Hexafluorocyclohexane Isomer Claims Most Polar Title | Chemical & Engineering News
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Web Date: March 24, 2015

Hexafluorocyclohexane Isomer Claims Most Polar Title

ACS Meeting News: Two-faced isomer has the highest dipole moment yet reported for a nonionic organic compound
Department: Science & Technology
News Channels: Materials SCENE, Organic SCENE
Keywords: fluorine, cyclohexane, deoxyfluorination, polarity, liquid crystal
Synthesis of all-cis hexafluorocyclohexane proceeds in 12 steps via a dianhydroinositol intermediate.
A reaction scheme showing the synthesis of all-cis hexafluorocyclohexane.
Synthesis of all-cis hexafluorocyclohexane proceeds in 12 steps via a dianhydroinositol intermediate.

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.”

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Joseph A. Castellano (March 25, 2015 4:35 PM)
As an old synthetic organic chemist, I also found this to be an impressive piece of work indeed. I did quite a bit of research with organic nitrogen-fluorine compounds back in the early 1960s and synthesized some interesting compounds (eg., 1,4-bis fluoriminocyclohexene) but none as elegant as the all-cis hexafluorocyclohexane. Bravo to David O'Hagan and his team!
Dr Ravindra V Singh (March 26, 2015 7:53 AM)
This is really very exciting news for scientific community. My heartfelt congratulations to Prof. O’Hagan and his team.

Great work!!

Warm regards,
Yahia Z. Hamada (March 26, 2015 12:14 PM)
Why the cyclohexane is depicted planner? Why they are not showing the chair conformation with the six fluorines?
Jacqueline Ruiz (April 1, 2015 11:07 AM)
Very Interesting. A good example for our Org Chem Students. Is it Planar?

J. Ruiz
Robert Lee (April 3, 2015 1:38 PM)
The hybridization of each of the c atoms would be sp3. This would indicate a bond angle of appx 109.5. The visualization of the molecule is not including the hydrogen and the perspective is from directly above so the angles are not visible. I would suggest that a rotation of the molecule to a side view would demonstrate the molecule is not planer. The x-ray crystal analysis should confirm this prediction.
Steve Ritter (April 6, 2015 9:20 AM)
The molecule adopts a chair conformation. You can view a ball-and-stick model of the single-crystal X-ray here: http://cen.acs.org/articles/93/i13/Molecule-Claims-Polar-Title.html
Steve Ritter (April 13, 2015 5:00 PM)
This work has now been reported in Nature Chemistry (http://www.nature.com/nchem/journal/vaop/ncurrent/full/nchem.2232.html), with a commentary by Nico Santschi and Ryan Gilmour (http://www.nature.com/nchem/journal/vaop/ncurrent/full/nchem.2240.html).
Ian Butler (April 14, 2015 12:37 PM)
What is the molecular symmetry of this chair form?

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