Boron trifluoride a versatile building block for other boron compounds

Boron trifluoride is the inorganic compound with the formula BF₃. This pungent colourless toxic gas forms white fumes in moist air. It is a useful Lewis acid and a versatile building block for other boron compounds.

Structure and bonding

The geometry of a molecule of BF3 is trigonal planar. Its D3h symmetry conforms with the prediction of VSEPR theory. The molecule has no dipole moment by virtue of its high symmetry. The molecule is isoelectronic with the carbonate anion, CO₃²⁻.

BF3 is commonly referred to as "electron deficient," a description that is reinforced by its exothermic reactivity toward Lewis bases.

In the boron trihalides, BX3, the length of the B-X bonds (1.30 Å) is shorter than would be expected for single bonds, and this shortness may indicate stronger B-X π-bonding in the fluoride. A facile explanation invokes the symmetry-allowed overlap of a p orbital on the boron atom with the in-phase combination of the three similarly oriented p orbitals on fluorine atoms. Others point to the ionic nature of the bonds in BF3.

 

Synthesis and handling

BF3 is manufactured by the reaction of boron oxides with hydrogen fluoride:

B₂O₃ + 6 HF → 2 BF₃ + 3 H₂O

Typically the HF is produced in situ from sulfuric acid and fluorite (CaF₂). Approximately 2300-4500 tonnes of boron trifluoride are produced every year.

On a laboratory scale, BF₃ is produced by the thermal decomposition of diazonium salts:

PhN₂BF₄ → PhF + BF₃ + N₂

Alternatively the chemical can be synthesized from Sodium tetrafluoroborate, Boron trioxide, and Sulfuric acid:

6 NaBF₄ + B₂O₃ + 6 H₂SO₄ → 8 BF₃ + 6 NaHSO₄ + 3 H₂O

Anhydrous boron trifluoride has a boiling point of −100.3 C and a critical temperature of −12.3 C, so that it can be stored as a refrigerated liquid only between those temperatures. Storage or transport vessels should be designed to withstand internal pressure, since a refrigeration system failure could cause pressures to rise to the critical pressure of 49.85 bar (4.985 MPa).

Boron trifluoride is corrosive. Suitable metals for equipment handling boron trifluoride include stainless steel, monel, and hastelloy. In presence of moisture it corrodes steel, including stainless steel. It reacts with polyamides. Polytetrafluoroethylene, polychlorotrifluoroethylene, polyvinylidene fluoride, and polypropylene show satisfactory resistance. The grease used in the equipment should be fluorocarbon based, as boron trifluoride reacts with the hydrocarbon-based ones.