Fluoroantimonic acid

Chemical composition

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

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, (such as ), and -adducts of fluoride, (such as ). Thus, the formula "" is a convenient but oversimplified approximation of the true composition.

Nevertheless, the extreme acidity of this mixture is evident from the inferior 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 in the same way water protonates to in aqueous acid. It is the fluoronium ion that accounts for fluoroantimonic acid's extreme acidity. The protons easily migrate through the solution, moving from to HF, when present, by the Grotthuss mechanism.

Two related products have been crystallized from mixtures, and both have been analyzed by single crystal X-ray crystallography. These salts have the formulas and . In both salts, the anion is . As mentioned above, is weakly basic; the larger anion is expected to be a still weaker base.

Acidity

Fluoroantimonic acid is the strongest superacid based on the measured value of its Hammett acidity function (H), which has been determined for various ratios of HF:. The H of HF is (Instead of around -11 as previously determined) Gillespie et al. accurately measured the Hammett acidity of a series of pentafluorides in anhydrous hydrogen fluoride in 1988, demonstrating that the anhydrous hydrogen fluoride solution of pentafluoride (i.e. "fluoroantimonic acid") has stronger acidity than the fluorosulfonic acid solution. Solutions of HF have H values ranging from to as the molar percentage of rises from to over . The lowest attained H is about −28 (although some sources have reported values below −31.)

The following H values provide a comparison to other superacids.

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

The H 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 pK 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 pK 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 pKs 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 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 . For example, has an experimentally determined GPA of 241 kcal/mol.

Reactions

Fluoroantimonic acid solution is so reactive that it is challenging to identify media where it is unreactive. Materials compatible as solvents for fluoroantimonic acid include sulfuryl chloride fluoride (), and sulfur dioxide (); some chlorofluorocarbons have also been used. Containers for are made of PTFE.

Fluoroantimonic acid solutions decompose when heated, generating free hydrogen fluoride gas and liquid antimony pentafluoride at a temperature of 40 C.

As a superacid, fluoroantimonic acid solutions protonate nearly all organic compounds, often causing dehydrogenation, or dehydration. In 1967, Bickel and Hogeveen showed that reacts with isobutane and neopentane to form carbenium ions: : :

It is also used in the synthesis of tetraxenonogold complexes.

Safety

is a highly corrosive substance that reacts violently with water. Heating it is dangerous as well, as it decomposes into toxic hydrogen fluoride.

References

References

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2. {{Sigma-Aldrich
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