Boron trichloride

Boron trichloride is the 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 the 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 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. 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 &minus; 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, 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, 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  &rarr;  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 aluminum, 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 BOClx compounds.

BCl3 is used as a reagent in the synthesis of organic compounds. Like the corresponding bromide, it cleaves C-O bonds in ethers.

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