Fluoroantimonic acid


Fluoroantimonic acid is an inorganic compound with the chemical formula . This substance is part of an extremely strong group of superacids known for being stronger than 100% pure sulfuric acid. It has been shown to protonate even hydrocarbons to afford pentacoordinate carbocations. The "canonical" composition of fluoroantimonic acid is prepared by treating liquid hydrogen fluoride with liquid antimony pentafluoride in a stoichiometric ratio of 2:1. Extreme caution needs to be in place when handling fluoroantimonic acid. It is exceptionally corrosive and can only be stored in containers lined with PTFE.

Structure

The reaction to produce fluoroantimonic acid results in formation of the fluoronium ion as a major species in equilibrium:
However, the speciation of "fluoroantimonic acid" is complex, and consists of a mixture of HF-solvated protons, +, and SbF5-adducts of fluoride. Thus, the formula "+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. 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.
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 still weaker.

Acidity

Fluoroantimonic acid is the strongest superacid based on the measured value of its Hammett acidity function, 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. The following H0 values show that fluoroantimonic acid is much stronger than other superacids. Increased acidity is indicated by smaller values of H0.
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.
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 of individual species present in the mixture have been calculated using density functional theory methods. For example, the ion-pair +•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 +•SbF6–. For example, H 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 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.

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:
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.

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 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. The only method of containment involves storage in a PTFE container as glass will dissolve upon contact. Safety gear must be worn at all times when handling or going anywhere near this corrosive substance. Fluoroantimonic acid can eat exposed flesh down to the bone while reacting violently with water present in human blood cells.