Identify the compound with the highest dipole moment among: (A) $\text{cis-1,2-dichloroethene}$, (B) $\text{trans-1,2-dichloroethene}$, (C) $\text{1,1-dichloroethene}$, (D) $\text{1,1,2,2-tetrachloroethene}$.

Solution

1. In $\text{trans-1,2-dichloroethene}$, the bond dipoles cancel out, resulting in a net dipole of zero.
2. In $\text{1,1,2,2-tetrachloroethene}$, the symmetry leads to a net dipole of zero.
3. In $\text{cis-1,2-dichloroethene}$, the bond dipoles add up at an angle of $60^{\circ }$.
4. In $\text{1,1-dichloroethene}$, the bond dipoles add up at an angle of approximately $120^{\circ }$ but the inductive effect of the two chlorines on the same carbon enhances the resultant vector.
5. The resultant dipole moment of $\text{1,1-dichloroethene}$ is higher than that of the cis isomer.
6. Hence the answer is (C).