Everything about Boron Trichloride totally explained
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Boron trichloride is a
chemical compound with the formula BCl
3. 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.
» B
2O
3 + 3 C + 3 Cl
2 → 2 BCl
3 + 3 CO
The synthesis is analogous to the
Kroll process for the conversion of titanium dioxide to titanium tetrachloride. In the laboratory BF
3 reacted with AlCl
3 gives BCl
3 via halogen exchange. BCl
3 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's symmetric and therefore the dipole moments associated with the bonds cancel each other out. Boron trichloride doesn't form dimers, although there's 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's some debate as to its extent. For example, BCl
3S(CH
3)
2 (CAS# 5523-19-3) is often employed as a conveniently handled source of BCl
3 because this solid (m.p. 88-90 °C) releases BCl
3:
» (CH
3)
2SBCl
3 (CH
3)
2S + BCl
3
When boron trichloride is passed at low pressure through devices delivering an electric discharge, diboron tetrachloride, Cl
2B-BCl
2, and tetraboron tetrachloride, formula B
4Cl
4, 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
Safety
BCl
3 is an aggressive reagent that releases
hydrogen chloride upon exposure to moisture or
alcohols. The
dimethyl sulfide adduct is safer to use, when possible.
Further Information
Get more info on 'Boron Trichloride'.
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