Fluoroboric acid

Fluoroboric acid

Canonical, skeletal formula of fluoroboric acid
Names
Preferred IUPAC name Tetrafluoroboric acid
Other names tetrafluoroboric acid, oxonium tetrafluoroboranuide, oxonium tetrafluoridoborate(1-)
Identifiers
CAS Number	(for H3OBF4), 16872-11-0 (solvent free), 80628-99-5 (for H5O2BF4) 14219-41-1 (for H3OBF4), 16872-11-0 (solvent free), 80628-99-5 (for H5O2BF4) N
ChEBI	CHEBI:38902 N
ChemSpider	26156 N
EC Number	240-898-3
Gmelin Reference	21702
Jmol 3D model	Interactive image Interactive image
MeSH	Fluoroboric+acid
PubChem	28118
RTECS number	ED2685000
UN number	1775
InChI InChI=1S/BF4H/c2-1(3,4)5/h2H N Key: YKRRMQXMWYXWJW-UHFFFAOYSA-N N
SMILES F[B](F)(F)[FH] [H+].[F--].F[B+](F)F
Properties
Chemical formula	BF4H
Molar mass	87.81 g·mol−1
Appearance	Colourless liquid
Melting point	−90 °C (−130 °F; 183 K)
Boiling point	130 °C (266 °F; 403 K)
Acidity (pKa)	-0.4
Basicity (pKb)	14.4
Vapor pressure	{{{value}}}
Related compounds
Related compounds	Hydrogen fluoride Triflic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Fluoroboric acid or tetrafluoroboric acid is an inorganic compound with the chemical formula H
₃OBF
₄.

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. With a strength comparable to nitric acid, fluoroboric acid 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

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 H₅O₂⁺, and the tetrahedral BF₄⁻ anion. The anion and cations are strongly hydrogen-bonded.

File:H3OBF4subunit.png

Subunit of crystal structure of H₃OBF₄ highlighting the hydrogen bonding between the cation and the anion.

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⁺ + BF₄⁻ + 2 H₂O

Anhydrous solutions can be prepared by treatment of aqueous fluoroboric acid with acetic anhydride.

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, while neat fluoroboric acid removes tert-butoxycarbonyl groups. Many reactive cations have been obtained using fluoroboric acid, e.g. tropylium tetrafluoroborate (C₇H₇⁺BF₄⁻), triphenylmethyl tetrafluoroborate (Ph₃C⁺BF₄⁻), triethyloxonium tetrafluoroborate (Et₃O⁺BF₄⁻), and benzenediazonium tetrafluoroborate (PhN₂⁺BF₄⁻).

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 unchanged.

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.

A 10% solution of fluoroboric acid, tin fluoroborate and thiourea is used to electroless tin plate the copper traces of printed circuit boards for protection and easier soldering.

Other fluoroboric acids

A series of fluoroboric acids is known in aqueous solutions. The series can be presented as follows:

• H[B(OH)₄]
• H[BF(OH)₃]
• H[BF₂(OH)₂]
• H[BF₃(OH)]
• H[BF₄]

See also

• Fluorosulfuric acid
• Fluoroantimonic acid