Stereochemistry of E2 Elimination

In the E2 elimination reaction, the carbon-hydrogen sigma bond and the carbon-leaving group sigma bond must lie in the same plane. This allows the orbitals to begin to overlap to form the pi bond as the bonds to the hydrogen and the leaving group are broken. There are two possible planar arrangements of these bonds: both on the same side of the C-C bond (syn-coplanar); or on opposite sides of the C-C bond (anti-coplanar).

Syn Elimination

In this case the bond from the carbon to the leaving group (green) and the bond from the other carbon to the hydrogen (blue) are syn-coplanar. The dihedral angle between these bonds is zero degrees. This conformation is eclipsed about the carbon-carbon bond.

Anti Elimination

In this case the bond from the carbon to the leaving group (green) and the bond from the other carbon to the hydrogen (blue) are anti-coplanar. The dihedral angle between these bonds is 180 degrees. This conformation is staggered about the carbon-carbon bond. Because this conformation is more stable than the eclipsed conformation required for syn elimination, anti elimination is preferred in E2 reactions.

Syn elimination is much less common but does occur when the leaving group and the hydrogen are held syn-coplanar in an eclipsed, or nearly eclipsed, conformation of a rigid compound. For an example, see the Elaboration Syn Eliminations on page 319.

In cyclohexane derivatives, anti elimination requires that the hydrogen and the leaving group be trans and in the conformation where they are both axial (trans-diaxial elimination).