| Fluoroantimonic acid | |
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Fluoroantimonic acid |
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| Identifiers | |
| CAS number | 16950-06-4 |
| PubChem | 21953576 |
| ChemSpider | 21241496 |
| EC number | 241-023-8 |
| Jmol-3D images | Image 1 |
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| Properties | |
| Molecular formula | SbHF6 |
| Molar mass | 236.76 g mol-1 |
| Exact mass | 235.902062306 g mol-1 |
| Appearance | Colourless liquid |
| Acidity (pKa) | −25 |
| Basicity (pKb) | 39 |
| Hazards | |
| R-phrases | R26, R29, R35 |
| Main hazards | Corrosive |
| NFPA 704 |
0
4
3
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| Related compounds | |
| Related acids | Antimony pentafluoride |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
Fluoroantimonic acid (HSbF6) is a mixture of hydrogen fluoride and antimony pentafluoride in various ratios.[1] The 1:1 combination forms the strongest known superacid, which has been demonstrated to protonate even hydrocarbons to afford carbocations and H2.[2]
The reaction of hydrogen fluoride (HF) and SbF5 is exothermic. HF, being a Lewis base, attacks the molecules of SbF5 to give an adduct. In the fluoroantimonic molecule, the anion is coordinated to the hydrogen, although the anion is formally classified as noncoordinating, because it is both a very weak nucleophile and a very weak base.
Despite the proton being called effectively "naked," it is in fact always attached to a fluorine through a very weak dative bond, similar to the hydronium cation.[3] However, the weakness of this bond accounts for the system's extreme acidity. Fluoroantimonic acid is 2×1019 (20 quintillion) times stronger than 100% sulfuric acid.[4]
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Two related products have been crystallised from HF-SbF5 mixtures, and both have been analyzed by single crystal X-ray crystallography. These salts have the formulas [H2F+][Sb2F11−] and [H3F2+][Sb2F11−]. In both salts the anion is Sb2F11−.[5] As mentioned above, SbF6− is classified as weakly basic; the larger monoanion Sb2F11− would be expected to be still weaker.
The following values are based upon the Hammett acidity function. Acidity is indicated by large negative values of H0.
This extraordinarily strong acid protonates nearly all organic compounds. In 1967, Bickel and Hogeveen showed that HF-SbF5 will remove H2 from isobutane and methane from neopentane:[6][7]
HF-SbF5 is rapidly and explosively decomposed by water. It reacts with virtually all known solvents.[1] Solvents that have been proven to be compatible with HF-SbF5 are SO2ClF and liquefied sulfur dioxide. Chlorofluorocarbons have also been used as solvents. Containers for HF-SbF5 are made of PTFE.