Refractive Index and Chemical Constitution | Refractivity
Refractive index of a liquid changes not only with the wavelength of light but also with the temperature. It can not, therefore, provide a satisfactory comparison of refractive powers of different liquids relative to their constitution. To eliminate the effect of temperature, Lorenz and Lorentz (1880) showed purely from theoretical considerations that-
where n is the refractive index, d the density and R is a constant which they defined as the Specific refraction or Refractivity. In order to compare the refractive powers of different liquids the specific refractions are multiplied by their respective molecular weights, the resulting products being termed the Molecular refraction or Molecular refractivity. Thus-
where M is the molecular weight and RM is the molecular refraction or molecular refractivity. The value of molecular refraction is characteristic of a liquid and remains constant at different temperatures.
Molecular refraction is essentially an additive property as is shown by the fact that the increase in molecular refraction of successive members of an homologous series is nearly constant.
Molar refraction of homologous series of normal alcohols are given in the table
| Formula | RM for D-line | Difference |
|---|---|---|
| CH3OH | 8.218 | - |
| CH3CH2OH | 12.839 | 4.621 |
| CH3CH2CH2OH | 17.515 | 4.676 |
| CH3CH2CH2CH2OH | 22.130 | 4.615 |
| CH3CH2CH2CH2CH2OH | 26.744 | 4.614 |
Owing to the additive nature of molecular refraction, it is possible from a study of molecular refraction of different compounds of known constitution to work out a series of constants, representing the atomic refractions (atomic weight x specific refraction) of various elements.
Like the parachor, molar refraction is constitutive as well and is influenced by the arrangement of atoms in the molecule or by such factors as unsaturation, ring closure, etc.
The atomic refraction of oxygen, for example, has different values in alcohols, ethers and ketones. Most of these atomic and structural molar contributions were determined by Eisenlohr and revised by Vogel. In the table below are given some of his values.
Table Molar refraction contribution for D-line-
| Fragments | D-line |
|---|---|
| CH2 increment | 4.647 |
| Hydrogen | 1.028 |
| Carbon | 2.591 |
| Chlorine | 5.844 |
| Bromine | 8.741 |
| Iodine | 13.954 |
| Oxygen | 1.746 |
| Methyl radical | 5.653 |
| Ethyl radical | 10.300 |
| Doble bond | 1.575 |
| Triple bond | 1.977 |
| 6 Carbon ring | -0.15 |
| 5 Carbon ring | -0.10 |
| 4 Carbon ring | 0.317 |
| C=O | 4.601 |
| -OH | 2.546 |
| -COOH | 7.226 |
| NO2 | 6.713 |
How the constants given above can be employed to confirm the structure of a compound may be illustrated by taking the example of benzene.
Calculation of molecular refraction of benzene-
Six Carbon atoms: 6 x 2.591 = 15.546
Six Hydrogen atoms: 6 × 1.028 = 6.1168
Three double bonds: 3 x 1.575 = 4.725
One six-membered ring = -0.15
RM = 25.289
The experimentally determined value 25.93 is near about the calculated value, which support to Kekule's formula for benzene. Thus the determination of molecular refractions of liquids affords an easy means of ascertaining their chemical constitution.
