Chapter 9, Problem 113AP

The compound 1,2-dichloroethane ${\text{(C}}_{\text{2}}{\text{H}}_{\text{4}}{\text{Cl}}_{\text{2}}\text{)}$ is nonpolar, while cis-dichloroethylene ${\text{(C}}_{\text{2}}{\text{H}}_{\text{2}}{\text{Cl}}_{\text{2}}\text{)}$ has a dipole moment: The reason for the difference is that groups connected by a single bond can rotate with respect to each other, but no rotation occurs when a double bond connects the groups. On the basis of bonding considerations, explain why rotation occurs in 1,2-dichloroethane but not in cis- dichloroethylene.