T H E FORMATION OF PHOSPHORUS PENTACHLORIDE FROM PHOSPHORUS TRICHLORIDE AND CHLORINE' BY H. AUSTIN TAYLOR
The results of recent work2 on gas reactions have led to the conclusion that nearly all chemical reactions are catalytic in nature, whether they are catalysed by the surface of the containing vessel or by the water present in the majority of reaction systems. The earlier ideas of the general stability of molecules have to be modified in view of the great influence of polar substances on various reactions, and a t the same time, the number of gaseous bimolecular reactions which are now known to occur in the gas phase without a catalyst in any form, is rapidly decreasing. It is of interest therefore to investigate some of the more well-known reactions which are at present assumed to be straight bimolecular reactions. An attempt has been made in this work to show that the formation of phosphorus pentachloride from the trichloride and chlorine is actually an uncatalysed reaction occurring in the gas phase and in absence of water vapour. The known properties of phosphorus trichloride and its general reactivity would lead one to expect that it required a very small amount of activation, if at all, and hence would probably react in the gaseous state. At the outset the intention was t o see if there was any difference between the amounts of phosphorus pentachloride which formed on a typical polar surface, glass, and on a typical non-polar surface such as paraffin wax, when the mixed vapours of phosphorus trichloride and chlorine were passed over both for the same length of time. With this end in view, a vessel was moulded in paraffin wax, Norrish having shown that on coating a glass vessel with wax the cracks in the latter offered sufficient glass surface for the reaction to proceed to some extent. The vessel consisted of a reaction chamber of approximately IOO cubic centimetres capacity, carrying at one end two side-tubes entering in opp'csite directions through which the trichloride and chlorine vapours respectively could be passed. At the other end of the wax vessel was a third tube as outlet for remaining gases. The third tube was sealed to a glass reaction chamber by melting the wax tube, so that the gases before finally leaving the apparatus passed first over the wax surface and then over the glass. The phosphorus trichloride was purified by distillation and boiled at 75.9OC. Chlorine was prepared by the action of hydrochloric acid on potassium permanganate, was washed free from any hydrochloric acid by passing through two gas washers containing water and dried by two gas washers containing sulphuric acid and finally over phosphorus pentoxide in a tube about 30 cm. in length. The resulting dry chlorine was liquefied in a spiral Contribution from the Laboratory of Physical Chemistry. Princeton University. *Inter a h Norrish: J. Chern. SOC.,123, 3006 (1923).
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gas bubbler immersed in a freezing mixture of solid carbon dioxide and ether. The phosphorus trichloride was placed in a similar spiral bubbler, maintained at room temperature and attached directly to one arm of the reaction chamber, the other arm being waxed to the chlorine holder. In this way the vapours of the two liquids could be mixed in the wax vessel by passing dry nitrogen through both bubblers, since the vapour pressures of both liquids are quite appreciable under the conditions cited. The nitrogen used was drawn from a cylinder of the compressed gas and passed successively through a drying tube of phosphorus pentoxide and a U tube containing glass wool, immersed in liquid air, to freeze out remaining traces of water vapour. By means of a by-pass (which was later sealed) in the chlorine bubbler, dry nitrogen, or nitrogen with some phosphorus trichloride vapour, could be passed through the whole apparatus, by first solidifying the chlorine temporarily in liquid air. Passing dry nitrogen in this manner through the whole apparatus for a period of twenty-four hours whilst repeatedly heating the glass portions which were made of Pyrex, the residual water on the surfaces could be more efficiently removed. The wax vessel on the other hand was subjected to a stream of nitrogen and phosphorus trichloride vapour for the same length of time. The small amount of water present may be assumed to react with the trichloride to give the solid phosphorus oxychloride POCl which itself possesses an appreciable vapour pressure at ordinary temperatures and would thus volatilise away. It is reasonably certain that, in this way the reaction system was dry: more certain is it that conditions were drier than held in the reaction studied by Norrish between ethylene and bromine, where water vapour was assumed absent. The first test was then made, by liquefying the chlorine again in carbon dioxide snow, and passing nitrogen through both liquids as described above. After a period of an hour and a half when a reasonable amount of the pentachloride was visible in the glass vessel the two reaction chambers were detached and weighed separately. Dry air was then blown through each to volatilise the phosphorus pentachloride and the empty vessels reweighed. The difference between the two weighings gave the amount of pentachloride in each of the two vessels. It was found that, the wax vessel contained 3.810g. whilst the glass vessel contained only 0.384g. That is, the reaction had apparently taken place almost entirely in the wax vessel. The reaction therefore appeared to be independent of the containing surface the presence of phosphorus pentachloride in the glass vessel being due to reaction between some molecules which had not reacted in the wax vessel, or to condensation in the glass vessel from the vapour phase. That such was actually the case was shown by replacing the wax vessel by an almost identical one of glass and repeating the experiment. Connections between the two glass reaction chambers was made with paraffin wax, and the same drying process observed as previously. It was found finally that after
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H. AUSTIN TAYLOR
passing the gases for half an hour' the first glass vessel which replaced the wax one used previously contained 0.6gg. of phosphorus pentachloride whilst only o.o3gg., formed in the second. Although no comparison can be made between the amounts of phosphorus pentachloride formed in a given time in both the wax and glass vessels owing to the difference in rates of nitrogen flow in the two cases, nevertheless the second result substantiates the fact that the major part of the reaction occurs when the vapours first mix, that is in the first reaction chamber, and since the polarity of the surface is apparently negligible the reaction must occur in the gaseous phase. If this view is correct the reaction ought to be visible owing to the formation of phosphorus pentachloride as a cloud. To test this, the two vapours in dry nitrogen were made to impinge on one another in a three litre flask. It was observed that immediately the gases were admitted to the flask a small amount of phosphorus pentachloride formed as a film on the glass and after a delay of 2 0 or 30 seconds the whole flask was suddenly filled with a white cloud which was composed of quite large particles of phosphorus pentachloride resembling fine snow flakes, which dropped to the bottom of the flask; the continued reaction occurred, still as a cloud, but of a much finer texture and thinner, being at times scarcely visible. The whole phenomenon resembled the precipitation of a supersaturated vapour, since it was further noticed that the cloud formation was initiated in the region where the first film of phosphorus pentachloride had formed, and travelled frqm there in a direction opposed to the normal direction of flow of the gases, towards the point at which the gases first impinged oh one another. The reason for this would appear to be, that the supersaturated vapour of phosphorus pentachloride formed where the phosphorus trichloride and chlorine impinged, spread under the force of the nitrogen stream towards the phosphorus pentachloride film on the flask, where precipitation was caused by particles of solid phosphorus pentachloride, the precipitation then travelling back throughout the whole supersaturated vapour. Succeeding reaction between phosphorus trichloride and chlorine would not show such a definite cloud formation as initially, since the numerous phosphorus pentachloride particles throughout the flask would prohibit the formation of further supersaturated vapour, precipitating it as formed. The rigorous process of drying adopted in the first experiments quoted, was not observed with the flask later employed, save that dry nitrogen and phosphorus trichloride were blown through it for a short time before the chlorine was admitted. Two possible explanations remain to be excluded, namely that dust particles, or water act as catalysts in the case of the cloud formation. The first experiments would exclude the possibility of water vapour acting as a catalyst, since phosphorus pentachloride was formed in its absence, 1 It was noticed in this second case that phosphorus pentachloride had begun t o cll,op from the first chamber into the second, and it seemed advisable to stop the experiment even after this short period.
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whilst the delay before the actual formation of the cloud would assist in the removal of traces of water by the phosphorus pentachloride first formed. the cloud then forming in a dry atmosphere. However, on introducing water intentionally by passing the chlorine vapours through water before mixing with the phosphorus trichloride it was found that the formation of a cloud was completely inhibited, the only evidence of reaction being the film on the walls of the vessel. Nor was it possible to produce the cloud again until the flask had been once more dried. Baker1, after repeated failures, succeeded once in obtaining a vapour density of phosphorus pentachloride greater than that for complete dissociation. This ill success even after vigorous drying indicates that water has little influence in the opposed reaction and therefore confirms the above conclusion. With regard to dust particles, the delay in cloud formation would also seem to preclude this possibility. The manner of propagation would also indicate the absence of effect due to dust particles since we can only assume that the dust particles are as likely to be situated in one spot as another. The course of the cloud formation as previously observed was confirmed by passing the gases, both in the same direction down a horizontal glass tube, two metres long and seventy five millimetres in diameter. A thin film of phosphorus pentachloride was first observed on the tube fifteen centimetres away from the inlet tubes, spreading slowly down the lower half of the tube throughout its whole length. After a period of about a minute a cloud started and spread thence in both directions namely back towards the inlet tubes and also down the tube in the normal direction of the gas flow It would appear therefore, that the explanation of the cloud formation offered above, namely by some phosphorus pentachloride which had already condensed on the glass surface, was true. From this it can only be concluded that the reaction between phosphorus trichloride and chlorine to give phosphorus pentachloride is a gas reaction at ordinary temperatures occurring in the gas phase and unaffected by catalysts. The speed of the reaction under the conditions cited precludes the evaluation of the rate of combination. It has been shown that liquid chlorine and phosphorus trichloride react violently at 87 -"C .
Summary Dry phosphorus trichloride and chlorine vapours in dry nitrogen react as completely in a wax vessel as in a glass one. 2. The phosphorus pentachloride formed gives rise to an easily supersaturated vapour which is precipitated as a white cloud by some phosphorus pentachloride condensed on the containing vessel. 3. The reaction is a true gaseous reaction being unaffected by surface catalysts as also by water. I n conclusion I desire to thank Professor H. S. Taylor a t whose suggestion the problem was'investigated and for advice and assistance during its progress. I.
Princeton, N . J .
J. Chem. SOC.,77, 648 (1900).
