Fluoroantimonic acid

Fluoroantimonic acid

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Names
IUPAC name Fluoroantimonic acid
Systematic IUPAC name Fluoranium hexafluorostibanuide Fluoranium hexafluoridoantimonate(1−)
Identifiers
CAS Number	16950-06-4 N
ChemSpider	32741664 N
EC Number	241-023-8
Jmol 3D model	Interactive image
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
Chemical formula	H 2SbF 7
Molar mass	256.765
Appearance	Colorless liquid
Density	2.885 g/cm3
Solubility	SO2ClF, SO2
Acidity (pKa)	−25
Basicity (pKb)	39
Vapor pressure	{{{value}}}
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 (systematically named fluoranium hexafluorostibanuide and fluoranium hexafluoridoantimonate(1−)) is an inorganic compound with the chemical formula H
₂FSbF
₆ (also written H
₂F[SbF
₆], 2HF•SbF₅, or simply HF-SbF₅). It is an ionic liquid created by reacting hydrogen fluoride (HF) with antimony pentafluoride (SbF₅) in a stoichiometric ratio of 2:1. It is the strongest known superacid, which has been demonstrated to protonate even hydrocarbons to afford carbocations and H₂. Similar acids can be created by using excess antimony pentafluoride.

The reaction to produce fluoroantimonic acid is:

2 HF $\rightleftharpoons$ H₂F⁺ + F⁻

SbF₅ + F⁻ → SbF₆⁻

The overall reaction is:

SbF₅ + 2 HF → SbF₆⁻ + H₂F⁺

The second reaction is not in equilibrium, therefore the overall reaction is not in equilibrium. The reaction is exothermic.

The F⁻, produced by autoionization of hydrogen fluoride, reacts with SbF₅ to yield the adduct SbF₆⁻ In the fluoronium ion, hydrogen fluoride is coordinated to the hydrogen ion. The anion is classified as noncoordinating and is both a very weak nucleophile and a very weak base.

The acid is often said to contain "naked protons", but the "free" protons are, in fact, always bonded to hydrogen fluoride molecules to make the fluoronium cations (similar to the hydronium cation in aqueous solution). It is the fluoronium ion that accounts for fluoroantomonic acid's extreme acidity. Fluoroantimonic acid is 10¹⁶ (10 quadrillion) times stronger than 100% sulfuric acid. The protons easily migrate through the solution, moving from H₂F⁺ to HF, when present, by the Grotthuss mechanism:

H₂F⁺ + HF $\rightleftharpoons$ HF + H₂F⁺

Fluoroantimonic acid thermally decomposes at higher temperatures, generating hydrogen fluoride gas.

Structure

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⁻. As mentioned above, SbF₆⁻ is weakly basic; the larger anion Sb₂F₁₁⁻ is expected to be still weaker.

The following values are based upon the Hammett acidity function. Increased acidity is indicated by smaller (in this case, more negative) values of H₀.

• Fluoroantimonic acid (1990) (H₀ Value = −31.3)
• Magic acid (1974) (H₀ Value = −19.2)
• Carborane acid (1969) (H₀ Value = −18.0)
• Fluorosulfuric acid (1944) (H₀ Value = −15.1)
• Triflic acid (1940) (H₀ Value = −14.9)

Applications

Main article: Superacid § Applications

This extraordinarily strong acid protonates nearly all organic compounds. In 1967, Bickel and Hogeveen showed that 2HF•SbF₅ will remove H₂ from isobutane and methane from neopentane to form carbenium ions:

(CH₃)₃CH + H⁺ → (CH₃)₃C⁺ + H₂

(CH₃)₄C + H⁺ → (CH₃)₃C⁺ + CH₄

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.

Safety

HF-SbF₅ is extremely corrosive, toxic, and moisture sensitive. Like most strong acids, fluoroantimonic acid can react violently with water, owing to the exothermic hydration. Consequently, it cannot be used in aqueous solution, only in hydrofluoric solution.

See also

• Fluoroboric acid
• Fluorosulfuric acid
• Hexafluorophosphoric acid