Phosphorus pentabromide

Phosphorus pentabromide is a reactive, yellow solid of formula PBr₅, which has the structure PBr₄⁺Br⁻ in the solid state but in the vapor phase is completely dissociated to PBr₃ and Br₂. Rapid cooling of this phase to 15 K leads to formation of the ionic species [PBr₄]⁺[Br₃]⁻.

It can be used in organic chemistry to convert carboxylic acids to acyl bromides. It is highly corrosive. It decomposes above 100 °C to give phosphorus tribromide and bromine:

Reversing this equilibrium to generate PBr₅ by addition of Br₂ to PBr₃ is difficult in practice because the product is susceptible to further addition to yield PBr₇.

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Phosphorus tribromide

Phosphorus tribromide is a colourless liquid with the formula PBr₃. It is a colourless liquid that 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.

Preparation

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.

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.

Phosphorus heptabromide

Phosphorus heptabromide is an inorganic compound with the formula PBr₇. It is one of the phosphorus bromides. At normal conditions, it forms red prismatic crystals. PBr₇ can be prepared by the sublimation of a mixture of phosphorus pentabromide and bromine.

The structure consists of the PBr₄⁺ cation paired with a tribromide (Br₃⁻) anion.

See also

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