The conformational preferences of polychlorocyclohexanes

Quantitative conformational analysis (Eliel, Stereochemistry of Organic Compounds, Wiley-Interscience, 1994) has been with us at least as long as Pitzer's landmark 1937 paper (J. Am. Chem. Soc., 1937, 59, 276) on ethane. Cyclohexanes have played a critical role in the quest for understanding. Notably, 1,2,3,4,5,6 hexachlorocyclohexane (C6H6Cl6) was apparently synthesized for the first time by Michael Faraday in 1825 (Philos. Trans. R. Soc., B, 1825, 115, 440). The gamma-1,2,3,4,5,6 hexachlorocyclohexane molecule subsequently acquired the common name lindane. Although banned or limited by many countries in 2006, nearly one billion tons of lindane has been manufactured and employed, mostly in agriculture, but also for treatment of human diseases such as lice. Although not as well characterized as lindane, other chlorocyclohexanes have been made and to some degree characterized. The pioneering experimental conformational studies by LeFevre and coworkers (J. Chem. Soc. B, 1970, 1608) of 1,2 dichlorocyclohexane, 1,1,2 trichlorocyclohexane, and 1,2,3,4,5,6 hexachlorocyclohexane are particularly noteworthy. The chlorocyclohexanes have also played a role in the development of molecular mechanics methods by Allinger and coworkers (J. Am. Chem. Soc., 1983, 105, 1716 and 1723). In the present research, we report the first systematic studies of all the chlorocyclohexanes, excluding those with two chlorines attached to a single carbon atom. We make careful comparisons with previous experimental and computational studies. A simple system is established to estimate the relative energies of the different isomers of a particular molecular species. Predicted dipole moments range from identically zero to 5.7 Debye.