Boiling Points of Halogenated Ethanes: An Explanatory Model Implicating Weak Intermolecular Hydrogen−Halogen Bonding

Abstract

This study explores via structural clues the influence of weak intermolecular hydrogen-halogen bonds on the boiling point of halogenated ethanes. The plot of boiling points of 86 halogenated ethanes versus the molar refraction (linked to polarizability) reveals a series of straight lines, each corresponding to one of nine possible arrangements of hydrogen and halogen atoms on the two-carbon skeleton. A multiple linear regression model of the boiling points could be designed based on molar refraction and subgroup structure as independent variables (R(2) = 0.995, standard error of boiling point 4.2 degrees C). The model is discussed in view of the fact that molar refraction can account for approximately 83.0% of the observed variation in boiling point, while 16.5% could be ascribed to weak C-X...H-C intermolecular interactions. The difference in the observed boiling point of molecules having similar molar refraction values but differing in hydrogen-halogen intermolecular bonds can reach as much as 90 degrees C.

Halogen Bonding in Iodo-perfluoroalkane/Pyridine Mixtures

“…The halogen may, therefore, interact with a nucleophile through σ-hole bonding, but it may also interact with an electrophile via its electronegative belt. Indeed, this is consistent with the very recent study by Beuchamp of the boiling points of 86 haloethanes, which considers the C−X···X interaction to be important . Further, the C−X···H−R bond angle for fluorine, which does not have a σ-hole, is 110−180°, but for the heavy halides, which have σ-holes, the C−X···H−R bond angles range from 90° to 130° .…”

“…Indeed, this is consistent with the very recent study by Beuchamp of the boiling points of 86 haloethanes, which considers the C−X···X interaction to be important . Further, the C−X···H−R bond angle for fluorine, which does not have a σ-hole, is 110−180°, but for the heavy halides, which have σ-holes, the C−X···H−R bond angles range from 90° to 130° . This is consistent with the electrophilic hydrogen interacting with the electronegative belt off-angle from the C−X axis.…”