Chromium(III) bromide is an inorganic compound with the chemical formula CrBr3. It is a dark colored solid that appears green in transmitted light but red with reflected light. It is used as a precursor to catalysts for the oligomerization of ethylene.
Synthesis
The compound is prepared in a tube furnace by the reaction of bromine vapor and chromium powder at 1000 °C. It is purified by extracting with absolute diethyl ether to remove any CrBr2, and is subsequently washed with absolute diethyl ether and absolute ethanol.1
2Cr + 3Br2 → 2CrBr3The effect of bromine on a highly heated mixture of chromium(III) oxide with coal:
Cr2O3 + 3Br2 + 3C → 2CrBr3 + 3COChemical properties
Analogous to the behavior of related chromium(III) halides, the tribromide dissolves in water to give CrBr3(H2O)3 only upon the addition of catalytic amounts of a reducing agent, which generates CrBr2.2 The reducing agent generates chromous bromide on the surface of the solid, which dissolves and re-oxidizes to Cr(III).
Chromium(III) bromide is reduced by hydrogen gas at 350-400 °C to give chromium(II) bromide:3
2 CrBr3 + H2 → 2 CrBr2 + 2 HBrOxidizes when heated in air:
2 CrBr3 + 3 O2 → 2 Cr2O3 + 6 Br2References
Brauer, Georg (1965) [1962]. Handbuch Der Präparativen Anorganischen Chemie [Handbook of Preparative Inorganic Chemistry] (in German). Vol. 2. Stuttgart; New York, New York: Ferdinand Enke Verlag; Academic Press, Inc. p. 1340. ISBN 978-0-32316129-9. Retrieved 2014-01-10. 978-0-32316129-9 ↩
Brauer, Georg (1965) [1962]. Handbuch Der Präparativen Anorganischen Chemie [Handbook of Preparative Inorganic Chemistry] (in German). Vol. 2. Stuttgart; New York, New York: Ferdinand Enke Verlag; Academic Press, Inc. p. 1340. ISBN 978-0-32316129-9. Retrieved 2014-01-10. 978-0-32316129-9 ↩
Brauer, Georg (1965) [1962]. Handbuch Der Präparativen Anorganischen Chemie [Handbook of Preparative Inorganic Chemistry] (in German). Vol. 2. Stuttgart; New York, New York: Ferdinand Enke Verlag; Academic Press, Inc. p. 1340. ISBN 978-0-32316129-9. Retrieved 2014-01-10. 978-0-32316129-9 ↩