Question

Which of the following compounds has zero dipole moment?
(A) $1,4-dichlorobenzene$
(B) $1,2-dichlorobenzene$
(C) $1,3-dichlorobenzene$
(D) $1-chloro-2-methylbenzene$

Answer 1

Hint: The bond dipole moment uses the idea of an electric dipole moment to measure the polarity of a chemical bond within a molecule. It occurs whenever there is a separation of positive and negative charges. The bond dipole μ is given by: $\mu =\delta d$

Complete step by step solution:
As there are two Cl groups present in ortho makes an acute angle according to the equation;
Dipole moment, D = $\sqrt{d_1^2{+d}_2^2{+d}_1{d}_{2}cosx}$
where, x = angle between the groups

A.
Here, the dipole moment is $0$ as ${Cos90}^{\circ }$ = $0$.

B.
The dipole moment of $1,2-dichlorobenzene$ is $2.54D$,so
C.
The dipole moment of $1,3-dichlorobenzene$ is $1.72D$.
D.
Here, there will be some dipole moment, not zero.
The order of dipole moment is: $1,2-dichlorobenzene$ $1,3-dichlorobenzene$ $1,4-dichlorobenzene$.
Trans-dichlorobenzene has zero dipole moment as it is symmetrical and the dipole moment will cancel out.

Hence, the correct option is A.

> Factors affecting dipole moment:
1) Polarity of molecule
2) Magnitude of charge
3) Geometry of molecule
4) The boiling point of the element.
5) Position of bond and lone pair of electrons in a molecule.
6) The dipole moment depends upon the bond length.

Note: The possibility to make a mistake is that you may choose option C. But in $1,3-dichlorobenzene$ they are not parallel and hence they will not cancel out completely, it has some dipole moment.