Fluoroantimonic acid

Chembox new
Name = Fluoroantimonic acid
ImageFile = Fluoroantimonic-acid-3D-vdW.png ImageName = Fluoroantimonic acid
IUPACName = Fluoroantimonic acid
OtherNames =
Section1 = Chembox Identifiers
CASNo = 16950-06-4
Section2 = Chembox Properties
Formula = HSbF₆
MolarMass = 236.808 g/mole
Appearance = colourless syrup
Density = g cm^–3
Solubility = decomposes
MeltingPt = °C at mmHg
BoilingPt = decomposes
pKa = −25
Section7 = Chembox Hazards
ExternalMSDS =
MainHazards = corrosive
FlashPt = non-flammable
RPhrases = R39/23/24/25-35
SPhrases =
Section8 = Chembox Related
Function = acids
OtherFunctn = HF
SbF₅
Magic acid
OtherCations = NaPF₆, NaSbF₆
OtherAnions = HBF₄

Fluoroantimonic acid HSbF₆ is a mixture of hydrogen fluoride and antimony pentafluoride in various ratios.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.] 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 (20 quintillion) times stronger than 100% sulfuric acid. The reaction of hydrofluoric acid and antimony pentafluoride proceeds as follows

tructure

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⁻. [cite journal | author = Mootz, D.; Bartmann, K. | title = The Fluoronium Ions H₂F⁺ and H₃F₂⁺: Characterization by Crystal Structure Analysis | journal = Angewandte Chemie, International Edition in English | year = 1988 | volume = 27 | pages = 391–392 | doi = 10.1002/anie.198803911] 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: [cite journal | author = Bickel, A. F.; Gaasbeek, C. J.; Hogeveen, H.; Oelderik, J. M.; Platteeuw, J. C. | title = Chemistry and spectroscopy in strongly acidic solutions: reversible reaction between aliphatic carbonium ions and hydrogen | journal = Chemical Communications | year = 1967 | volume = 1967 | pages = 634–5 | doi = 10.1039/C19670000634] [cite journal | author = Hogeveen, H.; Bickel, A. F. | title = Chemistry and spectroscopy in strongly acidic solutions: electrophilic substitution at alkane-carbon by protons | journal = Chemical Communications | year = 1967 | volume = 1967 | pages = 635–6 | doi = 10.1039/C19670000635] :(CH₃)₃CH + H⁺ → (CH₃)₃C⁺ + H₂:(CH₃)₄C + H⁺ → (CH₃)₃C⁺ + CH₄

afety

HF-SbF₅ is rapidly and explosively decomposed by water. It reacts with virtually all known solvents. Solvents that have been proven to be compatible with HF-SbF₅ are SO₂ClF and liquified sulfur dioxide. Chlorofluorocarbons have also been used as solvents. Containers forHF-SbF₅ are made of PTFE.

References