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 in excess of a quadrillion times stronger than 100% pure sulfuric acid, depending on the proportion of its ingredients. It has been shown to protonate even hydrocarbons to afford pentacoordinate carbocations (carbonium ions). Extreme care must be taken when handling fluoroantimonic acid. It is exceptionally corrosive, but can be stored in containers lined with PTFE (Teflon). Chemical composition. 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, [ (such as ), and SbF-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 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 basicity at all, is in fact the strongest Brønsted base in the mixture, protonating  to HF in the same way water protonates to HO in aqueous acid. 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 HF to HF, when present, by the Grotthuss mechanism. Two related products have been crystallized from HF-SbF 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"), which has been determined for different ratios of HF:SbF. While the "H" of pure HF is −15, addition of just 1 mol % of SbF lowers it to around −20. However, further addition of SbF results in rapidly diminishing returns, with the "H" reaching −21 at 10 mol%. The use of an extremely weak base as indicator shows that the lowest attainable "H", even with  > 50 mol % SbF, is somewhere between −21 and −23. The following "H" 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". 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. Sources often confuse the "H" value of fluoroantimonic acid with its p"K". 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 p"K" 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" 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"s 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 [HF]· 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 [HF]·. For example, H(CHBCl) 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·SbF will remove H from isobutane and methane from neopentane to form carbenium ions: It is also used in the manufacture of tetraxenonogold compounds. Materials compatible with fluoroantimonic acid as a solvent include SOClF, and sulfur dioxide; some chlorofluorocarbons have also been used. Containers for HF/SbF are made of PTFE. Safety. HF/SbF 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. Safety gear must be worn at all times when handling or going anywhere near this corrosive substance, as fluoroantimonic acid can protonate nearly every compound in the human body.