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Selenium Tetrabromide, SeBr4

Selenium Tetrabromide, SeBr4, is formed by the action of excess of bromine on selenium or on selenium monobromide, the latter preferably being in solution in carbon disulphide, in order to moderate the vigour of the process.

When selenium dioxide is dissolved in aqueous hydrobromic acid, selenium tetrabromide is also formed:

SeO2 + 4HBr SeBr4 + 2H2O.

Its presence may be shown by the separation of potassium selenibromide, K2SeBr6, on addition of concentrated potassium bromide solution.

Selenium tetrabromide is a reddish-brown crystalline powder which decomposes to some extent on warming, giving bromine and the monobromide, but much sublimes unchanged.

With sufficient water the tetrabromide is decomposed into the dioxide and hydrobromic acid. Carbon disulphide and chloroform dissolve it to a slight extent.

With the alkali bromides and alkyl ammonium bromides selenium tetrabromide can form crystalline additive compounds of the type M2SeBr6, termed selenibromides. Indeed, by the action of bromine on finely divided selenium in the presence of concentrated hydrobromic acid, a solution is obtained which gradually deposits deep red crystals of hydrogen selenibromide, H2SeBr6. The solution of this body in hydrobromic acid on dilution with water undergoes decomposition with liberation of colloidal selenium. Two reactions probably occur concurrently:

(1) H2SeBr6 + 3H2O = H2SeO3 + 6HBr,
and
(2) H2SeBr6 + H2O = Se + Br2 + 3HBr + HBrO.

The potassium and ammonium selenibromides have been prepared by dissolving selenium dioxide in hydrobromic acid and adding either potassium or ammonium bromide. The selenibromides are orange-red in colour and are decomposed by water into selenious acid, hydrogen bromide and the alkali bromide, yielding colourless solutions.

Selenium tetrabromide and sulphur trioxide yield the additive compound SeBr4.2SO3 in the form of bright yellow needles which at 170° C. form the yellow compound SeOBr2.SO3.
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