Fluoroantimonic Acid
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Fluoroantimonic acid is a mixture of
hydrogen fluoride Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...
and
antimony pentafluoride Antimony pentafluoride is the inorganic compound with the formula Sb F5. This colorless, viscous liquid is a strong Lewis acid and a component of the superacid fluoroantimonic acid, formed upon mixing liquid HF with liquid SbF5 in 1:1 ratio. It ...
, containing various cations and anions (the simplest being and ). This mixture is a
superacid In chemistry, a superacid (according to the original definition) is an acid with an acidity greater than that of 100% pure sulfuric acid (), which has a Hammett acidity function (''H''0) of −12. According to the modern definition, a superacid i ...
stronger than pure
sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
, by many orders of magnitude, according to its Hammett acidity function. It even protonates some
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...
s to afford pentacoordinate
carbocation Carbocation is a general term for ions with a positively charged carbon atom. In the present-day definition given by the IUPAC, a carbocation is any even-electron cation with significant partial positive charge on a carbon atom. They are further ...
s ( carbonium ions). Like its precursor
hydrogen fluoride Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...
, it attacks glass, but can be stored in containers lined with
PTFE Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene, and has numerous applications because it is chemically inert. The commonly known brand name of PTFE-based composition is Teflon by Chemours, a spin-off fro ...
(Teflon) or PFA.


Chemical composition

Fluoroantimonic acid is formed by combining
hydrogen fluoride Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...
and
antimony pentafluoride Antimony pentafluoride is the inorganic compound with the formula Sb F5. This colorless, viscous liquid is a strong Lewis acid and a component of the superacid fluoroantimonic acid, formed upon mixing liquid HF with liquid SbF5 in 1:1 ratio. It ...
: :SbF5 + 2 HF + 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, (such as ), and SbF5-adducts of fluoride, [(SbF5)''n''Fsup>– (such as ). Thus, the formula "" 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 the solution. Hydrogen fluoride, a weak acid in aqueous solution that is normally not thought to have any appreciable Brønsted base">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 and . In both salts, the anion is . As mentioned above, is weakly basic; the larger anion is expected to be a still weaker base.


Acidity

Fluoroantimonic acid is the strongest
superacid In chemistry, a superacid (according to the original definition) is an acid with an acidity greater than that of 100% pure sulfuric acid (), which has a Hammett acidity function (''H''0) of −12. According to the modern definition, a superacid i ...
based on the measured value of its Hammett acidity function (''H''0), which has been determined for various ratios of HF:SbF5. The ''H''0 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 the
fluorosulfonic acid Fluorosulfuric acid (IUPAC name: sulfurofluoridic acid) is the inorganic compound with the chemical formula . It is one of the strongest acids commercially available. It is a tetrahedral molecule and is closely related to sulfuric acid, , substitu ...
solution. A solution of HF containing 1 mol % of SbF5 is −20. The ''H''0 is −21 for 10 mol%. For > 50 mol % SbF5, the ''H''0 is between −21 and −23. The lowest attained H0 is about −28 (although some sources have reported to reach below −31.) The following ''H''0 values show that fluoroantimonic acid is stronger than other superacids. Increased acidity is indicated by lower (in this case, more negative) values of ''H''0. * Fluoroantimonic acid (−23 > ''H''0 > −28) *
Magic acid Magic acid () is a superacid consisting of a mixture, most commonly in a 1:1 molar ratio, of fluorosulfuric acid () and antimony pentafluoride (). This conjugate Brønsted acid, Brønsted–Lewis acid, Lewis superacid system was developed in the 1 ...
(''H''0 = −23) * Carborane acid (''H''0 < −18) * Fluorosulfuric acid (''H''0 = −15) * Triflic acid (''H''0 = −15) * Perchloric acid (''H''0 = −13) Of the above, only the carborane acids, whose ''H''0 could not be directly determined due to their high melting points, may be stronger acids than fluoroantimonic acid. The ''H''0 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 p''K''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 p''K''a 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 p''K''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 2Fsup>+· 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. However, certain carborane superacids have GPAs lower than that of 2Fsup>+·. For example, H(CHB11Cl11) has an experimentally determined GPA of 241 kcal/mol.


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 A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...
include SO2ClF, and
sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...
; some
chlorofluorocarbon Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly Halogenation, halogenated hydrocarbons that contain carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F). They are produced as volatility (chemistry), volat ...
s have also been used. Containers for HF/SbF5 are made of
PTFE Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene, and has numerous applications because it is chemically inert. The commonly known brand name of PTFE-based composition is Teflon by Chemours, a spin-off fro ...
. Fluoroantimonic acid solutions decompose when heated, generating free
hydrogen fluoride Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...
gas and liquid
antimony pentafluoride Antimony pentafluoride is the inorganic compound with the formula Sb F5. This colorless, viscous liquid is a strong Lewis acid and a component of the superacid fluoroantimonic acid, formed upon mixing liquid HF with liquid SbF5 in 1:1 ratio. It ...
at a temperature of 40 °C. As a
superacid In chemistry, a superacid (according to the original definition) is an acid with an acidity greater than that of 100% pure sulfuric acid (), which has a Hammett acidity function (''H''0) of −12. According to the modern definition, a superacid i ...
, fluoroantimonic acid solutions protonate nearly all
organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
s, often causing dehydrogenation, or dehydration. In 1967, Bickel and Hogeveen showed that 2HF·SbF5 reacts with isobutane and
neopentane Neopentane, also called 2,2-dimethylpropane, is a double-branched-chain alkane with five carbon atoms. Neopentane is a flammable gas at room temperature and pressure which can condense into a highly volatile liquid on a cold day, in an ice bat ...
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 Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...
. With superacids that are fuming and toxic, proper personal protective equipment should be used. In addition to the obligatory
glove A glove is a garment covering the hand, with separate sheaths or openings for each finger including the thumb. Gloves protect and comfort hands against cold or heat, damage by friction, abrasion or chemicals, and disease; or in turn to provide a ...
s and
goggles Goggles, or safety glasses, are forms of protective eyewear that usually enclose or protect the area surrounding the eye in order to prevent particulates, water or chemicals from striking the eyes. They are used in chemistry laboratories and ...
, the use of a
face shield A face shield, an item of personal protective equipment, aims to protect the wearer's entire face (or part of it) from hazards such as Projectile, flying objects and road debris, chemical splashes (in laboratory, laboratories or in Industry (eco ...
and
respirator A respirator is a device designed to protect the wearer from inhaling hazardous atmospheres including lead, lead fumes, vapors, gases and particulate matter such as dusts and airborne pathogens such as viruses. There are two main categories o ...
are also required. Regular lab gloves are not recommended, as this acid can react with the gloves. Safety gear must be worn at all times when handling or going anywhere near the substance, as fluoroantimonic acid can protonate every compound in the human body.


See also

* Fluoroboric acid * Fluorosulfuric acid * Hexafluorophosphoric acid


References

{{Fluorine compounds Antimony(V) compounds Fluoro complexes Inorganic compounds Superacids Mineral acids Ionic liquids