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

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.

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.