Question

Dipole moment of chlorobenzene is lower than that of cyclohexyl chloride. Give reason:

Answer 1

Think about what dipole moment actually is. Dipole moment is seen almost everywhere where there is a difference in charges/polarity and there is a significant distance between them.

Complete answer:
Let us start by knowing what dipole moment actually is. Dipole moment is defined as the product of the charge and the perpendicular or shortest distance between the charges. It is a scalar quantity that means, dipole moment has no direction associated with it.
Mathematically,
$\mu \text{ }=\text{ }Q\text{ }\times \text{ }d$, where Q is the charge and d is the perpendicular distance
Dipole moment is used in many places. For example, dipole moment is used to find out whether a compound is polar or not.
A compound having more dipole moments is more ionic. The formula for finding ionic character is:
$[\mu \left( real \right)\div \mu \left( observed \right)]\times 100$
It is seen that molecules which have a plane of symmetry have null net dipole moment. It is so because dipole moments being scalar quantities, they can get cancelled out in opposite directions.
More the dipole moment, more the bond strength and more the boiling point of the substance. Generally compounds which are ionic in nature have a high boiling point and are less volatile.
In chlorobenzene, the C atom of the C-Cl bond is $s{{p}^{2}}$ hybridized and has more s character and electronegativity. In cyclohexyl chloride, the C atom of the C-Cl bond is $s{{p}^{3}}$ hybridized, has less s character and less electronegative. Hence, the C-Cl bond in chlorobenzene possesses less polarity than the C-Cl bond present in cyclohexyl chloride. Also, in chlorobenzene, the lone pair of electrons on the Cl atom resonates with the benzene ring. There is a decrease in the magnitude of partial negative charge on Cl atoms as this gives partial double bond character to C-Cl bond . However, in the other compound, there is a single bond of carbon chlorine. Also C-Cl single bond has higher
bond length than with C-Cl bond with partial double bond character. Hence, chlorobenzene has lower dipole moment than cyclohexyl chloride

Note:
Do not get confused with the direction of the dipole moment. Dipole moment is a scalar quantity and does not have any direction. But once it is a scalar quantity, it can get subtracted or added. For example, in the molecule $B{{H}_{3}}$, dipoles cancel out each their and resultant dipole is less, but in case of $_{{}}N{{H}_{3}}$, dipoles add to each other, as they are in the same direction and resultant dipole is not 0.