Fluoroantimonic acid

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, 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.[2] 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.[3] 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.[4] As mentioned above, SbF⁻
₆ is weakly basic; the larger anion Sb
₂F⁻
₁₁ is expected to be still weaker.

Fluoroantimonic acid is the strongest superacid 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.[5][6][7] The following H₀ values show that fluoroantimonic acid is much stronger than other superacids.[8] 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.[8][9]

Sources often confuse the H₀ value of fluoroantimonic acid with its pK_a. 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 pK_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 pK_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.[2] (Solution-phase pK_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 [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.[10] 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.[11]

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. [12]

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:[13][14]

(CH₃)₃CH + H⁺ → (CH₃)₃C⁺ + H₂

(CH₃)₄C + H⁺ → (CH₃)₃C⁺ + CH₄

It is also used in the manufacture of tetraxenon gold 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.

HF-SbF₅ is an extremely corrosive and toxic substance that is sensitive to moisture.[9] As with most strong acids, fluoroantimonic acid can react violently with water due to the exothermic hydration. Only hydrogen fluoride can be used as a solvent for the acid, given that an aqueous solution can not be used. Heating fluoroantimonic acid is dangerous as well, as it decomposes into toxic fluorine gas.[15] The only method of containment involves storage in a PTFE container as glass will dissolve upon contact.[15] Safety gear must be worn at all times when handling or going anywhere near this corrosive substance, as fluoroantimonic acid can eat exposed flesh down to the bone while reacting violently with the water present in human cells.[15]

• Fluoroboric acid
• Fluorosulfuric acid
• Hexafluorophosphoric acid