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Phosphorus pentabromide is the inorganic compound with the formula PBr5. It is a yellow solid that is reactive toward water and hydroxyl-containing compounds.

The structure [PBr4]+Br− (tetrabromophosphonium bromide) in the solid state, but in the vapor phase it dissociates into PBr3 and Br2. Rapid cooling of this phase to 15 K give the salt phosphorus heptabromide (tetrabromophosphonium tribromide [PBr4]+[Br3]−).[2] Reversing this equilibrium to generate PBr5 by addition of Br2 to PBr3 is difficult in practice because the product is susceptible to further addition to yield phosphorus heptabromide [PBr4]+[Br3]−.[3]

Organic chemistry

It can be used in organic chemistry to convert carboxylic acids to acyl bromides. It is highly corrosive. It strongly irritates skin, eyes and causes severe skin burns.[1] It decomposes above 100 °C to give phosphorus tribromide and bromine:[4]

PBr5 → PBr3 + Br2

In organic synthesis, triphenylphosphine dibromide, (C6H5)3PBr2, is often used in place of PBr5.

References

  1. ^ a b c d e f “Phosphorus pentabromide”.
  2. ^ Corbridge, D. E. C. (2013). Phosphorus: Chemistry, Biochemistry and Technology, Sixth Edition. CRC Press. p. 154. ISBN 978-1-4398-4088-7.
  3. ^ Popov, A. I.; Skelly, N. E. (1954). “Spectrophotometric Study of Phosphorus Pentabromide in Various Solvents”. J. Am. Chem. Soc. 76 (15): 3916–3919. doi:10.1021/ja01644a014.
  4. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. doi:10.1016/C2009-0-30414-6. ISBN 978-0-08-037941-8.