Phosphorus Chlorobromides

Phosphorus trichloride is only partially miscible with bromine; two layers are formed, which, on the addition of iodine, combine with evolution of heat and the formation of a reddish solid resembling PBr₅:—

PCl₃ + 2½Br₂ + ½I₂ = PBr₅ + ICl₃

A mixture of PCl₃ and Br₂ in molecular proportions when moderately cooled (to 10° C. or thereabouts) deposited crystals having the composition PCl₃Br₂. These melted at 35° C. with decomposition and separation into two layers. The compound resembles phosphorus pentabromide (q.v.) in appearance. When brought into contact with water it is said to give first HOBr, HBr and PCl₃, also POBr₃ and POCl.

At lower temperatures still more bromine can be combined, giving compounds such as PCl₂Br₃ by displacement of chlorine and PCl₂Br₅.

Many other addition compounds have been reported by various workers. By mixing bromine with phosphorus trichloride, heating on the water-bath and then cooling to about -5° C. brown needle-shaped crystals have been obtained, the analysis of which leads to the formula PCl₃Br₈. By allowing this compound to stand with PCl₃ in a sealed tube for some days red crystals separated which had the composition PCl₃Br₄.