The Dielectric Constant of Dichlorethane, Dibromethane, Chlorobromethane, and Diacetyl; and the Phenomenon of Free Rotation

Measurements of the dielectric constant of dichlorethane, recently published by the author, indicate a considerable temperature variation of the electric moment, as had been predicted by various authors, because of the possibility of free rotation around the simple C-C bond, and a simultaneous dipole interaction between the two mutually rotating parts of the molecule. From these results one would predict a similar temperature variation in other symmetrically substituted ethanes containing large characteristic moments such as those of C-Cl, C-Br, and C=O. In order to verify this supposition the present measurements were made on dibromethane, chlorobromethane, and diacetyl. The results prove to be similar to those for dichlorethane, and hence consistent with the predicted behavior. The variations in electric moment (×${10}^{18}$ c.g.s.e.s.u.) for the four substances are as follows: dichlorethane (previously published) 1.12-1.54 in the temperature interval 305-544°K; dibromethane 0.94-1.10 in the interval 339-436°K; chlorobromethane 1.09-1.28 in the interval 339-436°K; and diacetyl 1.25-1.48 in the interval 329-504°K. A short discussion is finally given of the evidence for free rotation in ethane obtained from the specific heats of ethylene and ethane, and the possible importance of the hydrogen atoms in the interactions of the dichlorethane molecule.