Fluoroantimonic acid

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Fluoroantimonic acid
Fluoroantimonic acid
General
Systematic name Fluoroantimonic acid
Other names
Molecular formula HSbF6
Acidic power −31.3
Molar mass 236.808 g/mole
Appearance colourless syrup
CAS number 16950-06-4
Properties
Density and phase g cm–3
Solubility in water decomposes
Freezing Point °C at mmHg
Boiling point decomposes
Acidity (pKa) −25
Hazards
MSDS
Main hazards corrosive
Flash point non-flammable
R/S statement R:
S:
RTECS number
Related compounds
Related acids HF
SbF5
Magic acid
Other cations NaPF6, NaSbF6
Other anion HBF4
Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)
Infobox disclaimer and references

Fluoroantimonic acid HSbF6 is a mixture of hydrogen fluoride and antimony pentafluoride in various ratios.[1] The 1:1 combination affords one of the strongest known superacids, which has been demonstrated to protonate even hydrocarbons to afford carbocations.

The reaction of hydrogen fluoride (HF) and SbF5 is exothermic. HF releases its proton (H+), and its conjugate base (F−) is sequestered by one of more molecules SbF5 to give the octahedral SbF6−. This anion is classified as noncoordinating, because it is both a very weak nucleophile and a very weak base. The proton effectively becomes "naked", which accounts for the system's extreme acidity. Fluoroantimonic acid is 2×1019 times stronger than 100% sulfuric acid. The reaction of hydrofluoric acid and antimony pentafluoride proceeds as follows

Image:fluoroantimonic acid.jpeg

Contents

[edit] Handling HF-SbF5

HF-SbF5 is rapidly and explosively decomposed by water. It reacts with virtually all known solvents.[1] A solvent that has proven compatible with HF-SbF5 is SO2ClF. Chlorofluorocarbons have also been used as solvents. Containers for HF-SbF5 are made of Teflon.

[edit] Structure of HF-SbF5

Two related products have been crystallised from HF-SbF5 mixtures, and both have been analyzed by single crystal X-ray crystallography. These salts have the formulas [H2F+][Sb2F11−] and [H3F2+][Sb2F11−]. In both salts the anion is Sb2F11−.[2] As mentioned above, SbF6− is classified as weakly basic; the larger monoanion Sb2F11− would be expected to be still weaker.

[edit] Comparison with other acids

The following values are based upon the Hammett acidity function. Acidity is indicated by large negative values of H0.

[edit] Remarkable reactions promoted by HF-SbF5

This extraordinarily strong acid protonates all organic compounds. In 1967, Bickel and Hogeveen showed that HF-SbF5 will remove H2 from isobutane and methane from neopentane:[3] [4]

(CH3)3CH + H+ → (CH3)3C+ + H2
(CH3)4C + H+ → (CH3)3C+ + CH4

[edit] References

  1. ^ a b Olah, G. A.; Prakash, G. K. S.; Wang, Q.; Li, X. “Hydrogen Fluoride–Antimony(V) Fluoride” in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
  2. ^ Mootz, D.; Bartmann, K. "The Fluoroniurn Ions H2F+ and H3F2+: Characterization by Crystal Structure Analysis" Angewandte Chemie, International Edition in English 1988, volume 27, pages 391ff.
  3. ^ Bickel, A. F.; Gaasbeek, C. J.; Hogeveen, H.; Oelderik, J. M.; Platteeuw, J. C. “Chemistry and spectroscopy in strongly acidic solutions: reversible reaction between aliphatic carbonium ions and hydrogen” Chemical Communications, 1967, 634-5. DOI: 10.1039/C19670000634.
  4. ^ Hogeveen, H.; Bickel, A. F. “Chemistry and spectroscopy in strongly acidic solutions: electrophilic substitution at alkane-carbon by protons" Chemical Communications, 1967, page 635-6. DOI: 10.1039/C19670000635.
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