The Variation of Dielectric Constant with Temperature. II. Electric Moments of the Ethylene Halides

Both the free and hindered rotation of atoms or groups of atoms about single chemical linkages may be studied with the Debye relation between the molar polarization, $P$, and the reciprocal of the absolute temperature, $\frac{1}{T}$. The quantitative treatment by one of us for the simplest case of a free rotation has been compared with the more recent and more general expressions of Eyring and of Zahn. For the case in which the relation between $P$ and $\frac{1}{T}$ is nonlinear the use of Meyer's equation has been suggested for the calculation of the electric moment of the molecule. An apparatus has been described for measuring the dielectric constant of a vapor as a function of temperature. This apparatus includes a comparatively cheap vapor condenser of Monel metal, a means of obtaining vapor at any desired pressure up to that characteristic of 100°C, and a method of measuring vapor pressure in an all-glass apparatus. An experimental procedure has been designed to minimize as far as possible the systematic errors due to simplifying assumptions in the capacity equations of the measuring circuit. The results of dielectric constant and density measurements on ethylene chloride and ethylene bromide have been recorded. The experimental values for the total polarizations of these compounds have been plotted against the reciprocals of the absolute temperature. These results have been utilized for the calculation of the several electric moments by the usual methods based on the linear equation of Debye and by the application of Meyer's equation. All of the methods for the calculation of electric moments were found to give results approaching each other at higher temperatures but tending to deviate more and more at lower temperatures. It has also been possible to calculate the characteristic moments of the C - Cl bond and the C - Br bond from the experimental data. The characteristic moments of the bonds were found to bear the same ratio to each other as the moments of the methyl compounds. It has been further shown that Meyer's equation is applicable to the results for vapors and for solutions now existent in the literature. The indications are that the difference between the experimental results for vapors and for solutions may be due to changes in internal structure produced in the molecule by the solvent.