Difference between revisions of "Phosphorus tribromide"

Latest revision as of 20:59, 2 December 2018

Phosphorus tribromide

Cloudy PBr3, freshly distilled
Names
IUPAC name Phosphorus tribromide
Other names Phosphorous bromide Phosphorus(III) bromide Tribromophosphine
Properties
Chemical formula	PBr3
Molar mass	270.69 g/mol
Appearance	Colorless fuming liquid
Odor	Pungent, acidic
Density	2.852 g/cm3 (at 15 °C)
Melting point	−41.5 °C (−42.7 °F; 231.7 K)
Boiling point	173.2 °C (343.8 °F; 446.3 K)
Solubility in water	Hydrolyzes
Solubility	Reacts with alcohols, carboxylic acids, esters Soluble in acetone, antimony(III) bromide, arsenic(III) chloride, bromoacetic acid, carbon disulfide, chloroform, liq. H2S, liq. SO2
Thermochemistry
Std enthalpy of formation (ΔfHo298)	-185 kJ/mol
Hazards
Safety data sheet	Sigma-Aldrich
Related compounds
Related compounds	Phosphorus pentabromide 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 tribromide is an inorganic colorless liquid with the formula PBr₃. It is a dense liquid which fumes in open air and is used in chemistry to obtain alkyl bromides.

Properties

Chemical

Phosphorus tribromide reacts with water to form hydrobromic acid and phosphorus acid:

PBr₃ + 3 H₂O → H₃PO₃ + 3 HBr

Physical

Phosphorus tribromide is colorless liquid, which fumes in air and immiscible with water, though it hydrolyzes slowly in it.

Availability

Phosphorus tribromide is sold by various suppliers, but due to its hazards it's almost impossible to obtain as private individual. It's best to make it yourself.

Preparation

Phosphorus tribromide can be made by reacting elemental bromine with red phosphorus. Make sure to add an excess of red P to prevent phosphorus pentabromide from forming.^[1]

To limit the small fire that happens when you first add red P to the bromine s well as hydrolysis, a good idea is to purge the installation with an inert gas, such as nitrogen, carbon dioxide or argon. While the reaction can be done in neat, a solvent can also be used, such as carbon tetrachloride, carbon disulfide or benzene. This route is recommended if you're substituting red P with white P.

Decomposition of phosphorus pentabromide will give PBr₃ and bromine. Adding small amounts of phosphorus to the mixture will greatly improve the yield.

Reaction of hydrobromic acid with phosphorus trichloride will give PBr₃.^[2]

Reaction of elemental bromine with phosphorus triiodide will also give phosphorus tribromide.^[3]

A less known route involves the reaction of phosphorus vapors with mercury(II) bromide. The reaction gives phosphorus tribromide and mercury vapors, which are very toxic.^[4]

Projects

• Make alkyl bromides
• α-bromination of carboxylic acids

Handling

Safety

Phosphorus tribromide fumes in air to release HBr fumes which are very corrosive and toxic. Wear proper protection when handling the compound.

Storage

PBr₃ should be kept in teflon sealed glass bottles, in a dry place. Schlenk flasks are also a good storage container.

Disposal

Phosphorus tribromide should be neutralized with a base, such as calcium hydroxide suspension outside or in a well ventilated area. Since PBr₃ is immiscible with water, it will sink to the bottom and not quickly hydrolyze. Lots of HBr fumes will be produced during neutralization. Sodium thiosulfate can also be used as neutralizing agent.

References

1. ↑ https://www.youtube.com/watch?v=mYDYtsJ4NbQ
2. ↑ Matignon, C.; Comptes Rendus Hebdomadaires des Seances de l'Academie des Sciences; vol. 130; (1900); p. 1393
3. ↑ Gladstone, J. H.; Philosophical Magazine (1798-1977); vol. 35; (1849); p. 345 - 355
4. ↑ Loewig, C.; Das Brom und seine chemischen Verhaeltnisse, Heidelberg 1829, p. 51

Relevant Sciencemadness threads

• Phosphorus Tribromide
• Synthesis of PBr3
• Preparation of PBr3?