Question

Refractive indices of aromatic molecules are higher than those of saturated hydrocarbons; for example, for benzene and cyclohexane, n=1.50112 and 1.4290 at 20C, respectivly. Can you suggest a reason for why n for C6H6 > n for C6H12?

Answer 1

The magnitude of the refractive index of a medium depends upon how easily the electron cloud in the molecule is affected by the electric field of the elecromagnetic radiation (light)

At optical frequencies, the movement of electrons induced by light (electromagnetic radiations) in saturated hydrocarbons is limited by the rigidity or stiffness of the electron cloud..(This statement is an over simplification of complicated quantum mechanical ideas.)

But in aromatic compounds where there is delocalization of electron cloud there is greater freedom of movement for the electron cloud. In other words the electrons move more easily in aromatic compounds. This gives greater permittivity and refractive index for aromatic compounds.

(Permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how much electric field (more correctly, flux) is 'generated' per unit charge in that medium.)