Fluoroboric acid

Chembox new
Name = Fluoroboric acid
ImageFile =
ImageName =
IUPACName = Tetrafluoroboric acid
OtherNames = Fluoroboric acid; Hydrogen Tetrafluoroborate; Hydrofluoroboric acid; Borofluoric acid
Section1 = Chembox Identifiers
CASNo = 16872-11-0
RTECS = ED2685000
Section2 = Chembox Properties
Formula = HBF₄
MolarMass = 87.8126 g/mol
Appearance = Clear liquid
Density = 1.36 g/mL, liquid
3.0 g/L, gas
Solubility = Miscible
MeltingPt = -90 °C
BoilingPt = 130 °C
pKa = -0.4
Section3 = Chembox Structure
CrystalStruct = N/A
Section7 = Chembox Hazards
ExternalMSDS = [http://www.jtbaker.com/msds/englishhtml/f2608.htm External MSDS]
MainHazards = Corrosive
FlashPt = nonflammable
RPhrases = 34
SPhrases = 26-36/37/38-45
Section8 = Chembox Related
OtherAnions = Hexafluorophosphoric acid, Triflic acid
OtherCpds = Potassium fluoroborate, nitrosonium fluoroborate, Hexafluorophosphate, Hydrogen fluoride

Fluoroboric acid is the chemical compound with the formula HBF₄. It is the conjugate acid of tetrafluoroborate. It is available commercially as a solution in water and other solvents such as diethyl ether. With a strength comparable to nitric acid, fluoroboric acid is a strong acid with a weakly coordinating, non-oxidizing conjugate base.

Production

Pure fluoroboric acid is unknown. Aqueous solutions of HBF₄ can be produced by combining dissolving boric acid in hydrofluoric acid solution in aqueous solution at 20-25 °C. [OrgSynth | author = Flood, D. T. | title = Fluorobenzene | collvol = 2 | collvolpages = 295 | prep = CV2P0295] Three equivalents of HF react to give the intermediate boron trifluoride and the fourth gives fluoroboric acid.:: B(OH)₃ + 4 HF → H₃O⁺ + BF₄^- + 2 H₂O

Aqueous solutions of fluoroboric acid can also be prepared by treating impure hexafluorosilicic acid with solid boric acid followed by removal of precipitated silicon dioxide.Fact|date=September 2007 Anhydrous solutions can be prepared by treatment with acetic anhydride. [Gregory K. Friestad, Bruce P. Branchaud, "Tetrafluoroboric Acid" in "Encyclopedia of Reagents for Organic Synthesis" 2001 John Wiley & Sons. DOI: 10.1002/047084289X.rt035.]

Salts

Fluoroboric acid is the principal precursor to fluoroborate salts, which are typically prepared by acid-base reactions. The inorganic salts are intermediates in the manufacture of flame-retardant materials, glazing frits, and in electrolytic generation of boron. HBF₄ is also used in aluminum etching and acid pickling.

Applications

Organic chemistry

HBF₄ is used as a catalyst in 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, while neatFact|date=September 2007 fluoroboric acid removes tert-butoxycarbonyl groups.

Galvanic cells

Aqueous HBF₄ is used as an electrolyte in galvanic cell oxygen sensor systems which consist of an anode, cathode, and oxygen-permeable membrane. The solution of HBF₄ is able to dissolve lead(II) oxide from the anode in the form of lead tetrafluoroborate while leaving the rest of the system

Metal plating

A mixture of CrO₃, HBF₄, and sulfonic acids in conjunction with a cathode treatment give tin-plated steel. Tin(I) fluoroborate/fluoroboric acid mixtures and organic reagents are used as the electrolyte in the cathode treatment of the tin plating process. Similar processes of electrodeposition and electrolytic stripping are used to obtain specific metal alloys.

Other fluoroboric acids

A series of fluoroboric acids is known in aqueous solutions. The series can be presented as follows [Greenwood&Earnshaw] :
* H [B(OH)₄]
* H [BF(OH)₃]
* H [BF₂(OH)₂]
* H [BF₃(OH)]
* H [BF₄]

References

* Albert, R.; Dax, K.; Pleschko, R.; Stutz, A. E. "Carbohydr. Res." 1985, "137", 282-290.
* Bandgar, B. P.; Patil, A. V.; Chavan, O.S. "J. Mol. Catal. A: Chem." 2006, "256", 99-105.
* Heintz, R. A.; Smith, J. A.; Szalay, P. S.; Weisgerber, A.; Dunbar, K. R. "Inorg. Synth." 2002, "33", 75.
* Housecroft, C E.; Sharpe, A. G. "Inorganic Chemistry." 2nd ed. New York: Pearson Prentice Hall, 2005. 307.
* Meller, A. "Gmelin Handbook of Inorganic Chemistry: Boron." Vol. 3. New York: Springer-Verlag, 1988. 301-310.
* Perry, D. L.; Phillips, S. L. "Handbook of Inorganic Compounds." Boca Raton: CRC Press, 1995. 1203.
* Wamser, C. A. "J. Am. Chem. Soc." 1948, "70", 1209-1215.
* Wilke-Doerfurt, E.; Balz, G. Z. "Z. Anorg. Allg. Chem." 1927, "159", 224.

External links

* [http://www.inchem.org/documents/icsc/icsc/eics1040.htm INCHEM: Fluoroboric Acid (ICSC)]