Question

Define the terms dipole moment and molar polarisability. Explain why ethyl chloride has dipole moment of 2.05 D but chlorobenzene has the dipole moment of 1.70 D.​

Answer 1

Answer:

Dipole Moment - A dipole moment is the turning force a fixed charge has on a dipole in a molecule. The moment depends on the charge at the end of the dipole and its distance from the charge at the other end of the dipole (i.e., the separation of charge).

Molar Polarization- The molar polarization of a gas is the sum of electronic, atomic and orientationally effects. For comparison with spectral sums involving electronic transitions, only this latter component is relevant.

I see two explanations

• Consider a resonance structure where the chlorine donates electrons to the alkene. This puts a formal positive charge on chlorine:
• Resonance forms of vinyl chloride
• This would be a very minor contributor given the electronegativities and the mismatch in orbital size. However, this is not possible in ethyl chloride at all.
• The more likely explanation is that an sp2-hybridized carbon is more electronegative than an sp3-hybridized carbon. Since the electronegativity of chlorine will be closer to the electronegativity of the carbons in vinyl chloride than the carbons of ethyl chloride, the C–Cl bond will be less polarized in vinyl chloride, giving a smaller dipole.

There are two factors:

• The bonding orbital of carbon in C2H3Cl is an sp2 hybrid orbital while C2H5Cl’s is an sp3 one. The former has a higher s-content and a lower p-content resulting in it being closer to the atom resulting in a shorter bond. A shorter bond means less charge separation.

• sp2 carbons are also more electronegative than sp3 carbons

Dipole moment depends not only on electronegativity differences but also bond lengths. All other factors held equal, the longer the bond, the greater the dipole moment.

Explanation: