Hexafluorobenzene

Hexafluorobenzene
Skeletal formula of hexafluorobenzene
Space-filling model of hexafluorobenzene
Names
Preferred IUPAC name
Hexafluorobenzene
Other names
Perfluorobenzene
Identifiers
3D model (JSmol)
Abbreviations HFB
1683438
ChEBI
ChemSpider
ECHA InfoCard 100.006.252 Edit this at Wikidata
EC Number
  • 206-876-2
101976
UNII
  • InChI=1S/C6F6/c7-1-2(8)4(10)6(12)5(11)3(1)9 checkY
    Key: ZQBFAOFFOQMSGJ-UHFFFAOYSA-N checkY
  • InChI=1/C6F6/c7-1-2(8)4(10)6(12)5(11)3(1)9
    Key: ZQBFAOFFOQMSGJ-UHFFFAOYAJ
  • Fc1c(F)c(F)c(F)c(F)c1F
Properties
C6F6
Molar mass 186.056 g·mol−1
Appearance Colorless liquid
Density 1.6120 g/cm3
Melting point 5.2 °C (41.4 °F; 278.3 K)
Boiling point 80.1 °C (176.2 °F; 353.2 K)
1.377
Viscosity cP (1.200 mPa•s) (20 °C)
0.00 D (gas)
Hazards
GHS labelling:
GHS02: Flammable
Warning
H225
P210, P233, P240, P241, P242, P243
Flash point 10 °C (50 °F; 283 K)
Related compounds
Related compounds
Benzene
Hexachlorobenzene
Polytetrafluoroethylene
Perfluorotoluene
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)
Infobox references

Hexafluorobenzene, HFB, C
6
F
6
, or perfluorobenzene is an organofluorine compound. In this derivative of benzene, all hydrogen atoms have been replaced by fluorine atoms. The technical uses of the compound are limited, although it has some specialized uses in the laboratory owing to distinctive spectroscopic properties.

Geometry of the aromatic ring

Hexafluorobenzene stands somewhat aside in the perhalogenbenzenes. When counting for bond angles and distances it is possible to calculate the distance between two ortho fluorine atoms. Also the non bonding radius of the halogens is known. The following table presents the results:

Formula Name Calculated
inter-halogen
distance, aromatic ring assumed planar
Twice nonbonding radius Consequent symmetry of the benzene
C6F6 hexafluorobenzene 279 270 D6h
C6Cl6 hexachlorobenzene 312 360 D3d
C6Br6 hexabromobenzene 327 390 D3d
C6I6 hexaiodobenzene 354 430 D3d

Hexafluorobenzene is the only perhalobenzene being planar, the other perhalobenzene species exhibiting buckling. As a consequence, in C6F6 the overlap between the p-orbitals is optimal versus the other perhalobenzenes, resulting in lower aromaticity of those compounds compared to C6F6.

Synthesis

The direct synthesis of hexafluorobenzene from benzene and fluorine has not been useful. Instead it is prepared by the reaction of alkali-fluorides with halogenated benzene:

C6Cl6 + 6 KF → C6F6 + 6 KCl

Reactions

Most reactions of hexafluorobenzene proceed with displacement of fluoride. One example is its reaction with sodium hydrosulfide to afford pentafluorothiophenol:

C6F6 + NaSH → C6F5SH + NaF

The reaction of pentafluorophenyl derivatives has been long puzzling for its mechanism. Independent of the substituent, they all exhibit a para directing effect. The new introduced group too has no effect on the directing behaviour. In all cases, a 1,4-disubstituted-2,3,5,6-tetrafluorobenzene derivative shows up. Finally, the clue is found not in the nature of the non-fluorine substituent, but in the fluorines themselves. The π-electropositive effect introduces electrons into the aromatic ring. The non-fluorine substituent is not capable of doing so. As charge accumulates at the ortho and para positions relative to the donating group, the ortho and para-positions relative to the non-fluorine substituent receive less charge, so are less negative or more positive. Furthermore, the non-fluorine substituent in general is more bulky than fluorine, so its ortho-positions are sterically shielded, leaving the para-position as the sole reaction site for anionic entering groups.

UV light causes gaseous HFB to isomerize to hexafluoro derivative of Dewar benzene.

Laboratory applications

Hexafluorobenzene has been used as a reporter molecule to investigate tissue oxygenation in vivo. It is exceedingly hydrophobic, but exhibits high gas solubility with ideal liquid gas interactions. Since molecular oxygen is paramagnetic it causes 19F NMR spin lattice relaxation (R1): specifically a linear dependence R1= a + bpO2 has been reported. HFB essentially acts as molecular amplifier, since the solubility of oxygen is greater than in water, but thermodynamics require that the pO2 in the HFB rapidly equilibrates with the surrounding medium. HFB has a single narrow 19F NMR signal and the spin lattice relaxation rate is highly sensitive to changes in pO2, yet minimally responsive to temperature. HFB is typically injected directly into a tissue and 19F NMR may be used to measure local oxygenation. It has been extensively applied to examine changes in tumor oxygenation in response to interventions such as breathing hyperoxic gases or as a consequence of vascular disruption. MRI measurements of HFB based on 19F relaxation have been shown to correlate with radiation response of tumors. HFB has been used as a gold standard for investigating other potential prognostic biomarkers of tumor oxygenation such as BOLD (Blood Oxygen Level Dependent), TOLD (Tissue Oxygen Level Dependent) and MOXI (MR oximetry) A 2013 review of applications has been published.

HFB has been evaluated as standard in fluorine-19 NMR spectroscopy.

Toxicity

Hexafluorobenzene may cause eye and skin irritation, respiratory and digestive tract irritation and can cause central nervous system depression per MSDS. The National Institute for Occupational Safety and Health (NIOSH) lists it in its Registry of Toxic Effects of Chemical Substances as neurotoxicant.

See also


This page was last updated at 2024-02-15 01:04 UTC. Update now. View original page.

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