In the preferred transition state of an E2 reaction, the dihedral angle between the C-H bond being broken and the C-X bond of the leaving group is

A $9 0^{\circ}$
B $18 0^{\circ}$
C $0^{\circ}$
D $6 0^{\circ}$

Answer: B

The anti-periplanar arrangement allows the C-H bonding orbital to overlap best with the C-X antibonding orbital as the new pi bond forms; this corresponds to a H-C-C-X dihedral angle of

18 0^{\circ}

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