Tetrafluoroboric acid

Structural formula
General
Surname	Tetrafluoroboric acid
other names	Tetrafluoroboric acid Fluoroboric acid Hydrofluoric acid
Molecular formula	HBF 4
Brief description	colorless to yellowish liquid with a pungent odor
External identifiers / databases
CAS number 16872-11-0 EC number 240-898-3 ECHA InfoCard 100.037.165 PubChem 28118 Wikidata Q412333
properties
Molar mass	87.81 g mol −1
Physical state	liquid
density	1.38 g cm −3
Melting point	−78 ° C
boiling point	130 ° C ( decomposition )
solubility	miscible with water
safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary danger H and P phrases H: 314 P: 260-264-280-303 + 361 + 353-305 + 351 + 338-310
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Tetrafluoroboric acid is an inorganic acid . Your anion is the tetrafluoroborate ion (BF ₄ ^- ). It is commercially available as a 45-50% solution .

presentation

The reaction of boric acid with 50% hydrofluoric acid is used as a large-scale synthesis of tetrafluoroboric acid .

${\displaystyle \mathrm {B(OH)_{3}+4\ HF\longrightarrow \ HBF_{4}+3\ H_{2}O} }$

Another possibility is the reaction of boron trifluoride with hydrofluoric acid .

${\displaystyle \mathrm {BF_{3}+\ HF\longrightarrow \ HBF_{4}} }$

properties

Tetrafluoroboric acid is only stable in solution. The aqueous solution is colorless and can be concentrated up to a concentration of about 50%. Geometrically, the boron atom is tetrahedrally surrounded by four fluorine atoms and forms a spherical anion. The boron atom is BF ₄ ^- ie sp ³ hybridized . Tetrafluoroboric acid has a pKa value of −0.4. In aqueous solution it is dissociated as H ₃ O ⁺ _(aq) and [BF ₄ ] ^- _(aq) .

use

Tetrafluoroboric acid can be used to synthesize fluorinated aromatics . For this purpose , diazoaryls are reacted with tetrafluoroboric acid in a Schiemann reaction , a variant of the Sandmeyer reaction .

It can also be used to split off the Boc protective group and as a catalyst for polymerizations . Tetrafluoroborate ions are weakly coordinating anions . Their salts can therefore easily exchange their anions for more complexing ones and for this reason they are often used to produce other salts. One example is copper (II) tetrafluoroborate, which is used to produce copper complexes.

Until a few years ago, tetrafluoroboric acid was widely used as the basis for acidic tin electrolytes . Due to the inevitable proportion of fluoride or hydrofluoric acid , these electrolytes pose a disposal problem. Therefore, electrolytes based on methanesulfonic acid or sulfuric acid are usually found today .

Molten alkali metal and ammonium tetrafluoroborates (such as sodium , potassium, and ammonium tetrafluoroborate ) are good solvents for metal oxides and are used in fluxes for soldering and brazing .

literature

• AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 91st – 100th, improved and greatly expanded edition. Walter de Gruyter, Berlin 1985, ISBN 3-11-007511-3 , p. 848.

Individual evidence

1. ↑ ^{a b c d e f g} Entry on tetrafluoroboric acid in the GESTIS substance database of the IFA , accessed on January 8, 2018(JavaScript required) .
2. ↑ Entry on Tetrafluoroboric acid in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on February 1, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .
3. ↑ G. Brauer (Ed.), Handbook of Preparative Inorganic Chemistry 2nd ed., Vol. 1, Academic Press 1963, pp. 221-222.
4. ↑ Toxicological evaluation of tetrafluoroboric acid and salts (PDF) at the professional association raw materials and chemical industry (BG RCI), accessed on August 22, 2012.