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

Feb 13, 2015

There are a couple of factors that contribute to the lower dipole moment for the $\text{C-Cl}$ 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 ${\text{2p}}^{2}$ hybridized, while in cyclohexyl chloride, that carbon is ${\text{sp}}^{3}$ 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 ${\text{sp}}^{2}$ 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 ${\text{sp}}^{3}$ 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

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As a result, the $\text{C-Cl}$ 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 $\text{C-Cl}$ 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 $\text{C-Cl}$ bond in chlorobenzene to be lower, or weaker, than the dipole moment of the same bond in cyclohexyl chloride.