Question

Which of the following would not have a dipole moment?
(i) $CC{l}_4$ (iii) $1,2$-dichloroacetylene
(ii) trans-$1,2$-dichloroethene (iv) $C{H}_{2}C{l}_2$

• A. (i) and (iv)
• B. (i), (ii) and (iv)
• C. (i), (ii) and (iii)
• D. (ii) and (iii)

Answer 1

The correct option is C (i), (ii) and (iii)

In carbon tetrachloride, the resulting dipole moment of three $C-Cl$ bonds is equal in magnitude and opposite in direction to the dipole moment of the fourth $C-Cl$ bond. Hence the molecule has no net dipole moment.
In trans-$1,2$-dichloroethane, the dipole vectors of two $C-Cl$ bonds are equal in magnitude and opposite in direction. Hence, they cancel each other.
Similarly the dipole vectors of two $C-H$ bonds are equal in magnitude and opposite in direction. Hence, they cancel each other.
Thus the molecule has no net dipole moment.
In $1,2$-dichloroacetylene, the dipole moment vectors of two $C-Cl$ bonds are equal in magnitude and opposite in direction. Hence, they cancel each other. Hence, the molecule has no net dipole moment.
In dichloromethane, the resultant vector of dipole moments of two $C-Cl$ bonds is not equal to the resultant vector of the dipole moments of two $C-H$ bonds. Hence, the molecule has net dipole moment.