fluoroboric acid

Fluoroboric acid or tetrafluoroboric acid (archaically, fluoboric acid) is an inorganic compound with the chemical formula ⁺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 (oxonium tetrafluoroborate), although more realistically, several water molecules solvate the proton: (H₂O)''_n''⁺BF₄⁻]. The ethyl ether solvate is also commercially available: (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).

It is mainly produced as a precursor to other fluoroborate salts.Gregory K. Friestad, Bruce P. Branchaud "Tetrafluoroboric Acid" E-Eros Encyclopedia of Reagents for Organic Synthesis. 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.

Structure and production

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: ⁺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 ₂F⁺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 , and the tetrahedral 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:
: B(OH)₃ + 4 HF → H₃O⁺ + + 2 H₂O
Anhydrous solutions can be prepared by treatment of aqueous fluoroboric acid with acetic anhydride.

Acidity

The acidity of fluoroboric acid is complicated by the fact that the name refers to several different species H(OEt₂)⁺BF, H₃O⁺BF, and HF^.BF₃ – each with a different acidity. The aqueous pK_a is quoted as −0.44. Titration of NBuBF in acetonitrile solution indicates that HBF₄, i.e., HF^.BF₃, has a pK_a of 1.6 in that solvent. Its acidity is thus comparable to that of fluorosulfonic acid.

Applications

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.

Organic chemistry

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 (), triphenylmethyl tetrafluoroborate (), triethyloxonium tetrafluoroborate (), and benzenediazonium tetrafluoroborate ().

Electroplating

Solutions of HBF₄ are used in the electroplating of tin and tin alloys. In this application, methanesulfonic acid is displacing the use of HBF₄. Fluoroboric acid is also used for high-speed electroplating of copper in fluoroborate baths.

Safety

HBF₄ is toxic and attacks skin and eyes. It attacks glass. It hydrolyzes, releasing corrosive, volatile hydrogen fluoride.

Other fluoroboric acids

A series of fluoroboric acids is known in aqueous solutions. The series can be presented as follows:
* H (OH)₄* H F(OH)₃* H F₂(OH)₂* H F₃(OH)* H F₄

See also

* Fluorosulfuric acid
* Fluoroantimonic acid

References

Further reading

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External links

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{{fluorine compounds

Tetrafluoroborates
Mineral acids