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Fluoroantimonic acid



Fluoroantimonic acid
IUPAC name Fluoroantimonic acid
Identifiers
CAS number 16950-06-4
Properties
Molecular formula HSbF6
Molar mass 236.808 g/mole
Appearance colourless syrup
Density g cm–3
Melting point

°C at mmHg

Boiling point

decomposes

Solubility in water decomposes
Acidity (pKa) −25
Hazards
Main hazards corrosive
R-phrases R39/23/24/25-35
Flash point non-flammable
Related Compounds
Other anions HBF4
Other cations NaPF6, NaSbF6
Related acids HF
SbF5
Magic acid
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 the strongest known superacid, which has been demonstrated to protonate even hydrocarbons to afford carbocations and H2.

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


Additional recommended knowledge

Contents

Structure

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.

Comparison with other acids

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

  • Fluoroantimonic acid (1990) (H0 Value = −31.3)
  • Magic acid (1974) (H0 Value = −19.2)
  • Carborane superacid (1969) (H0 Value = −18.0)
  • Fluorosulfuric acid (1944) (H0 Value = −15.1)
  • Triflic acid (1940) (H0 Value = −14.9)

Applications

This extraordinarily strong acid protonates nearly 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

Safety

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.

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. (1988). "The Fluoronium Ions H2F+ and H3F2+: Characterization by Crystal Structure Analysis". Angewandte Chemie, International Edition in English 27: 391ff.
  3. ^ Bickel, A. F.; Gaasbeek, C. J.; Hogeveen, H.; Oelderik, J. M.; Platteeuw, J. C. (1967). "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. (1967). "Chemistry and spectroscopy in strongly acidic solutions: electrophilic substitution at alkane-carbon by protons". Chemical Communications 1967: 635-6. doi:10.1039/C19670000635.
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Fluoroantimonic_acid". A list of authors is available in Wikipedia.
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