The stereochemistry of the dihaloethanes, dihaloethenes and the dihalocyclohexanes

The stereochemical features of the dihaloethanes and the dihalocyclohexanes have attracted significant attention over the years. While the dichloro- and the dibromo-compounds seem to display the stereochemical preferences that we would normally predict for them, the analogous difluorocompounds do not. More specifically, the 1,2-difluorocompounds seem to prefer conformations in which the fluorines are gauche. This 'gauche effect' has been examined by using molecular orbital calculations. A re-examination of the conformational properties of these molecules, and their stereo-electronic effects, using the molecular-mechanics-based modeling program STR3DI32.EXE, has enabled-us to conclude that the stereochemical properties of these 1,2-difluoroalkanes are best seen as resulting from the. participation of the fluorines in significant C-H hydrogen bonding, both intra- and intermolecular. (C) 2003 Elsevier Science B.V. All rights reserved.