Boron trichloride, BCl₃

For the synthesis, impure boron (prepared by heating the oxide with magnesium and boiling the resulting mass with hydrochloric acid) is employed. It is heated to dull redness in a stream of dry hydrogen, allowed to cool in the gas, and, after displacing the hydrogen with chlorine, heated in chlorine to redness. The product is strongly cooled and protected from moisture, shaken with mercury or silver powder to remove chlorine, and fractionally distilled to eliminate hydrogen chloride and silicon chloride. Boron trichloride is also produced when boron sesqui-oxide is heated for some days at 150° with phosphorus pentachloride.

Boron trichloride is a colourless liquid of high refractive index and normal vapour density (Wohler and Deville). At 0° C. its density is 1.43386; its coefficient of expansion is large. It melts at -107° C. and boils at 12.5° C.; its vapour pressure is as follows: -

°C.	mm.	°C.	mm.	°C.	mm.	°C.	mm.	°C.	mm.
-80	4	-60	18	-40	67	-20	197	0	477
-75	6	-55	26	-35	89	-15	251	5	579
-70	9	-50	37	-30	116	-10	314	10	695
-65	13	-45	51	-25	151	-5	387	12.4	753

One gram-molecule of the trichloride, when decomposed by water, produces 79.2 Cals. Boron trichloride is without action on sodium below 150°, zinc below 200°, and sulphur below 250°, but it reacts with sulphur trioxide at 120°, producing sulphuryl chloride and boron sesqui-oxide (Gustavson).

When boron trichloride vapour is passed into liquid ammonia, excess of which is then removed at -23°, 15 molecules of ammonia are fixed for each molecule of the chloride used. The products are boron araide and an ammonia addition-product of ammonium chloride: -

BCl₃ + 15NH₃ = B(NH₂)₃ + 3NH₄Cl.3NH₃.

If the ammonia be removed at 0°, only 6 molecules are fixed, since the dissociation pressure of NH₄Cl.3NH₃ exceeds 760 mm. at 0°. When the reaction takes place above 0° C., boron imide, B₂(NH)₃, is produced: -

2B(NH₂)₃ = B₂(NH)₃ + 3NH₃.

According to Besson, boron trichloride forms a white, solid compound, BCl₃.PH₃, with phosphine. The following double compounds have also been described, besides various addition-compounds with organic substances: BCl₃.NOCl, BCl₃.POCl₃, BCl₃.CNCl, and BCl₃.HCN.

When boron trichloride is introduced into the afterglow of active nitrogen, a pale bluish-green glow is developed, easily distinguished from the green colour of the boric acid flame, and a white, amorphous solid that contains both boron and nitrogen (probably boron nitride) is deposited.