In an E2 reaction, what geometric arrangement is required between the leaving group and the hydrogen being abstracted?

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

Answer: (D)

E2 eliminations are concerted and stereospecific, so the hydrogen that is abstracted and the leaving group must be anti-periplanar. This means they lie opposite each other along the C–C bond and in the same plane, giving a dihedral angle of about 180 degrees. Why that orientation matters is about orbital alignment: the base withdraws the proton as the C–H bond electrons are donated into forming the C=C bond while the leaving group leaves. The anti arrangement provides the proper overlap between the C–H sigma bond and the C–L g sigma* orbital, allowing the bond breaking and bond forming to occur in one smooth, lower-energy transition state. In cyclic systems, this translates to a trans-diaxial arrangement for the reaction to proceed. If the hydrogen and leaving group aren’t anti, the necessary orbital alignment isn’t achieved, and the reaction either slows dramatically or follows a different pathway.