View Article Online / Journal Homepage / Table of Contents for this issue STERN : ACTION OF BROMINE ON PHOSPHORUS TRICHLORIDE. 815 LXXVIL-On the Action of Bromine on Phosphorus Trichloride. By A. L. STERN,Scholar in the Chemical Laboratory of the Mason College, Birniiugham. THEaction of bromine on trichloride of phosphorus was first investi- gated by Gladstone (Phil.Mag., 35, 1849, 353-354), when attempt- ing to form a compound of phosphorus containing both halogens, that is, a chlorobromide of phosphorus. He found that when bromine was Published on 01 January 1886. Downloaded by Temple University 26/10/2014 14:23:36. poured into phosphorus trichloride, it sank to the bottom and two layers were formed, the upper consisting of a solution of bromine in the trichloride and the lower of a solution of the trichloride in bromine. These two layers could not be made to mix, but on adding a little iodine combination immediately took place with develop- ment of much heat, and on cooling red crystalline masses separated, resembling phosphorus pentabromide with excess of bromine. These crystals having been freed as far as possible from the mother-liquor, were analysed by decomposing them with water and precipitating with silver nitrate. Their percentage composition was found to be- Bromine ...................... 88.6 Chlorine ...................... 6.4 Phosphorus (by difference) ....... 5.0 This Gladstone regarded not as a compound of both halogens, but View Article Online 83 6 STERN ON THE ACTION OF merely as phosphorus pentabromide contaminated with a little chloride, the pentabromide requiring-. Bromine.. ............. 92-45 per cent. Phosphorus ........... 7.55 ,, The reaction is probably PCI, + Br,I = PBre + IC1,. Gladstone also tried other methods for forming the chloro-bromide but without success. Wichelhaus (ArtnnnZen, Supp. 6, 277) states that phosphorus tri- chloride and bromine combine with development of heat, and that the product when cooled by a powerful freezing mixture deposits crystals, which he assumed to be PCI,Br,. On exposure to the ordinary tem- perature, however, these melt, and the liquid separates into two layers. Friedel and Ladenburg (BUZZ. Xoc. Chim., 8, 1867, 146) used a mixture of PCl,+Br,, and found it reacted as if it was PClBBr2. Michaelis (Bey., 5, 9) found that the chlorobromide was stable at ordinary temperatures. By exposing a mixture of the trichloride with bromine in the proportion PCl, : Brz in a tube for some time to a winter temperature, he obtained the compound PCl,Br, (the analysis gave PCl,Br, + 5.2 per cent. PCls), decomposing into PCl, and Br, at 35". On placing crystals of this or even of some of the mother-liquor in a mixture of phosphorus trichloride and bromine, the lower layer immediat>ely solidified, the compound PCl,Br4 sepa- rating out, just as a supersaturated solution solidifies when a crystal of the salt is dropped into it. On treating this compound with PCI, Published on 01 January 1886. Downloaded by Temple University 26/10/2014 14:23:36. he found it to be unaltered. Prinvanlt (Compt. Tend., 74, 868) found that a homogeneous liquid was obtained on gradually adding bromine to phosphorus trichloride. On heating this on a water-bath until bromine was no longer evolved, a red liquid was left, which crystallised at -4" or - 5", forming brown oily needles, and it is stated that this compound can be distilled un- changed below go", producing a colourless vapour of the formula PCl,Br,. Quick distillation above go", however, decomposes it, crys- t'als of PCl,Br, being obtained in the receiver ; if this is dissolved in cold PCl,, and the solution boiled, yellow ci-ystals of PClaBr separate on cooling. Prinvault also obtained PCI,Br4 thus :- PCI,Br, + PCL = PCl,Br, + PCl4Br, PC1,Br7 + PCltBr = 2PCl,Br4. Michaelis (Ber., 5, 414) has repeated the above experiments, and View Article Online BROMINE ON PHOSPHORUS TRICHLORIDE. 817 obtained different results. When 2Br2 was added to PCZ, there appeared after strong heating a homogeneous oily liquid, which on cooling crystallieed, giving brown needles with a green reflex, of the composition approximately represented by the formula PCI,Br, ; he found that this compound could not be distilled without decompo- sition, and that its vapour was of the colour of bromine. Michaelis considers these compounds to have a similar constitution to PCI,,ICl and PC1,,Fe,Cl6 ; while Prinvault holds them to be compounds of PBr, with BrC1. With a view to the more complete investigation of this reaction, the following experiments were performed. Phosphorus trichloride mas mixed with bromine (free from iodine) in various proportions, the mixture allowed to &and? and the crystals which were deposited were analysed. Three series of experiments were performed. Phosphorus trichloride and bromine were run from two burettes into small stoppered bottles ; heat was developed, and on cooling crystals were deposited ; in order to obtain them free from the mother-liquor, the contents of the bottles were poured into funnels, the stems of which were plugged with asbestos, and the funnels were then placed in a closed vessel over soda-lime. In order to obtain them even approximately free from the mother-liquor, they had to drain for at least 24 hours ; during that time decomposition seemed to take place, and on attempting to transfer them from the funnel to a small weighing tube, they deliquesced so rapidly that it was useless to analyse them. A process had therefore to be devised by means of which the crystals could be freed from the mother-liquor and weighed without exposing them to the atmosphere. This was clone as follows. Published on 01 January 1886. Downloaded by Temple University 26/10/2014 14:23:36. A glass tube about inch in diameter and 10 inches long, was drawn out into a narrow neck at one end and the bottom of this sealed off. The narrow neck having been plugged with asbestos, measured quantities of phosphorus trichloiide and bromine were run in at the top from a dry burette, the lower sealed part of the tube being occasionally cooled to moderate the heat produced by the reaction, otherwise the PC1, boiled. The tube was then sealed off at the top. In order to separate the crystals which formed on cooling, the two ends of the tube were broken and a current of air dried by sulphuric acid was drawn through the tube by means of a filter-pump in communication with the lower end. When the mother-liquor had been drawn off, both ends of the tube were re-sealed, and the whole weighed. The crystals were then dissolved in solution of sulphurous acid by breaking off one end of the tube under the surface of the solution, and allowing the liquid to rise in the tube. The tube was then 3KS View Article Online 818 STERN ON THE ACTION OF washed, dried, and re-weighed, together with the ends broken off. The difference in weight gave the amount taken for analysis. The solution was made up to a definite volume, and to a measured quantity of this, decinormal silver nitrate added as long as a precipi- tate formed. The phosphorus was determined by difference. The trichloride and the bromine were mixed in four different proportions. (i.) PCI, : Br2.-Heat was developed on mixing, and two layers were formed, the lower dark-red and the upper light-red; after standing a few hours the lower one suddenly solidified to a brick-red crystallin e mass. (ii.) PC13 : 2Br2.-More heat was developed on mixing than in the former case, and a homogeneous red liquid was formed ; a quantity of dark-red crystals separating out on cooling. (iii.) PCI, : 3Br2.-A considerable amount of heat was developed on mixing, and a homogeneous red liquid was formed ; on sealing iip and shaking, a quantity of dark-red crystals separated. In each of these three experiments, the mother-liquor was drawn off on the following day ; temperature about 13". (iv.) PC13 : 6Br2.-A considerabIe amount of heat was developed on mixing; a homogeneous red liquid was formed, and dark-red crystals separated out on cooling. These were left in contact with the mother-liquor for 31 days, in order to see if all the chlorine could be displaced by prolonged contact with excess of bromine. This, however, was not the case. As the phosphorus in this series was merely estimated by difference, and as no precautions were taken to eliminate moisture from the bromine, or phosphorous acid from the trichloride, and as the use of Published on 01 January 1886. Downloaded by Temple University 26/10/2014 14:23:36. asbestos as a filtering material might be open to objection, a second series of analyses was performed. In this series, the bromine was first tested for iodine, and found to be free from it, then shaken up, and left in contact with strong sulphuric acid, aud finally distilled into a dry tube similar in construction to the one previously described, except that the asbestos was replaced by platinum gauze fused into the tube. The phosphorus trichloride was also distilled directly into the tube. In the former series, the liquids were measured from a, burette, in this one by marking lengths on the tubes proportional to the required volume. The mode of analysis was slightly modified; the crystals were decomposed as before by dilute sulphurous acid, and the solution made up to a definite volume. To determine the chlorine and bromine, silver nitrate was added to a portion of the solution and the precipitate of Ag(C1,Br) weighed; a portion of this was then transferred to a View Article Online BROMINE ON PHOSPHORUS TRICHLORIDE.