Question

Which of the following compounds will have a zero dipole moment?

• A. 1,1-dichloroethylene
• B. trans-1,2-dichloroethylene
• C. cis-1,2-dichloroethylene
• D. none of the above compound

Answer 1

The correct option is B trans-1,2-dichloroethylene

Trans has no (or zero) dipole moment than cis isomer.
Cis isomer has more dipole moment than trans isomer because it has two similar groups on same side of double bond. So the dipole gets added, thus cis isomer is more polar than trans.
In the trans isomer, the terminal groups are on the opposite sides of the double bond. So, here the dipole moments of the trans isomer is zero. Hence, trans-1,2-dichloroethylene has zero dipole moment.
1,1-dichloroethylene also has some dipole moment because it has two similar groups on same side of double bond. So the dipole gets added.