Fluoroantimonic acid
Chemical compound
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Chemical composition
Fluoroantimonic acid is formed by combining hydrogen fluoride and antimony pentafluoride:
- SbF5 + 2 HF ⇌ SbF−6 + 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, [(HF)nH]+ (such as H3F+2), and SbF5-adducts of fluoride, [(SbF5)nF]− (such as Sb4F−21). Thus, the formula "[H2F]+[SbF6]−" 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 H2F+ in the same way water protonates to H3O+ 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 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 [H2F+][Sb2F−11] and [H3F+2][Sb2F−11]. In both salts, the anion is Sb2F−11. As mentioned above, SbF−6 is weakly basic; the larger anion Sb2F−11 is expected to be a still weaker base.
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Acidity
Fluoroantimonic acid is the strongest superacid based on the measured value of its Hammett acidity function (H0), which has been determined for various ratios of HF:SbF5. The H0 of HF is −15.1±0.1 Solutions of HF have H0 values ranging from −20 to −22±1 as the molar percentage of SbF5 rises from 1% to over 50%. The lowest attained H0 is about −28 (although some sources have reported values below −31.)
The following H0 values provide a comparison to other superacids.
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.
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 (GPA) of individual species present in the mixture have been calculated using density functional theory methods. (Solution-phase pKas 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 1,060 kJ/mol. For comparison, the commonly encountered superacid triflic acid, TfOH, is a substantially weaker acid by this measure, with a GPA of 1,250 kJ/mol. However, certain carborane superacids have GPAs lower than that of [H2F]+·[SbF6]−. For example, H(CHB11Cl11) has an experimentally determined GPA of 1,010 kJ/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 (SO2ClF), and sulfur dioxide (SO2); some chlorofluorocarbons have also been used. Containers for HF−SbF5 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 (104 °F).
As a superacid, fluoroantimonic acid solutions protonate nearly all organic compounds, often causing dehydrogenation, or dehydration. In 1967, Bickel and Hogeveen showed that 2HF·SbF5 reacts with isobutane and neopentane to form carbenium ions:
- (CH3)3CH + H+ → (CH3)3C+ + H2
- (CH3)4C + H+ → (CH3)3C+ + CH4
It is also used in the synthesis of tetraxenonogold complexes.
Safety
HF−SbF5 is a highly corrosive substance that reacts violently with water. Heating it is dangerous as well, as it decomposes into toxic hydrogen fluoride.