3,119,666 United States Patent Office Patented Jan. 28, 1964 1. 2 solvent is immaterial, since in practical operation, I re 3,119,666 use the solvent for the reaction after the suspended phos METHOS FOR THE PREPARATION OF phorus pentabromide is removed therefrom by filtration PHOSPHORUS PENTABROMIDE or by centrifuging after the reaction is completed. There Richard C. Nametz, St. Louis, Mich., assignor to fore, upon the first use of the solvent it becomes satu Michigan Chemical Corporation, St. Louis, Mich., a rated with the small amount of phosphorus pentabromide corporation of Michigan which it will dissolve, and will dissolve no more of the No Drawing. Fied Nov. 12, 1957, Ser. No. 695,548 product upon reuse. 12 Claims. (C. 23-205) In this method, I utilize substantially equimolar quan This invention relates to an improved method for the O tities of bromide and of phosphorus tribromide, together production of phosphorus pentabromide. with an amount of the co-solvent for bromine and phos Phosphorus pentabromide (phosphoric bromide) is a phorus tribromide relative to the quantities of the re known compound having a melting point above 100 C., actants used which is within the range which will suspend at which temperature it decomposes to form phosphorus the phosphorus pentabromide as a slurry which can be tribromide with the evolution of bromine. Due to its 5 readily stirred and which at the end of the reaction can heat sensitivity, phosphorus pentabromide cannot be puri be readily filtered, but which does not provide an un wieldly bulk of material to handle. fied by distillation and is difficult to purify due to its I have found that it is conventient to use an amount of highlyPhosphorus reactive pentabromidenature. has heretofore been pre the co-solvent within the range of about 400 ml. to about 20 1000 ml. and preferably within the range of about 600 pared by direct addition of bromine to phosphorus tri ml. to about 700 ml. for each gram mole of phosphorus bromide. Phosphorus pentabromide is a solid at the ten pentabromide produced. peratures used for its formation by this reaction, which In carrying out this method, I may dissolve the phos must be below those at which it tends to decompose to phorus tribromide to be reacted in the entire quantity revert to phosphorus tribromide with the release of bro 25 of the co-solvent to be used and add the bromine to this nine. In the course of this reaction, the reaction mix Solution while maintaining anhydrous conditions. Again, ture solidifies to a solid mass which is difficult, if not I may dissolve the bromine in the co-solvent and add this impossible, to stir. This solid mass consists of the Solid Solution to the phosphorus tribromide. However, I have phosphorus pentabromide, and entrapped liquid phos found that it is desirable to dissolve the phosphorus tri phorus tribromide. When the remainder of the bromine 30 bromine in a portion of the inert co-solvent and the necessary to convert the unreacted, entrapped phosphorus bromine in the remainder cf the co-solvent to be used in tribromide is added to this solid mass, it forms a more the reaction and add the bromine solution to the phos or less segregated liquid phase, which cannot react effi phorus tribromide solution. In this manner, I am able ciently with the remainder of the phosphorus tribromide. to minimize the proportion of the co-solvent present at These phase relationships between the desired solid 35 the beginning of the reaction, and progressively increase pentabromide product, the liquid bromine and the liquid the proportion present as the reaction proceeds and greater phosphorus tribromide, together with the heat-sensitivity proportions become necessary to keep the solid phos of the phosphorus pentabromide, have made its com phorus pentabromide being produced in the form of a mercial production quite difficult and generally uneco suspension in the reaction mixture and avoid the setting nomical. 40 up of the reaction mixture as a solid mass. It is the object of this invention to provide a method By this method, I find that I can obtain a substantially for the preparation of phosphorus pentabromide which is quantitative reaction with a yield approximating 97-98% efficient and economical to operate, and which produces a of phosphorus pentabromide of high purity. As bromine product of high quality. is added to the solution of the phosphorus tribromide, the Other objects of this invention and its various advan phosphorus pentabromide crystallizes from the solution tageous features will become apparent from the descrip giving a readily filterable suspension. After an equi tion which follows. molar quantity of bromine has been added to the solution By the method in accordance with this invention, I pro of phosphorus tribromide, the phosphorus pentabromide duce phosphorus pentabromide by reacting phosphorus is removed from the solution by filtration after being tribromide with bromide in the presence of an inert Sol 50 cooled to a temperature of about 20° C. The resulting vent for both the phosphorus tribromide and the bromine, coarse yellow crystals are then subjected to vacuum dis which is not a solvent for phosphorus pentabromide, while tillation to remove the residual solvent. The filtrate con maintaining anhydrous reaction conditions and keeping sisting of the co-solvent, any unreacted bromine or phos the reaction mixture at a temperature within the range phorus tribromide which it may carry, together with any of Inabout general, 30° C. the to inertabout volatile, 35 C. halogenated aliphatic hy 55 phosphorus pentabromide dissolved therein may be re drocarbons are suitable solvents for use in this method. used without distillation or other purification in the pro Carbon tetrachloride, ethylene dichloride, tetrachloro duction of another lot of the phosphorus pentabromide. ethylene, chloroform, methylene chloride and similar The method in accordance with this invention will be compounds are entirely suitable, and I prefer carbon tetra specifically illustrated by the following example. chloride since its volatility and other characteristics make 60 Example it convenient to use in the reaction. I specified, in the foregoing, that the inert solvent for A solution of 271 grams (1.0 mole) of phosphorus tri bromine and phosphorus tribromide which I utilize is a bromide in 500 ml. of carbon tetrachloride was prepared. non-solvent for phosphorus pentabromide, it will be fully A solution of 160 grams (1.0 mole) bromine in 150 ml. understood that I use the term "non-solvent” in a rela 65 of carbon tetrachloride was added to this phosphorus tri tive sense. I prefer that the solubility of the phosphorus bromide solution while maintaining the mixture at a tem pentabromide in the solvent be quite low. However, a perature of 20° C. to 30° C. and maintaining anhydrous slight solubility of the phosphorus pentabromide in the reaction conditions by the use of phosphorus pentoxide - 3,119,666 - - - eS - 4 drying tube. The phosphorus pentabromide started to to produce a suspension of phosphorus pentabromide in separate from the reaction mixture in the form of yellow the said solvent, filtering the phosphorus pentabromide crystals, when approximately one-half of the bromine from the said suspension and removing the residual sol solution had been added. At the end of the addition of vent from the phosphorus pentabromide filtrate by vacu the carbon tetrachloride solution of bromine, the reaction um distillation. mixture was cooled to a temperature of about 20 C. and 6. A method for the production of phosphorus penta was found to be readily filterable. The coarse yellow bromide which comprises dissolving phosphorus tribro crystals separated from the solution by filtration were mide in an anhydrous chlorinated lower aliphatic hydro subjected to vacuum distillation under an absolute pres carbon solvent, slowly adding thereto about an equimolar sure of 5 millimeters to remove the residual carbon tetra O proportion of bromine dissolved in the same solvent while chloride and give the final product. maintaining anhydrous conditions of reaction and utiliz In the foregoing I have specifically exemplified the ing a total amount of the said solvent which keeps the method in accordance with this invention and set-forth phosphorus pentabromide formed by the reaction in sus various details as to the method. However, it will be ension throughout the course of the reaction and pre understood by those skilled in the art, that numerous 15 vents the reaction mixture from setting-up as a massive deviations may be made from the exact procedure set Solid, and upon the completion of the reaction separating forth without departing from the spirit of my invention or the phosphorus pentabromide from the said solvent by the scope of the claims which follow. filtration. I claim: 7. A method for the production of phosphorus penta 1. A method for the production of phosphorus penta 20 bromide which comprises reacting phosphorus tribromide bromide which comprises reacting phosphorus tribromide and bromine under anhydrous conditions while in solu and bromine under anhydrous conditions in the presence tion in carbon tetrachloride to produce a suspension of of an anhydrous chlorinated lower aliphatic hydrocarbon phosphorus pentabromide in the carbon tetrachloride, as a solvent for the bromine and the phosphorus tribro filtering the phosphorus pentabromide from the said sus mide in which phosphorus pentabromide is no more than pension and removing the residual carbon tetrachloride slightly soluble to produce a suspension of phosphorus from the said phosphorus pentabromide by vacuum dis pentabromide in the said solvent, separating the phos tillation. phorus pentabromide from the said solvent, and remov 8. A method for the production of phosphorus penta ing the residual solvent from the phosphorus pentabro bromide which comprises dissolving phosphorus tribro mide.