Phosphorus tribromide

Phosphorus tribromide is a colourless liquid with the formula PBr₃. The liquid fumes in moist air due to hydrolysis and has a penetrating odour. It is used in the laboratory for the conversion of alcohols to alkyl bromides.

PBr₃ is prepared by treating red phosphorus with bromine. An excess of phosphorus is used in order to prevent formation of PBr₅:

Because the reaction is highly exothermic, it is often conducted in the presence of a diluent such as PBr₃. Phosphorus tribromide is also generated in situ from red phosphorus and bromine.

Phosphorus tribromide, like PCl₃ and PF₃, has both properties of a Lewis base and a Lewis acid. For example, with a Lewis acid such as boron tribromide it forms stable 1 :1 adducts such as Br₃B · PBr₃. At the same time PBr₃ can react as an electrophile or Lewis acid in many of its reactions, for example with amines.

An important reaction of PBr₃ is with alcohols, where it replaces an OH group with a bromine atom to produce an alkyl bromide. All three bromides can be transferred.

Several detailed procedures are available. In some cases, triphenylphosphine/Br₂ is superior to PBr₃.

The mechanism for a primary alcohol involves formation of a phosphorous ester (to form a good leaving group), followed by an S_N2 substitution.

Because of the S_N2 substitution step, the reaction generally works well for primary and secondary alcohols, but fails for tertiary alcohols. If the reacting carbon centre is chiral, the reaction usually occurs with inversion of configuration at the carbon alpha to the alcohol, as is usual with an S_N2 reaction.