The compound 1,2-dichloroethane (C2H4Cl2) is nonpolar, while cis-dichloroethylene (C2H2Cl2) 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.

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CI CI H-C-C-H 1,2-dichloroethane Cl CI cis-dichloroethylene