Boron Trichloride - Production and Reactions

Production and Reactions

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 CO

The carbothermal 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. BCl3 is a trigonal planar molecule like the other boron trihalides, and has a bond length of 175pm.

Boron trichloride does not form dimers, although NMR studies of mixtures of boron trihalides shows the presence of mixed halides. 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. For example, BCl3 · S(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)2S · BCl3 ⇌ (CH3)2S + BCl3

When boron trichloride is passed at low pressure through devices delivering an electric discharge, diboron tetrachloride, 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 prepared by the redistribution reaction of BCl3 with organotin reagents:

2 BCl3 + R4Sn → 2 RBCl2 + R2SnCl2

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