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 Fluoroantimonic acid stored in a PFA bottle | |
| Names | |
|---|---|
| IUPAC name Fluoroantimonic acid | |
| Systematic IUPAC name Hexafluoroantimonic acid | |
Other names
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| Identifiers | |
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3D model (JSmol) | |
| ChemSpider |
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| ECHA InfoCard | 100.037.279 |
| EC Number |
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| Properties | |
| Molar mass | 236.756 g/mol |
| Appearance | Colorless liquid |
| Density | 2.885 g/cm3 |
| Boiling point | 40 °C (104 °F; 313 K) (decomposes) |
| soluble | |
| Solubility insulfuryl chloride fluoride | soluble |
| Solubility insulfur dioxide | soluble |
| Vapor pressure | 19 hPa (18 °C (64 °F; 291 K)) |
| Hazards[1] | |
| GHS labelling:[2] | |
   | |
| Danger | |
| H300+H310+H330,H314,H411 | |
| P260,P262,P264,P270,P271,P273,P280,P284,P301+P310+P330,P301+P330+P331,P302+P350,P303+P361+P353,P304+P340+P310,P305+P351+P338+P310,P362,P391,P403+P233,P405,P501 | |
| NFPA 704 (fire diamond) | |
Threshold limit value (TLV) | 0.5 mg/m3 (TWA) |
| NIOSH (US health exposure limits):[2] | |
PEL (Permissible) | 0.5 mg/m3 |
REL (Recommended) | 0.5 mg/m3 (TWA) |
| Related compounds | |
Relatedacids | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Fluoroantimonic acid is a mixture ofhydrogen fluoride andantimony pentafluoride, containing various cations and anions, the simplest beingfluoronium (H2F+ andFluoroantimonate (SbF−6]]). The mixture is the strongest knownsuperacid, stronger than puresulfuric acid by many orders of magnitude, according to itsHammett acidity function. It evenprotonates somehydrocarbons to afford pentacoordinatecarbocations (carbonium ions).[3] Like its precursorhydrogen fluoride, it attacks glass, but can be stored in containers lined withPTFE (Teflon) orPFA.[citation needed]
Fluoroantimonic acid is formed by combininghydrogen fluoride andantimony pentafluoride:
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 asH3F+2), andSbF5-adducts of fluoride,[(SbF5)nF]− (such asSb4F−21). Thus, the formula "[H2F]+[SbF6]−" is a convenient but oversimplified approximation of the true composition.[4]
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 appreciableBrønsted basicity at all, is in fact the strongest Brønsted base in the mixture, protonating toH2F+ in the same way water protonates toH3O+ in aqueous acid. It is the fluoronium ion that accounts for fluoroantimonic acid's extreme acidity. The protons easily migrate through the solution, moving fromH2F+ to HF, when present, by theGrotthuss mechanism.[5]
Two related products have been crystallized fromHF−SbF5 mixtures, and both have been analyzed by single crystalX-ray crystallography. These salts have the formulas[H2F+] [Sb2F−11] and[H3F+2] [Sb2F−11]. In both salts, the anion isSb2F−11. As mentioned above,SbF−6 is weakly basic; the larger anionSb2F−11 is expected to be a still weaker base.[6]
Fluoroantimonic acid is the strongestsuperacid based on the measured value of itsHammett acidity function (H0), which has been determined for various ratios of HF:SbF5. TheH0 of HF is−15.1±0.1 (Instead of around -11 as previously determined) Gillespie et al. accurately measured the Hammett acidity of a series of pentafluorides in anhydrous hydrogen fluoride in 1988, demonstrating that the anhydrous hydrogen fluoride solution of pentafluoride (i.e. "fluoroantimonic acid") has stronger acidity than thefluorosulfonic acid solution.[7] Solutions of HF haveH0 values ranging from−20 to−22±1 as the molar percentage ofSbF5 rises from1% to over50%. The lowest attainedH0 is about −28 (although some sources have reported values below −31.)[8][9]
The followingH0 values provide a comparison to other superacids.[10]
| Acidities of selected superacids[a] | ||
|---|---|---|
| Compound | H0 (high value) | H0 (low value) |
| Fluoroantimonic acid | −23 | −28 |
| Magic acid | −23 | |
| Carborane acid | −18 | - |
| Fluorosulfuric acid | −15 | |
| Triflic acid | −15 | |
| Perchloric acid | −13 | |
Of the above, only thecarborane acids, whoseH0 could not be directly determined due to their high melting points, may be stronger acids than fluoroantimonic acid.[10][11]
TheH0 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.[citation needed]
Thegas-phase acidity (GPA) of individual species present in the mixture have been calculated using density functional theory methods.[4] (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 superacidtriflic acid, TfOH, is a substantially weaker acid by this measure, with a GPA of 1,250 kJ/mol.[12] However, certaincarborane superacids have GPAs lower than that of[H2F]+·[SbF6]−. For example,H(CHB11Cl11) has an experimentally determined GPA of 1,010 kJ/mol.[13]
Fluoroantimonic acid solution is so reactive that it is challenging to identify media where it is unreactive. Materials compatible as solvents for fluoroantimonic acid includesulfuryl chloride fluoride (SO2ClF), andsulfur dioxide (SO2); somechlorofluorocarbons have also been used. Containers forHF−SbF5 are made ofPTFE.[citation needed]
Fluoroantimonic acid solutions decompose when heated, generating freehydrogen fluoride gas and liquidantimony pentafluoride at a temperature of 40 °C (104 °F).[citation needed]
As asuperacid, fluoroantimonic acid solutions protonate nearly allorganic compounds, often causing dehydrogenation, or dehydration. In 1967, Bickel and Hogeveen showed that2HF·SbF5 reacts withisobutane andneopentane to formcarbenium ions:[14][15]
It is also used in the synthesis oftetraxenonogold complexes.[16]
HF−SbF5 is a highly corrosive substance that reacts violently with water. Heating it is dangerous as well, as it decomposes into toxichydrogen fluoride.
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