Fluoroantimonic acid is an inorganic compound with the chemical formula H
6 (also written H
6], 2HF·SbF5, or simply HF-SbF5). It is an extremely strong acid, easily qualifying as a superacid. The Hammett acidity function, H0, has been measured 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.
|Systematic IUPAC name
3D model (JSmol)
|Main hazards||Extremely corrosive, Violent hydrolysis|
|GHS Signal word||Danger|
|H300, H310, H314, H330, H411|
|P260, P264, P273, P280, P284, P301+310|
|NFPA 704 (fire diamond)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
The "canonical" composition of fluoroantimonic acid is prepared by treating liquid hydrogen fluoride (HF) with liquid antimony pentafluoride (SbF5) in a stoichiometric ratio of 2:1. It is the strongest superacid based on measured H0 value. Only the carborane acids, whose H0 could not be directly determined due to their high melting points, may be stronger acids than fluoroantimonic acid. It has been shown to protonate even hydrocarbons to afford pentacoordinate carbocations (carbonium ions). It is exceptionally corrosive and can only be stored in containers lined with Teflon.
Fluoroantimonic acid thermally decomposes when heated, generating free hydrogen fluoride gas and liquid antimony pentafluoride. At temperatures as low as 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 .
The reaction to produce fluoroantimonic acid results in formation of the fluoronium ion as a major species in equilibrium:
- SbF5 + 2 HF ⇄ SbF−
6 + H2F+
However, as noted below, the speciation of "fluoroantimonic acid" is complex, and consists of a mixture of HF-solvated protons, [(HF)nH]+ (e.g., H3F2+), and SbF5-adducts of fluoride (e.g., Sb4F21–). Thus, the formula "[H2F]+SbF6–" is a convenient but oversimplified approximation of the true composition. Nevertheless, the extreme acidity of this mixture is evident from the exceptionally poor proton-accepting ability of the species present in solution (hydrogen fluoride, normally not thought to have any appreciable Brønsted basicity at all, is in fact the strongest Brønsted base in the mixture). 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. 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:
- H2F+ + HF ⇌ HF + H2F+
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. (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]+•SbF6– 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. However, certain carborane superacids have GPAs lower than that of [H2F]+•SbF6–. For example, H(CHB11Cl11) has an experimentally determined GPA of 241 kcal/mol.
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
11] and [H
11]. In both salts, the anion is Sb
11. As mentioned above, SbF−
6 is weakly basic; the larger anion Sb
11 is expected to be still weaker.
The following values show that fluoroantimonic acid is much stronger than other superacids based upon the Hammett acidity function. Increased acidity is indicated by smaller (in this case, more negative) values of H0.
This extraordinarily strong acid dehydrogenates nearly all organic compounds. In 1967, Bickel and Hogeveen showed that 2HF·SbF5 will remove H2 from isobutane and methane from neopentane to form carbenium ions:
- (CH3)3CH + H+ → (CH3)3C+ + H2
- (CH3)4C + H+ → (CH3)3C+ + CH4
It is also used in the manufacture of tetraxenon gold compounds.
HF-SbF5 is extremely corrosive, toxic, and moisture sensitive. Like most strong acids, fluoroantimonic acid can react violently with water, owing to the exothermic hydration. Consequently, it cannot be used in aqueous solution, only in hydrogen fluoride as solvent.
Fluoroantimonic acid cannot be stored in glass, as it will dissolve it. It must be stored in a PTFE (polytetrafluoroethylene) container.
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