Chemistry of density: extension and structural origin of Carnelley's rule in chloroethanes

Abstract

Low-density liquids and solids, with all intermolecular contacts longer than the sum of van der Waals radii, are formed by all ethanes chlorinated at one locant: CH₂ClCH₃, CHCl₂CH₃ and CCl₃CH₃. The concepts of molecular symmetry described by Carnelley and that of point groups have been compared. Carnelley's rule, when applied to liquid and solid chloroethanes clearly reveals the density dependence on the presence of intermolecular Cl⋯Cl and H⋯Cl short contacts, or their absence due to steric hindrances of overcrowded substituents. At 2.62 GPa, CH₂ClCH₃ freezes directly into phase II, with molecules arranged into layers with short Cl⋯Cl, H⋯Cl and H⋯H contacts. Only for CH₂ClCH₃, both the low-density phase at low temperature and closely-packed phase above 2.62 GPa have been observed.