Phosphorus pentabromide

Phosphorus pentabromide is a reactive, yellow solid of formula PBr₅, which has the structure [PBr₄]⁺Br− (tetrabromophosphonium bromide) in the solid state but in the vapor phase is completely dissociated to PBr₃ and Br₂. Rapid cooling of this phase to 15 K leads to formation of the ionic species phosphorus heptabromide (tetrabromophosphonium tribromide [PBr₄]⁺[Br₃]⁻).^[2]

It can be used in organic chemistry to convert carboxylic acids to acyl bromides. It is highly corrosive. It strongly irritates skin and eyes.^[1] It decomposes above 100 °C to give phosphorus tribromide and bromine:^[3]

PBr₅ → PBr3 + Br2

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]⁻.^[4]

References

^ a b c d e f "Phosphorus pentabromide".
^ Corbridge, D. E. C. (2013). Phosphorus: Chemistry, Biochemistry and Technology, Sixth Edition. CRC Press. p. 154. ISBN 978-1-4398-4088-7.
^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
^ 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.