Fluoroantimonic acid

Fluoroantimonic acid

Fluoroantimonic acid stored in a PFA bottle
Names
IUPAC name Fluoroantimonic acid
Systematic IUPAC name Hexafluoroantimonic acid
Other names Fluoroantimonic(V) acid Hydrogen fluoroantimonate Fluoronium Fluoroantimonate Fluoronium Hexafluoroantimonate
Identifiers
CAS Number	16950-06-4 (HSbF6) N
3D model (JSmol)	Interactive image
ChemSpider	32741664 N
ECHA InfoCard	100.037.279
EC Number	241-023-8
PubChem CID	11118066 wrong formula
CompTox Dashboard (EPA)	DTXSID10894750
InChI InChI=1S/FH2.6FH.Sb/h1H2;6*1H;/q+1;;;;;;;+5/p-6 N Key: HBGBSIVYTBPVEU-UHFFFAOYSA-H N
SMILES [FH2+].F[Sb-](F)(F)(F)(F)F
Properties
Molar mass	236.756 g/mol
Appearance	Colorless fuming liquid
Density	2.885 g/cm3
Boiling point	40 °C (104 °F; 313 K) (decomposes)
Solubility in water	Reacts explosively
Solubility	SO2ClF, SO2
Related compounds
Related acids	Antimony pentafluoride Hydrogen fluoride Magic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony penta­fluoride, containing various cations and anions (the simplest being H
₂F⁺
and Sb F⁻
₆). This mixture is a superacid that, in terms of corrosiveness, is trillions of times stronger than pure sulfuric acid when measured by its Hammett acidity function. It even protonates some hydro­carbons to afford pentacoordinate carbo­cations (carbonium ions).^[1] Like its precursor hydrogen fluoride, it attacks glass, but can be stored in containers lined with PTFE (Teflon) or PFA.

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.^[2] 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 H₂F⁺ in the same way water protonates to H₃O⁺ 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.^[3] 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 [H
₂F⁺
] [Sb
₂F⁻
₁₁] and [H
₃F⁺
₂] [Sb
₂F⁻
₁₁]. In both salts, the anion is Sb
₂F⁻
₁₁.^[4] As mentioned above, SbF⁻
₆ is weakly basic; the larger anion Sb
₂F⁻
₁₁ is expected to be a still weaker base.

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. A solution of HF containing 1 mol % of SbF₅ is −20. The H₀ is −21 for 10 mol%. For > 50 mol % SbF₅, the H₀ is between −21 and −23. The lowest attained H₀ is about -28.^[5][6][7] The following H₀ values show that fluoroantimonic acid is stronger than other superacids.^[8] Increased acidity is indicated by smaller (in this case, more negative) values of H₀.

• Fluoroantimonic acid (−23 > H₀ > −28)
• Magic acid (H₀ = −23)
• Carborane acid (H₀ < −18)
• Fluorosulfuric acid (H₀ = −15)
• Triflic acid (H₀ = −15)
• Perchloric acid (H₀ = −13)

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.^[8][9]

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 pK_a 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 pK_a value is not well-defined.^{[citation needed]}

The gas-phase acidity (GPA) of individual species present in the mixture have been calculated using density functional theory methods.^[2] (Solution-phase pK_as 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]⁺·SbF^–
₆ 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.^[10] However, certain carborane superacids have GPAs lower than that of [H₂F]⁺·SbF^–
₆. For example, H(CHB₁₁Cl₁₁) has an experimentally determined GPA of 241 kcal/mol.^[11]

Reactions

Fluoroantimonic acid solution is so reactive that it is challenging to identify media with which it is unreactive. 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.^{[citation needed]}

Fluoroantimonic acid solutions decompose when heated, generating free hydrogen fluoride gas and liquid antimony pentafluoride at a temperature of 40 °C.^[12]

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:^[13][14]

(CH₃)₃CH + H⁺ → (CH₃)₃C⁺ + H₂

(CH₃)₄C + H⁺ → (CH₃)₃C⁺ + CH₄

It is also used in the synthesis of tetraxenonogold complexes.^[15]

Safety

Fluoroantimonic acid

Hazards
Occupational safety and health (OHS/OSH):
Main hazards	extremely corrosive, toxic, violent hydrolysis, oxidizer
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H240, H271, H290, H300+H310+H330, H314, H350, H410, H411, H441
Precautionary statements	P260, P264, P273, P280, P284, P301+P310
NFPA 704 (fire diamond)	4 0 4 W OX

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 gas. With superacids that are fuming and toxic, proper personal protective equipment should be used. In addition to the obligatory gloves and goggles, the use of a face shield and respirator are also required. Regular lab gloves are not recommended, as this acid can react with the gloves.^[9] Safety gear must be worn at all times when handling or going anywhere near this corrosive substance, as fluoroantimonic acid can protonate every compound in the human body.^[16]

See also

• Fluoroboric acid
• Fluorosulfuric acid
• Hexafluorophosphoric acid

References