Fluoroantimonic acid

Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony penta­fluoride, containing various cations and anions (the simplest being H
2F+
 and SbF−
6). It is a "superacid". In terms of its pKa, fluoroantimonic acid is 1015 times more acidic than sulfuric acid. It even protonates some hydro­carbons to afford pentacoordinate carbo­cations (carbonium ions).[1] Fluoro­antimonic acid is corrosive. For example, it cannot be contained directly in glass carboys, as it attacks glass,[2] but can be stored in containers lined with PTFE (Teflon).

Fluoroantimonic acid
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.037.279
EC Number
  • 241-023-8
CompTox Dashboard (EPA)
  • InChI=1S/FH2.6FH.Sb/h1H2;6*1H;/q+1;;;;;;;+5/p-6 N
    Key: HBGBSIVYTBPVEU-UHFFFAOYSA-H N
  • [FH2+].F[Sb-](F)(F)(F)(F)F
Properties
Appearance Colorless liquid
Density 2.885 g/cm3
Solubility SO2ClF, SO2
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
 Extremely corrosive, Violent hydrolysis
GHS labelling:
Danger
Hazard statements
 H300, H310, H314, H330, H411
Precautionary statements
 P260, P264, P273, P280, P284, P301+P310
NFPA 704 (fire diamond)
4
0
3
Related compounds
Related acids
 Antimony pentafluoride
Hydrogen fluoride
Magic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Chemical composition

Fluoroantimonic acid is formed by combining hydrogen fluoride and antimony pentafluoride:

SbF5 + 2 HF ⇌ SbF−
6 + H2F+

The speciation (i.e., the inventory of components) of "fluoroantimonic acid" is complex. Spectroscopic measurements show that fluoroantimonic acid consists of a mixture of HF-solvated protons, [(HF)nH]+ (such as H3F+2), and SbF5-adducts of fluoride, [(SbF5)nF]– (such as Sb4F−21). Thus, the formula "[H2F]+[SbF6]−" is a convenient but oversimplified approximation of the true composition.[3] Nevertheless, the extreme acidity of this mixture is evident from the exceptionally poor proton-accepting ability of the species present in solution. Hydrogen fluoride, a weak acid in aqueous solution that is normally not thought to have any appreciable Brønsted basicity at all, is in fact the strongest Brønsted base in the mixture, protonating to H2F+ in the same way water protonates to H3O+ in aqueous acid. As a result, the acid is often said to contain "naked protons", though the "free" protons are, in fact, always bonded to hydrogen fluoride molecules.[4] It is the fluoronium ion that accounts for fluoroantimonic acid's extreme acidity. The protons easily migrate through the solution, moving from H2F+ to HF, when present, by the Grotthuss mechanism.

Two related products have been crystallized from HF-SbF5 mixtures, and both have been analyzed by single crystal X-ray crystallography. These salts have the formulas [H
2F+
][Sb
2F−
11] and [H
3F+
2][Sb
2F−
11]. In both salts, the anion is Sb
2F−
11.[5] As mentioned above, SbF−
6 is weakly basic; the larger anion Sb
2F−
11 is expected to be weaker still.

Acidity

Fluoroantimonic acid is the second strongest superacid, second only to helonium, based on the measured value of its Hammett acidity function (H0), which has been determined for different ratios of HF:SbF5. While the H0 of pure HF is −15, addition of just 1 mol % of SbF5 lowers it to around −20. However, further addition of SbF5 results in rapidly diminishing returns, with the H0 reaching −21 at 10 mol%. The use of an extremely weak base as indicator shows that the lowest attainable H0, even with > 50 mol % SbF5, is somewhere between −21 and −23.[6][7][8] The following H0 values show that fluoroantimonic acid is much stronger than other superacids.[9] Increased acidity is indicated by smaller (in this case, more negative) values of H0.

  • Fluoroantimonic acid (−23 < H0 < −21)
  • Magic acid (H0 = −19.2)
  • Carborane acid (H0 < −18)
  • Fluorosulfuric acid (H0 = −15.1)
  • Triflic acid (H0 = −14.9)

Of the above, only the carborane acids, whose H0 could not be directly determined due to their high melting points, may be stronger acids than fluoroantimonic acid.[9][10]

Sources often confuse the H0 value of fluoroantimonic acid with its pKa. The H0 value measures the protonating ability of the bulk, liquid acid, and this value has been directly determined or estimated for various compositions of the mixture. The pKa on the other hand, measures the equilibrium of proton dissociation of a discrete chemical species when dissolved in a particular solvent. Since fluoroantimonic acid is not a single chemical species, its pKa value is not well-defined.

The gas-phase acidity (GPA) of individual species present in the mixture have been calculated using density functional theory methods.[3] (Solution-phase pKas of these species can, in principle, be estimated by taking into account solvation energies, but do not appear to be reported in the literature as of 2019.) For example, the ion-pair [H2F]+·SbF–
6 was estimated to have a GPA of 254 kcal/mol. For comparison, the commonly encountered superacid triflic acid, TfOH, is a substantially weaker acid by this measure, with a GPA of 299 kcal/mol.[11] However, certain carborane superacids have GPAs lower than that of [H2F]+·SbF–
6. For example, H(CHB11Cl11) has an experimentally determined GPA of 241 kcal/mol.[12]

Reactions

Fluoroantimonic acid thermally decomposes when heated, generating free hydrogen fluoride gas and liquid antimony pentafluoride. At a temperature of 40 °C, fluoroantimonic acid will release HF into the gas phase. Antimony pentafluoride liquid can be recovered from fluoroantimonic acid by heating and releasing HF into the gas phase. [13]

Applications
Main article: Superacid § Applications

This extraordinarily strong acid protonates nearly all organic compounds, often causing dehydrogenation, or dehydration. In 1967, Bickel and Hogeveen showed that 2HF·SbF5 will remove H2 from isobutane and methane from neopentane to form carbenium ions:[14][15]

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

It is also used in the manufacture of tetraxenonogold compounds.

Materials compatible with fluoroantimonic acid as a solvent include SO2ClF, and sulfur dioxide; some chlorofluorocarbons have also been used. Containers for HF/SbF5 are made of PTFE.

Safety

HF/SbF5 is an extremely corrosive and toxic substance that is sensitive to moisture.[10] As with most strong acids, fluoroantimonic acid can react violently with water due to exothermic hydration. Heating fluoroantimonic acid is dangerous as well, as it decomposes into toxic hydrogen fluoride gas.[2] The main method of containment involves storage in a PTFE container as it dissolves glass and many other materials.[2] Safety gear must be worn at all times when handling or going anywhere near this corrosive substance, as fluoroantimonic acid can protonate every compound in the human body.[2]

See also
  • Fluoroboric acid
  • Fluorosulfuric acid
  • Hexafluorophosphoric acid

References
  1. Olah, G. A. (2001). A Life of Magic Chemistry: Autobiographical Reflections of a Nobel Prize Winner. John Wiley and Sons. pp. 100–101. ISBN 978-0-471-15743-4.
  2. Anne Marie Helmenstine, Ph.D. "What Is the World's Strongest Superacid?". ThoughtCo. Retrieved April 13, 2022.
  3. Esteves, Pierre M.; Ramírez-Solís, Alejandro; Mota, Claudio J. A. (March 2002). "The Nature of Superacid Electrophilic Species in HF/SbF5: A Density Functional Theory Study". Journal of the American Chemical Society. 124 (11): 2672–2677. doi:10.1021/ja011151k. ISSN 0002-7863. PMID 11890818.
  4. Klein, Michael L. (October 25, 2000). "Getting the Jump on Superacids" (PDF). Pittsburgh Supercomputing Center (PSC). Archived from the original (PDF) on May 31, 2012. Retrieved 2012-04-15.
  5. Mootz, Dietrich; Bartmann, Klemens (March 1988). "The Fluoronium Ions H2F+ and H
    3    F+
    2    : Characterization by Crystal Structure Analysis". Angewandte Chemie International Edition. 27 (3): 391–392. doi:10.1002/anie.198803911.
  6. Superacid chemistry. Olah, George A. (George Andrew), 1927–2017., Olah, George A. (George Andrew), 1927–2017. (2nd ed.). Hoboken, N.J.: Wiley. 2009. ISBN 9780470421543. OCLC 391334955.{{cite book}}: CS1 maint: others (link)
  7. Olah, G. A. (2005). "Crossing Conventional Boundaries in Half a Century of Research". Journal of Organic Chemistry. 70 (7): 2413–2429. doi:10.1021/jo040285o. PMID 15787527.
  8. In ref. 2 (2005), Olah estimates that HF-SbF5 may reach H0 values as low as –28. On the other hand, in ref. 1 (2009), Olah cites one method that estimated H0 values down to –27 for FSO3H-SbF5 at 90% SbF5, but indicates that more reliable experimentally determined equilibrium constants do not support H0 values lower than about –24 for either magic acid or fluoroantimonic acid.
  9. Gillespie, R. J.; Peel, T. E. (1973-08-01). "Hammett acidity function for some superacid systems. II. Systems sulfuric acid-[fsa], potassium fluorosulfate-[fsa], [fsa]-sulfur trioxide, [fsa]-arsenic pentafluoride, [sfa]-antimony pentafluoride and [fsa]-antimony pentafluoride-sulfur trioxide". Journal of the American Chemical Society. 95 (16): 5173–5178. doi:10.1021/ja00797a013. ISSN 0002-7863.
  10. Olah, G. A.; Prakash, G. K. Surya; Wang, Qi; Li, Xing-ya (15 April 2001). "Hydrogen Fluoride–Antimony(V) Fluoride". Encyclopedia of Reagents for Organic Synthesis. New York: John Wiley and Sons. doi:10.1002/047084289X.rh037m. ISBN 9780470842898.
  11. Koppel, Ilmar A.; Burk, Peeter; Koppel, Ivar; Leito, Ivo; Sonoda, Takaaki; Mishima, Masaaki (May 2000). "Gas-Phase Acidities of Some Neutral Brønsted Superacids: A DFT and ab Initio Study". Journal of the American Chemical Society. 122 (21): 5114–5124. doi:10.1021/ja0000753. ISSN 0002-7863.
  12. Meyer, Matthew M.; Wang, Xue-bin; Reed, Christopher A.; Wang, Lai-Sheng; Kass, Steven R. (2009-12-23). "Investigating the Weak to Evaluate the Strong: An Experimental Determination of the Electron Binding Energy of Carborane Anions and the Gas phase Acidity of Carborane Acids". Journal of the American Chemical Society. 131 (50): 18050–18051. doi:10.1021/ja908964h. ISSN 0002-7863. PMID 19950932. S2CID 30532320.
  13. Oelderik, Jan (December 1966). "Werkwijze ter bereiding van halogeenverbindingen van vijfwaardig antimoon". Netherlands Patent Application. NL 6508096 A.
  14. 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 (13): 634–635. doi:10.1039/C19670000634.
  15. Hogeveen, H.; Bickel, A. F. (1967). "Chemistry and spectroscopy in strongly acidic solutions: electrophilic substitution at alkane-carbon by protons". Chemical Communications. 1967 (13): 635–636. doi:10.1039/C19670000635.
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