Why is the dipole moment of the C-Cl bond in chlorobenzene lower than that of cyclohexyl chloride?

There are a couple of factors that contribute to the lower dipole moment for the #"C-Cl"# bond in chlorobenzene.

Take a look at the two molecules, chlorobenzene and cyclohexyl chloride

Notice that the chlorine atoms are attached to carbon atoms that have a different orbital hybridization. In chlorobenzene, the carbon to which chlorine is attached is #"2p"^2# hybridized, while in cyclohexyl chloride, that carbon is #"sp"^3# hybridized.

That has an impact on dipole moment because the greater the s character of the hybrid orbital, the more electronegative that carbon atom will be. The #"sp"^2# hybridized carbon will have less stendency to release electrons to the chlorine atom, which in turn will make the dipole moment weaker when compared with the #"sp"^3# hybridized carbon's bond with chlorine.

Another important factor is the fact that chlorobenzene is resonance-stabilized, which means that the lone pairs of electrons on the chlorine atom will be delocalized on the benzene ring

.

As a result, the #"C-Cl"# single bond will acquire some double bond character. Double bonds are shorter than single bonds, which has a direct impact on dipole moment because dipole moment depends on charge and distance.

By comparison, the #"C-Cl"# is a pure single bond, which means, of course, that it will be longer.

These two factors are what determine the dipole moment for the #"C-Cl"# bond in chlorobenzene to be lower, or weaker, than the dipole moment of the same bond in cyclohexyl chloride.