Phosphorus tribromide - PBr3
This is a sample of 4 ml of phosphorus tribromide, sealed in an ampoule, which protects it from moisture. Phosphorus tribromide is very moisture sensitive and extremely corrosive. It is one of the strongest brominating agents, which can replace -OH groups from organic molecules by -Br atoms, e.g. it can convert organic acids of structure R-C(O)OH to the acid chlorides R-C(O)Br.
Phosphorus tribromide is the most stable bromide of the element phosphorus. Other stable bromides are phosphorus pentabromide and phosphorus heptabromide.
When phosphorus tribromide is added to
water, copious amounts of hydrogen bromide are formed and phosphorous
acid dissolves in the water. Phosphorus tribromide fumes strongly in
contact with air, due to reaction of its vapor with water vapor.
The picture below shows a more detailed view of the liquid in the ampoule.
Absolutely pure phosphorus tribromide
is said to be colorless. This sample is very pale yellow. It is
homemade, from excess red phosphorus and bromine, with the liquid
distilled from the excess red phosphorus. The pale yellow color may be
due to trace amounts of phosphorus pentabromide dissolved in the liquid.
Storage of phosphorus tribromide is difficult, albeit not as difficult as storage of phosphorus pentabromide. The liquid can best be stored in glass ampoules, but it can also be stored in a bottle with a very well sealed cap and a PTFE liner.