Fluoroantimonic acid

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

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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

Main article: Superacid#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.