bond exactly as the C–LG bond breaks, giving the most favorable transition state with maximum overlap and minimal torsional strain. In cyclic systems, this requirement often forces the leaving group and the β-hydrogen into axial positions to achieve the necessary anti relationship; if the β-H is not anti to the leaving group, the overlap is poor and the reaction proceeds slowly or not at all by E2. So, the geometry the reaction needs is anti-periplanar.