# Boron trichloride

Boron trichloride is the inorganic compound with the formula BCl$_3$. This colorless gas is a reagent in organic synthesis. It is highly reactive towards water.

## Production and structure

Boron reacts with halogens to give the corresponding trihalides. Boron trichloride is, however, produced industrially by chlorination of boron oxide and carbon at 501 °C.
    B$_2$O$_3$ + 3 C + 3 Cl$_2$ → 2 BCl$_3$ + 3 CO
The carbothermic reaction is analogous to the Kroll process for the conversion of titanium dioxide to titanium tetrachloride. One consequence of this synthesis route is that samples of boron trichloride are often contaminated with phosgene.
In the laboratory BCl$_3$ can be prepared by treating with AlCl$_3$ with BF$_3$, a halide exchange reaction.
BCl$_3$ is a trigonal planar molecule like the other boron trihalides. The B–Cl bond length is 175 pm. A degree of π-bonding has been proposed to explain the short B− Cl distance, although there is some debate as to its extent. BCl$_3$ 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 AlCl$_3$ and GaCl$_3$, which form dimers or polymers with 4 or 6 coordinate metal centres.

## Reactions

BCl$_3$ hydrolyzes readily to give hydrochloric acid and boric acid:
    BCl$_3$ + 3 H$_2$O → B(OH)$_3$ + 3 HCl
Alcohols behave analogously giving the borate esters, e.g. trimethyl borate.
As a strong Lewis acid, BCl$_3$ forms adducts with tertiary amines, phosphines, ethers, thioethers, and halide ions. Adduct formation is often accompanied by an increase in B-Cl bond length. BCl$_3$•S(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$_3$)$_2$S·BCl$_3$ ⇌ (CH$_3$)$_2$S + BCl$_3$
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 BCl$_3$ with organotin reagents:
    2 BCl$_3$ + R$_4$Sn → 2 RBCl$_2$ + R$_2$SnCl$_2$

### Reduction

Reduction of BCl$_3$ to elemental boron is conducted commercially in the laboratory, when boron trichloride can be converted to diboron tetrachloride by heating with copper metal:
    2 BCl$_3$ + 2 Cu → B$_2$Cl$_4$ + 2 CuCl
B$_4$Cl$_4$ 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 (disproportionates) 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.
    n B$_2$Cl$_4$ → B$_n$Cl$_n$ + n BCl$_3$
The compounds with formulas B$_8$Cl$_8$ and B$_9$Cl$_9$ 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. Aluminium 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 BCl$_3$. It has been used in the field of high energy fuels and rocket propellants as a source of boron to raise BTU value. BCl$_3$ is also used in plasma etching in semiconductor manufacturing. This gas etches metal oxides by formation of a volatile BOCl$_x$ and M$_x$O$_y$Cl$_z$ compounds.
BCl$_3$ is used as a reagent in the synthesis of organic compounds. Like the corresponding bromide, it cleaves C-O bonds in ethers.

## Safety

BCl$_3$ is an aggressive reagent that can form hydrogen chloride upon exposure to moisture or alcohols. The dimethyl sulfide adduct (BCl$_3$SMe$_2$), which is a solid, is much safer to use, when possible, but H$_2$O will destroy the BCl$_3$ portion while leaving dimethyl sulfide in solution.