Fluoroboric acid ortetrafluoroboric acid (archaically,fluoboric acid) is aninorganic compound with the simplifiedchemical formulaH+[BF4]−. Solvent-free tetrafluoroboric acid (H[BF4]) has not been reported. The term "fluoroboric acid" usually refers to a range of compounds includinghydronium tetrafluoroborate ([H3O]+[BF4]−), which are available as solutions. Theethyl ether solvate is also commercially available, where the fluoroboric acid can be represented by the formula[H((CH3CH2)2O)n]+[BF4]−, wheren is 2.
It is mainly produced as a precursor to other fluoroborate salts.[3] It is a strongacid. Fluoroboric acid is corrosive and attacks the skin. It is available commercially as a solution in water and othersolvents such asdiethyl ether. It is a strong acid with aweakly coordinating, non-oxidizing conjugate base.[2] It is structurally similar toperchloric acid, but lacks the hazards associated withoxidants.
PureH[BF4] has not been described. The same holds true for the superacids that are known by the simplified formulasH[PF6] andH[SbF6].[4][5] However, a solution ofBF3 in HF is highly acidic, having an approximate speciation of[H2F]+[BF4]− (fluoronium tetrafluoroborate) and aHammett acidity function of −16.6 at 7 mol %BF3, easily qualifying as a superacid.[6] Although the solvent-freeH[BF4] has not been isolated, its solvates are well characterized. These salts consist of protonated solvent as a cation, e.g.,H3O+ andH5O+2, and the tetrahedralBF−4 anion. The anion and cations are strongly hydrogen-bonded.[7]
Subunit of crystal structure of[H3O]+[BF4]− highlighting the hydrogen bonding between the cation and the anion
Aqueous solutions ofH[BF4] are produced by dissolvingboric acid in aqueoushydrofluoric acid.[8][9] Three equivalents of HF react to give the intermediate boron trifluoride and the fourth gives fluoroboric acid:
B(OH)3 + 4 HF → H3O+ + BF−4 + 2 H2O
An anhydrous fluoroboric acid solution can be prepared by adding aqueous fluoroboric acid to an excess ofacetic anhydride at 0°C, which produces a solution of fluoroboric acid,acetic acid, and residual acetic anhydride.[10]
The acidity of fluoroboric acid is complicated by the fact that its name refers to a range of different compounds, e.g.[H(CH3CH2)2O]+[BF4]− (dimethyloxonium tetrafluoroborate),[H3O]+[BF4]− (oxonium tetrafluoroborate), andHF·BF3 (hydrogen fluoride-boron trifluoride 1:1 adduct) – each with a different acidity. The aqueouspKa is quoted as −0.44.[3]Titration of[N((CH2)3CH3)4]+[BF4]− (tetrabutylammonium tetrafluoroborate) inacetonitrile solution indicates thatH[BF4], i.e.,HF·BF3, has a pKa of 1.6 in that solvent. Its acidity is thus comparable to that offluorosulfonic acid.[2]
Fluoroboric acid is the principal precursor tofluoroborate 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 glazingfrits, and in electrolytic generation ofboron.H[BF4] is also used in aluminum etching and acid pickling.
Solutions ofH[BF4] are used in the electroplating of tin and tin alloys. In this application,methanesulfonic acid is displacing the use ofH[BF4].[11] Fluoroboric acid is also used for high-speedelectroplating of copper in fluoroborate baths.[12]
^abcKütt, A., et al., "Equilibrium Acidities of Superacids", J. Org. Chem. 2010, volume 76, pp. 391-395.doi:10.1021/jo101409p
^abcGregory K. Friestad, Bruce P. Branchaud "Tetrafluoroboric Acid" E-Eros Encyclopedia of Reagents for Organic Synthesis.doi:10.1002/047084289X.rt035
^Juhasz, Mark; Hoffmann, Stephan; Stoyanov, Evgenii; Kim, Kee-Chan; Reed, Christopher A. (2004-10-11). "The Strongest Isolable Acid".Angewandte Chemie International Edition.43 (40):5352–5355.doi:10.1002/anie.200460005.ISSN1433-7851.PMID15468064.
^Olah, George A.; Surya Prakash, G. K.; Sommer, Jean; Molnar, Arpad (2009-02-03).Superacid chemistry. Olah, George A. (George Andrew), 1927-2017,, Olah, George A. (George Andrew), 1927-2017. (2nd ed.). Hoboken, N.J.ISBN9780471596684.OCLC191809598.{{cite book}}: CS1 maint: location missing publisher (link)
^Mootz, D.; Steffen, M. "Crystal structures of acid hydrates and oxonium salts. XX. Oxonium tetrafluoroborates H3OBF4, [H5O2]BF4, and [H(CH3OH)2]BF4",Zeitschrift für Anorganische und Allgemeine Chemie 1981, vol. 482, pp. 193-200.doi:10.1002/zaac.19814821124
^Wudl, F.; Kaplan, M. L., "2,2′-Bi-1,3-Dithiolylidene (Tetrathiafulvalene, TTF) and its Radical Cation Salts" Inorg. Synth. 1979, vol. 19, 27.doi:10.1002/9780470132500.ch7
^abBalaji, R.; Pushpavanam, Malathy (2003). "Methanesulphonic acid in electroplating related metal finishing industries".Transactions of the Imf.81 (5):154–158.doi:10.1080/00202967.2003.11871526.S2CID91584456.
Albert, R.; Dax, K.; Pleschko, R.; Stütz, A. E. (1985). "Tetrafluoroboric acid, an efficient catalyst in carbohydrate protection and deprotection reactions".Carbohydrate Research.137:282–290.doi:10.1016/0008-6215(85)85171-5.
Bandgar, B. P.; Patil, A. V.; Chavan, O. S. (2006). "Silica supported fluoroboric acid as a novel, efficient and reusable catalyst for the synthesis of 1,5-benzodiazepines under solvent-free conditions".Journal of Molecular Catalysis A: Chemical.256 (1–2):99–105.doi:10.1016/j.molcata.2006.04.024.
Heintz, R. A.; Smith, J. A.; Szalay, P. S.; Weisgerber, A.; Dunbar, K. R. (2002).Homoleptic Transition Metal Acetonitrile Cations with Tetrafluoroborate or Trifluoromethanesulfonate Anions. Inorganic Syntheses. Vol. 33. pp. 75–83.doi:10.1002/0471224502.ISBN9780471208259.
Housecroft, C. E.; Sharpe, A. G. (2004).Inorganic Chemistry (2nd ed.). Prentice Hall. p. 307.ISBN978-0-13-039913-7.
Meller, A. (1988). "Boron".Gmelin Handbook of Inorganic Chemistry. Vol. 3. New York: Springer-Verlag. pp. 301–310.
Perry, D. L.; Phillips, S. L. (1995).Handbook of Inorganic Compounds (1st ed.). Boca Raton: CRC Press. p. 1203.ISBN9780849386718.
Wamser, C. A. (1948). "Hydrolysis of Fluoboric Acid in Aqueous Solution".Journal of the American Chemical Society.70 (3):1209–1215.doi:10.1021/ja01183a101.
Wilke-Dörfurt, E.; Balz, G. (1927). "Zur Kenntnis der Borfluorwasserstoffsäure und ihrer Salze".Zeitschrift für Anorganische und Allgemeine Chemie.159 (1):197–225.doi:10.1002/zaac.19271590118.
