Phosphorus pentabromide

Phosphorus pentabromide

Names
Other names Pentabromophosphorane Phosphorus(V) bromide
Properties
Chemical formula	PBr5
Molar mass	430.49 g/mol
Appearance	Yellow crystalline solid
Density	3.61 g/cm3
Melting point	103.7 °C (218.7 °F; 376.8 K) (decomposes)
Boiling point	Decomposes
Solubility in water	Reacts
Solubility	Reacts with alcohols, carboxylic acids Soluble in AsCl3, bromine, liq. HBr, liq. HCl, liq. H2S, phosphoryl chloride, liq. SO2 sparingly soluble in benzene, carbon disulfide, carbon tetrachloride
Thermochemistry
Std enthalpy of formation (ΔfHo298)	-253.6 kJ/mol
Hazards
Safety data sheet	Sigma-Aldrich
Related compounds
Related compounds	Phosphorus tribromide Phosphoryl bromide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Phosphorus pentabromide is a reactive, unstable, yellow solid chemical compound with the formula PBr₅. This compound has the structure PBr₄⁺Br⁻ in the solid state, but in the vapor phase is completely dissociated to PBr₃ and Br₂.

Properties

Chemical

Phosphorus pentabromide rapidly hydrolyzes in water to release hydrobromic and phorphorous acids.

Addition of elemental phosphorus to this liquid chemical will cause it to convert to PBr₃.

Upon excess bromination or cooling below 15 K, it converts to phosphorus heptabromide, which is a red solid.

Physical

Phosphorus pentabromide is a yellowish crystalline solid, which breaks down to PBr₃ and Br₂ upon heating. PBr₅ readily hydrolyzes in moist air. It is soluble in anh. HCl, HBr, H₂S, but less so in benzene, CS₂, carbon tet.

Availability

PBr₅ is sometimes sold by chemical suppliers, but due to its hazards it's difficult to get hold of.

Preparation

Can be prepared by reacting PBr₃ and bromine in equimolar amounts, at room temperature. Excess bromine will cause it to convert to PBr₇. Petroleum ether can be used as a reaction solvent.

During the synthesis of phosphorus tribromide, small amounts of phosphorus pentabromide will be formed. To separate the pentabromide, the PBr₃ and excess bromine are removed with a vacuum pump and condensed in a cold trap.

Projects

• Make phosphoryl bromide
• Make alkyl bromides

Handling

Safety

Phosphorus tribromide reacts with water and its fumes are very corrosive, as well as toxic. Wear proper protection when handling the compound.

Storage

In Schlenk flasks, at low temperatures. Periodically check for decomposition.

Disposal

Phosphorus pentabromide should be neutralized with a base, such as calcium hydroxide suspension outside or in a well ventilated area. Lots of HBr fumes will be produced during neutralization. Sodium thiosulfate can also be used as neutralizing agent, as it neutralizes bromine more effectively than bases.