Arsenic tribromide is an inorganic compound with the formula AsBr3, it is a bromide of arsenic. This pyramidal molecule is the only known binary arsenic bromide. AsBr3 is noteworthy for its very high refractive index of approximately 2.3. It also has a very high diamagnetic susceptibility. The compound exists as colourless deliquescent crystals that fume in moist air.
Preparation Arsenic tribromide is optimally prepared by treating arsenic(III) oxide with hot concentrated hydrobromic acid.
As2O3 + 6 HBr → 2 AsBr3 + 3 3 H2O It can also be prepared by the direct bromination of arsenic powder. Alternatively, arsenic(III) oxide can be treated with a mixture of elemental sulfur and bromine:
2 As2O3 + 3 S + 6 Br2 → 4 AsBr3 + 3 SO2 Arsenic tribromide hydrolyzes readily. It is soluble in hydrocarbons.
Bromides of arsenic AsBr5 is not known, although the corresponding phosphorus compound PBr5 is well characterized. AsBr3 is the parent for a series of hypervalent anionic bromoarsenates including [As2Br8]2−, [As2Br9]3−, and [As3Br12]3−. Organoarsenic bromides (CH3)2AsBr and (CH3)AsBr2 are formed efficiently by the copper-catalyzed reaction of methyl bromide with hot arsenic metal. This synthesis is similar to the direct process used for the synthesis of methyl chlorosilanes.
Safety Arsenic tribromide is highly toxic. It is a carcinogen and a teratogen.
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