Boron trichloride

Production and structure

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 carbothermic reaction 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.

A degree of π-bonding has been proposed to explain the short B− Cl distance although there is some debate as to its extent. It does not dimerize, although NMR studies of mixtures of boron trihalides shows the presence of mixed halides. The absence of dimerisation contrasts with the tendencies of AlCl₃ and GaCl₃, which form dimers or polymers with 4 or 6 coordinate metal centres.

Reactions

BCl₃ hydrolyzes readily to give hydrochloric acid and boric acid:

BCl₃ + 3 H₂O → B(OH)₃ + 3 HCl

Alcohols behave analogously giving the borate esters, e.g. trimethyl borate.

As a strong Lewis acid, BCl₃ forms adducts with tertiary amines, phosphines, ethers, thioethers, and halide ions. Adduct formation is often accompanied by an increase in B-Cl bond length. 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

The mixed aryl and alkyl boron chlorides are also of known. 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

Reduction

Reduction of BCl₃ to elemental boron is conducted commercially (see below). In the laboratory, when boron trichloride can be converted to diboron tetrachloride by heating with copper metal:

2 BCl₃ + 2 Cu → B₂Cl₄ + CuCl

B₄Cl₄ can also be prepared in this way. Colourless diboron tetrachloride (m.p. -93 °C) is 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.

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.

Safety

BCl₃ is an aggressive reagent that can form hydrogen chloride upon exposure to moisture or alcohols. The dimethyl sulfide adduct (BCl₃SMe₂), which is a solid, is much safer to use, when possible, but H₂O will destroy the BCl₃ portion while leaving dimethyl sulfide in solution.