Boron trichloride

Chembox new
Name = Boron trichloride
ImageFile = Boron-trichloride-2D.png ImageName = Boron trichloride
ImageFile1 = Boron-trichloride-3D-vdW.png ImageName1 = Boron trichloride
IUPACName = Boron trichloride
OtherNames = Boron(III) chloride
Section1 = Chembox Identifiers
CASNo = 10294-34-5
RTECS = ED1925000
Section2 = Chembox Properties
Formula = BCl₃
MolarMass = 117.17 g/mol
Appearance = Colorless gas,
fumes in air
Density = 1.43 g/ml (O °C)
Solubility = "decomp."
MeltingPt = −107.3 °C
BoilingPt = 12.5 °C
Section3 = Chembox Structure
MolShape = Trigonal planar ("D"_3h)
Dipole = zero
Section7 = Chembox Hazards
ExternalMSDS =
EUClass = Very toxic (T+)
RPhrases = R14, R26/28, R34
SPhrases = S1/2, S9, S26, S28,
S36/37/39, S45
FlashPt = Non-flammable
Section8 = Chembox Related
OtherAnions = Boron trifluoride
Boron tribromide
Boron triiodide
OtherCations = Aluminium chloride
Gallium chloride
OtherCpds = Boron oxide
Carbon tetrachloride

Boron trichloride is a chemical compound with the formula BCl₃. This colorless gas is a valuable reagent in organic synthesis. It is also dangerously reactive.

Production and properties

Boron reacts with halogens to give the corresponding trihalides. Boron trichloride is, however, produced industrially by direct chlorination of boron oxide and carbon at 500 °C.:B₂O₃ + 3 C + 3 Cl₂ → 2 BCl₃ + 3 COThe synthesis is analogous to the Kroll process for the conversion of titanium dioxide to titanium tetrachloride. In the laboratory BF₃ reacted with AlCl₃ gives BCl₃ via halogen exchange.Greenwood&Earnshaw] BCl₃ is a trigonal planar molecule like the other boron trihalides, and has a bond length of 175pm. It has a zero dipole moment because it is symmetric and therefore the dipole moments associated with the bonds cancel each other out. Boron trichloride does not form dimers, although there is some evidence that may indicate dimerisation at very low temperatures (20°K). NMR studies of mixtures of boron trihalides shows the presence of mixed halides which may indicate a four centre intermediate e.g. a dimer. The absence of dimerisation contrasts with the other trihalides of group 13 which contain 4 or 6 coordinate metal centres, for example see AlCl₃ and GaCl₃. A degree of π-bonding has been proposed to explain the short B− Cl distance although there is some debate as to its extent.

BCl₃ is a Lewis acid readily forming adducts with tertiary amines, phosphines, ethers, thioethers, and halide ions. [cite journal| author = W. Gerrard and M. F. Lappert| title = Reactions Of Boron richloride With Organic Compounds| journal = Chemical Reviews| year = 1958| volume = 58| pages = 1081–1111| doi = 0.1021/cr50024a003] For example, BCl₃S(CH₃)₂ (CAS# 5523-19-3) is often employed as a conveniently handled source of BCl₃ because this solid (m.p. 88-90 °C) releases BCl₃::(CH₃)₂SBCl₃$overrightarrow\{leftarrow\}$ (CH₃)₂S + BCl₃

When boron trichloride is passed at low pressure through devices delivering an electric discharge, diboron tetrachloride, [Wartik, T.; Rosenberg, R.; Fox, W. B. "Diboron tetrachloride" Inorganic Syntheses 1967, volume X, pages 118-126.] Cl₂B-BCl₂, and tetraboron tetrachloride, formula B₄Cl₄, are formed. Colourless diboron tetrachloride (m.p. -93 °C) has a planar molecule in the solid, (similar to dinitrogen tetroxide, but in the gas phase the structure is staggered. It decomposes at room temperatures to give a series of monochlorides having the general formula (BCl)_"n", in which "n" may be 8, 9, 10, or 11; the compounds with formulas B₈Cl₈ and B₉Cl₉ are known to contain closed cages of boron atoms.

The mixed aryl and alkyl boron chlorides are also of interest. Phenylboron dichloride is commercially available. Such species can be prepare by the reaction of BCl₃ with organotin reagents::2 BCl₃ + R₄Sn → 2 RBCl₂ + R₂SnCl₂

Uses

Boron trichloride is a starting material for the production of elemental boron. It is also used in the refining of aluminium, magnesium, zinc, and copper alloys to remove nitrides, carbides, and oxides from molten metal. It has been used as a soldering flux for alloys of aluminium, iron, zinc, tungsten, and monel. Aluminum castings can be improved by treating the melt with boron trichloride vapors. In the manufacture of electrical resistors, a uniform and lasting adhesive carbon film can be put over a ceramic base using BCl₃. It has been used in the field of high energy fuels and rocket propellants as a source of boron to raise BTU value. BCl₃ is also used in plasma etching in semiconductor manufacturing. This gas etches metal oxides by formation of a volatile BOCl_x compounds.

BCl₃ is used as a reagent in the synthesis of organic compounds. Like the corresponding bromide, it cleaves C-O bonds in ethers. [Yamamoto, Y.; Miyaura, N. "Boron Trichloride" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.]

afety

BCl₃ is an aggressive reagent that releases hydrogen chloride upon exposure to moisture or alcohols. The dimethyl sulfide adduct is safer to use, when possible.

References

*cite journal
author = D. R. Martin
title = Coordination compounds of boron trichloride. I - A review
journal = Chemical Reviews
year = 1944
volume = 34
pages = 461–473
doi = 10.1021/cr60109a005
* cite journal
author = Kabalka GW, Wu ZZ, Ju YH
title = The use of organoboron chlorides and bromides in organic synthesis
journal = Journal of Organometallic Chemistry
year = 2003
volume = 680
pages = 12–22
doi = 10.1016/S0022-328X(03)00209-2

External links

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