The molecular formula C2H2Cl2 corresponds to dichloroethene, which can exist in three different structural isomers: cis-dichloroethene, trans-dichloroethene, and the less common form called vinylidene chloride.
The distinction in polarity between these structures primarily comes down to their geometric arrangement and how this affects the dipole moments of the individual bonds within each molecule.
Cis-Dichloroethene: In this isomer, the two chlorine atoms are on the same side of the carbon-carbon double bond. This arrangement creates a net dipole moment because the bond dipoles from the C-Cl bonds do not cancel each other out, leading to a polar molecule.
Trans-Dichloroethene: Conversely, in trans-dichloroethene, the chlorine atoms are located on opposite sides of the double bond. This geometry allows the bond dipoles to cancel each other out, resulting in a molecule that has no net dipole moment, making it nonpolar.
Vinylidene Chloride: This structure has the chlorines attached to the same carbon atom, which can also lead to a nonpolar or moderately polar state depending on the exact arrangement of the bonds. Nevertheless, it typically exhibits some degree of polarity due to the way the atoms are distributed.
In summary, the reason that one of the three structures for C2H2Cl2 is nonpolar while the other two are polar is mainly due to the spatial arrangement of the chlorine atoms and how this affects the overall dipole moment of the molecule. The cis and trans orientations result in different electronegativity distributions and ultimate molecular polarity.