Boron tribromide, BBr₃

Boron tribromide, BBr₃, its preparation and purification may be carried out as described for the chloride.

Boron tribromide is a colourless, mobile liquid which boils at 90.5° C. and solidifies to a colourless mass which melts at -46° C. Its density is 2.6499 at 0° C. and 2.6175 at 16.5°. The vapour density is normal; the vapour pressure is as follows: -

°C.	mm.	°C.	mm.	°C.	mm.	°C.	mm.	°C.	mm.
-50	0.7	-20	5.5	10	32	40	130	70	390
-40	1.5	-10	10 5	20	54	50	193	80	540
-30	3	0	19	30	85	60	280	90	730

Boron tribromide reacts with ammonia at 0° to form boron imide.

2BBr₃ + 27NH₃ = B₂(NH)₃ + 6NH₄Br.3NH₃.

With phosphine, boron tribromide forms a white, solid substance of the formula BBr₃.PH₃.9 The corresponding arsenic compound BBr₃.AsH₃, prepared by the direct union of arsine and boron tribromide at - 100°, is a very unstable, white, amorphous solid, which, on keeping in the dark at 0°, slowly decomposes into boron tribromide, arsenic, and hydrogen. The following addition-compounds are also known: BBr₃.PBr₃, BBr₃.PBr₅, 2BBr₃.PCl₃, 2BBr₃.PCl₆, and 2BBr₃.P₂I₄.