Phosphorus tribromide

Preparation

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

2 P + 3 Br₂ → 2 PBr₃

Because the reaction is highly exothermic, it is often conducted in the presence of a diluent such as PBr₃.

Reactions

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.

The most 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.

PBr₃ + 3 ROH → 3 RBr + HP(O)(OH)₂

The mechanism (shown for a primary alcohol) involves formation of a phosphorus 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 alcohol alpha carbon, as is usual with an S_N2 reaction.

In a similar reaction, PBr₃ also converts carboxylic acids to acyl bromides.

PBr₃ + 3 RCOOH → 3 RCOBr + HP(O)(OH)₂

Applications

The main use for phosphorus tribromide is for conversion of primary or secondary alcohols to alkyl bromides, as described above. PBr₃ usually gives higher yields than hydrobromic acid, and it avoids problems of carbocation rearrangement- for example even neopentyl bromide can be made from the alcohol in 60% yield.

Another use for PBr₃ is as a catalyst for the α-bromination of carboxylic acids. Although acyl bromides are rarely made in comparison with acyl chlorides, they are used as intermediates in Hell-Volhard-Zelinsky halogenation. Initially PBr₃ reacts with the carboxylic acid to form the acyl bromide, which is more reactive towards bromination. The overall process can be represented as

On a commercial scale, phosphorus tribromide is used in the manufacture of pharmaceuticals such as alprazolam, methohexital and fenoprofen. It is also a potent fire suppression agent marketed under the name PhostrEx.

Precautions

PBr₃ evolves corrosive HBr, is toxic, and reacts violently with water and alcohols.

In reactions that produce phosphorous acid as a by-product, when working up by distillation be aware that this can decompose above about 160 °C to give phosphine which can cause explosions in contact with air.