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Phosphorus tribromide, like PCl 3 and PF 3, 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 Br 3 B · PBr 3. At the same time PBr 3 can react as an electrophile or Lewis acid in many of its reactions, for example with amines.
The reaction is initiated by addition of a catalytic amount of PBr 3, after which one molar equivalent of Br 2 is added. PBr 3 converts the carboxylic OH to the acyl bromide. The acyl bromide tautomerizes to an enol, which reacts with the Br 2 to brominate at the α position.
Phosphoryl bromide is prepared by the reaction between phosphorus pentabromide and phosphorus pentoxide: [4] [5]. 3 PBr 5 + P 2 O 5 → 5 POBr 3. It can also be prepared via the slow addition of liquid bromine to phosphorus tribromide at 0 °C, followed by the slow addition of water and vacuum distillation of the resulting slurry.
Phosphorus pentachloride, phosphorus pentabromide, and phosphorus heptabromide are ionic in the solid and liquid states; PCl 5 is formulated as PCl 4 + PCl 6 –, but in contrast, PBr 5 is formulated as PBr 4 + Br −, and PBr 7 is formulated as PBr 4 + Br 3 −. They are widely used as chlorinating and brominating agents in organic chemistry.
Phosphorus pentabromide is a reactive, yellow solid of formula P Br 5, which has the structure [PBr 4] + Br − (tetrabromophosphonium bromide) in the solid state but in the vapor phase is completely dissociated to PBr 3 and Br 2.
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