Boron halogen

Boron forms halogen compounds of the type BX₃. The fluoride, chloride, and bromide can be prepared by the direct union of their elements, but not the iodide. The chloride and bromide can also be prepared by heating to redness in a stream of either chlorine or bromine an intimate mixture of boron sesqui-oxide and carbon: -

B₂O₃ + 3C + 3X₂ = 2BX₃ + 3CO.

The boron halides have none of the properties of salts. At the ordinary temperature the fluoride is a gas; the liquid chloride boils at 12.5° C.; the bromide is a volatile liquid, and the iodide a solid of low melting-point. Each is rapidly decomposed by water, the change being expressed, except for the fluoride, by the (irreversible) equation: -

BX₃ + 3H₂O = H₃BO₃ + 3HX.

Besson has described the bromo-iodides of boron, BBr₂I and BBrI₂. They are colourless liquids which boil at 125° and 180° respectively. A mixture of these compounds and boron tri-iodide is obtained when hydrogen iodide acts upon boron tribromide at a high temperature.

Two oxychlorides of boron, BOCl and BOCl₃, have been described, but it is doubtful whether they really exist. The first is stated by Gustavson to be obtained as a white, gelatinous solid when boron sesqui-oxide and boron trichloride are heated together at 150° in a sealed tube.