Fluoroantimonic Acid

Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony pentafluoride, containing various cations and anions (the simplest being and ). This substance is a superacid that can be over a billion times stronger than 100% pure sulfuric acid in terms of its protonating ability measured by Hammett function. It even protonates some hydrocarbons to afford pentacoordinate carbocations ( carbonium ions). Fluoroantimonic acid is corrosive. For example, it cannot be contained directly in glass carboys, as it attacks glass, but can be stored in containers lined with PTFE (Teflon).

Chemical composition

Fluoroantimonic acid is formed by combining hydrogen fluoride and antimony pentafluoride:
:SbF₅ + 2 HF + 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, (such_as_),_and_SbF₅-adducts_of_fluoride,_[(SbF₅)''_n''Fsup>–_(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_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_ (such_as_),_and_SbF₅-adducts_of_fluoride,_[(SbF₅)''_n''Fsup>–_(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_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_base">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_"hydrogen#Protons_and_acids.html" ;"title="Brønsted_base.html" ;"title="SbF₅)''_n''F.html" ;"title="(such as ), and SbF₅-adducts of fluoride, [(SbF₅)''_n''F">(such as ), and SbF₅-adducts of fluoride, [(SbF₅)''_n''Fsup>– (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 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 base">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 "hydrogen#Protons and acids">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.

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 and . In both salts, the anion is . As mentioned above, is weakly basic; the larger anion is expected to be weaker still.

Acidity

Fluoroantimonic acid is the strongest superacid based on the measured value of its Hammett acidity function (''H''₀), which has been determined for different ratios of HF:SbF₅. While the ''H''₀ 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 ''H''₀ reaching −21 at 10 mol%. The use of an extremely weak base as indicator shows that the lowest attainable ''H''₀, even with > 50 mol % SbF₅, is somewhere between −21 and −23. The following ''H''₀ values show that fluoroantimonic acid is much stronger than other superacids. Increased acidity is indicated by smaller (in this case, more negative) values of ''H''₀.
*Fluoroantimonic acid (−23 < ''H''₀ < −21)
*Magic acid (''H''₀ = −19.2)
*Carborane acid (''H''₀ < −18)
*Fluorosulfuric acid (''H''₀ = −15.1)
*Triflic acid (''H''₀ = −14.9)

Of the above, only the carborane acids, whose ''H''₀ could not be directly determined due to their high melting points, may be stronger acids than fluoroantimonic acid.

Sources often confuse the ''H''₀ value of fluoroantimonic acid with its p''K''_a. The ''H''₀ 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 p''K''_a 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 p''K''_a 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 p''K''_as 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 ₂Fsup>+· 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 ₂Fsup>+·. For example, H(CHB₁₁Cl₁₁) has an experimentally determined GPA of 241 kcal/mol.

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.

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:
:(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. 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. The main method of containment involves storage in a PTFE container as it dissolves glass and many other materials.

See also

*Fluoroboric acid
*Fluorosulfuric acid
*Hexafluorophosphoric acid

References

{{Fluorine compounds

Antimony(V) compounds
Fluoro complexes
Inorganic compounds
Superacids
Mineral acids
Ionic liquids