Chemists have made the all-cis isomer of 1,2,3,4,5,6-hexafluorocyclohexane, a crystalline ring molecule with all six fluorines pointing “up” and all six hydrogens pointing “down.” This compound is remarkable for being the most polar nonionic compound now known to exist, according to David O’Hagan of the University of St. Andrews, in Scotland. O’Hagan, whose team synthesized the compound, described the chemistry behind the molecule this week during a Division of Fluorine Chemistry symposium at the American Chemical Society national meeting in Denver.
The deceptively simple molecule is in reality a complex synthetic target because it has nine possible isomers that can adopt up to 15 conformations, O’Hagan said. Orienting all the fluorine atoms on one face of the ring in the all-cis isomer was a particular challenge because the fluorines all want to repel each other—the all-cis isomer has twice the strain energy as any other isomer.
O’Hagan’s lab previously prepared the “four up-two down” isomer in 2012, starting from benzene and using a combination of oxidation, fluorination, and deoxyfluorination steps. To make the “six up” isomer, O’Hagan, Neil S. Keddie, and colleagues devised a similar strategy starting from myo-inositol, a cyclohexane with a hydroxyl group on each ring carbon.
The new molecule, which adopts the classic chair conformation of cyclohexane, is electropositive on the hydrogen face of the ring and highly electronegative on the fluorine face, leading to a dipole moment of 6.2 debyes. Many ionic compounds have higher dipole moments, but the O’Hagan team’s search for a more polar aliphatic or aromatic compound came up empty.
The high polarity makes fluorinated cyclohexanes amenable to stacking together in layers to prepare materials such as liquid crystals, O’Hagan said. Benzene and hexafluorobenzene are known to stack together in alternating layers, he notes, and it appears the all-cis hexafluorocyclohexane provides a fusion of that arrangement within a single ring.
“O’Hagan and coworkers’ preparation of the facially polarized hexafluorocyclohexane is an impressive synthetic achievement,” said Stephen G. DiMagno, an expert in polar hydrophobic effects at the University of Nebraska, Lincoln. “The strained stereoisomer demonstrates, in one concise X-ray crystal snapshot, that the dipolar C–F bond of fluorocarbons will find any weak favorable electrostatic interaction in preference to interaction with another C–F bond.”