Fluoroboric acid or tetrafluoroboric acid (archaically, fluoboric acid) is an inorganic compound with the chemical formula [H+][BF4−], where H+ represents the solvated proton. The solvent can be any suitably Lewis basic entity. For instance, in water, it can be represented by H
4 (oxonium tetrafluoroborate), although more realistically, several water molecules solvate the proton: [H(H2O)n+][BF4−]. The ethyl ether solvate is also commercially available: [H(Et2O)n+][BF4−], where n is most likely 2. Unlike other strong acids like H2SO4 or HClO4, the pure unsolvated substance does not exist (see below).
|Preferred IUPAC name
tetrafluoroboric acid, oxonium tetrafluoroboranuide, oxonium tetrafluoridoborate(1-)
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||87.81 g·mol−1|
|Melting point||−90 °C (−130 °F; 183 K)|
|Boiling point||130 °C (266 °F; 403 K)|
|Acidity (pKa)||~1.8 (MeCN solution)|
|P260, P264, P280, P301+P330+P331, P303+P361+P353, P304+P340, P305+P351+P338, P310, P321, P363, P405, P501|
|NFPA 704 (fire diamond)|
|Safety data sheet (SDS)||External MSDS|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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It is mainly produced as a precursor to other fluoroborate salts. It is a strong acid. Fluoroboric acid is corrosive and attacks the skin. It is available commercially as a solution in water and other solvents such as diethyl ether. It is a strong acid with a weakly coordinating, non-oxidizing conjugate base. It is structurally similar to perchloric acid, but lacks the hazards associated with oxidants.
Pure HBF4 has been described as a "nonexistent compound", as a sufficiently 'naked' proton is expected to abstract a fluoride from the tetrafluoroborate ion to give hydrogen fluoride and boron trifluoride: [H+][BF4–] → HF + BF3. (The same holds true for the superacids that are known by the simple formulas HPF6 and HSbF6.) However, a solution of BF3 in HF is highly acidic, having an approximate speciation of [H2F+][BF4–] and a Hammett acidity function of –16.6 at 7 mol % BF3, easily qualifying as a superacid. Although the solvent-free HBF4 has not been isolated, its solvates are well characterized. These salts consist of protonated solvent as a cation, e.g., H3O+ and H
2, and the tetrahedral BF−
4 anion. The anion and cations are strongly hydrogen-bonded.
Aqueous solutions of HBF4 are produced by dissolving boric acid in aqueous hydrofluoric acid. Three equivalents of HF react to give the intermediate boron trifluoride and the fourth gives fluoroboric acid:
The acidity of fluoroboric acid is complicated by the fact that the name refers to several different species H(OEt2)+BF−
4, and HF.BF3 – each with a different acidity. The aqueous pKa is quoted as −0.44. Titration of NBu+
4 in acetonitrile solution indicates that HBF4, i.e., HF.BF3, has a pKa of 1.6 in that solvent. Its acidity is thus comparable to that of fluorosulfonic acid.
Fluoroboric acid is the principal precursor to fluoroborate salts, which are typically prepared by treating the metal oxides with fluoroboric acid. The inorganic salts are intermediates in the manufacture of flame-retardant materials and glazing frits, and in electrolytic generation of boron. HBF4 is also used in aluminum etching and acid pickling.
HBF4 is used as a catalyst for alkylations and polymerizations. In carbohydrate protection reactions, ethereal fluoroboric acid is an efficient and cost-effective catalyst for transacetalation and isopropylidenation reactions. Acetonitrile solutions cleave acetals and some ethers. Many reactive cations have been obtained using fluoroboric acid, e.g. tropylium tetrafluoroborate (C
4), triphenylmethyl tetrafluoroborate (Ph
4), triethyloxonium tetrafluoroborate (Et
4), and benzenediazonium tetrafluoroborate (PhN+
A series of fluoroboric acids is known in aqueous solutions. The series can be presented as follows: