The Dielectric Constant of Formic, Acetic, and Propionic Acids, and the Electric Moment of Complex Molecules

Abstract

The temperature and pressure variation of the dielectric constant of formic and propionic acid vapors has been studied experimentally. The behavior is similar to that previously found for acetic acid. For formic acid it is possible to calculate from Coolidge's vapor density measurements the amount of association into double molecules, and to explain the apparent departures from Debye's theory in terms of such association. It is then seen that the apparently linear pressure curves can be explained by the compensation of several factors in the measurements. By assuming that the optical part of the polarization is doubled on association and taking this value from refractivity measurements, values for the electric moment of the single and double molecules of formic acid are obtained, 1.51×${10}^{-18\mathrm{}}$ and 0.99×${10}^{-18\mathrm{}}$ c.g.s.e.s.u., respectively. Although no reliable vapor density measurements exist for acetic acid it is assumed that the anomaly in this case also is due to association rather than to the previously suggested effect of vibrational quantization. The previous data are reinterpreted accordingly. By using the optical refractivity as before a value is obtained from the high temperature measurements for the electric moment of the single molecule of acetic acid, 1.73×${10}^{-18\mathrm{}}$; and similarly for propionic acid, 1.74×${10}^{-18\mathrm{}}$ c.g.s.e.s.u. The electric moment of complex molecules is then discussed with reference to directed valence and to internal free rotation around single bonds. The possibility of the importance of interaction between external rotation and internal free rotation is suggested. The question of the interpretation of measured electric moments is raised for the case where free rotation causes a time variation of electric moment. In this connection the experimental values of electric moment are discussed. The values of the electric moment of these acids indicate that the OH group is tightly bound as supposed by Eucken and Meyer. A structure for the double molecule of formic acid is suggested, with the four oxygen molecules at the corners of a rectangle, the planes of the two constituent single molecules at an angle of about 120°, and the OH groups bound tightly in positions of minimum potential energy. This structure does not seem to be inconsistent with the observed electric moment.