Some Refictions
and Properties
of the
Phosphorus Sulfides J O H N C . P K R N K R T , Research D i r e c t o r , a n d J . H O W A R D B R O W N , Assistant Research Director, O l d b u r y Klectro-Chemical C o . , N i a g a r a Kails, Ν . Υ .
The history of the compounds of phosphorus and sulfur has b e e n called "A C o m e d y o f E r r o r s " as recently as the early 1 9 0 0 ' s · · . Intensive investigation and more reliable criteria have brought order and at present four different compositions are recognized J. HE reaction between phosphorus and sulfur was discovered in 1740 by A. S. Marggraf, who noted that the union is very vigorous—almost explosive—at elevated temperatures. This discovery preceded the birth of modern chemistry by a gen eration or two; it is therefore not sur prising that the early literature on the phosphorus-sulfur compounds contains numerous errors. In the course of the century or so following the discovery of the reaction there were described no less than 14 phosphorus-sulfur compositions ranging from P4S to P2S12 which were claimed as definite compounds. Intensive investigation and more reliable criteria have reduced the number of compositions recognized as chemical entities until, at present, there are only four. Their com positions are represented by the formulas P4S3, P4S6, P4S7, and P4S10. The chaotic state of knowledge of this subject even in the early part of the twentieth century led Mellor, in his ''Comprehensive Treatise on Inorganic and Theoretical Chemistry," to preface the chapter on the phosphorus sulfides with the following quotation: "The history of the compounds of phosphorus and sulfur is truly a comedy of errors. Compounds diligently investigated and carefully described by one worker could not be prepared by another worker or else are represented as mixtures.—B. Herscovici." All of the phosphorus sulfides may be prepared by direct combination of phos phorus or of the lowest sulfide, P4S3, with sulfur. In cases involving the thermal reaction combination is apt to be incom plete unless fairly high temperatures are used; a further complication is due to partial dissociation of the higher sulfides at elevated temperatures. I t is therefore ]X)ssible to have unreacted mixtures of the lower sulfides with sulfur which, lack ing suitable criteria, could be and un doubtedly often were mistaken for com pounds. Early in the present century the chem istry of the phosphorus-sulfur compounds was clarified by the work of several in VOLUME
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vestigators, among whom Alfred Stock was outstanding for the volume and accuracy of his contributions. With the assistance of numerous co-workers he published, dur ing the period 1905-13, a series of papers in which he pointed out for the first time, and repeatedly, that only three com pounds—viz., P4S3, P4S7, and P4S10— could be made by the thermal reaction of phosphorus with sulfur. He also estab lished the currently accepted formulas by molecular weight determinations and careful analyses; his critical data' are among the most reliable we have for these compounds. It is not greatly to the ' discredit of Alfred Stock that he overdid the clarifi cation; he failed to include the composi tion P4S.s in his list of compounds. The existence of this compound was reported by Boulouch in 1904, but his results were not fully confirmed until 1935, when Treadwell and Beeli prepared and studied the product in somewhat greater detail. This compound is the least familiar of the phosphorus sulfides and has been com paratively little investigated. It is the only member of the series which has not been commercialized. The physical properties of the four phos phorus sulfides are shown in Table I. The commercial history of the phos phorus sulfides begins very early in the twentieth century. We have found refer ences in the German literature to "com mercial phosphorus pentasulfide" and to "crude phosphorus sesquisulfide" which indicate that these were chemical com modities as early as 1903. It was not,
however, until about 1909 that phos phorus sesquisulfide attained any real importance; phosphorus pentasulfidc was not manufactured on a large scale until the mid-twenties; phosphorus heptasulfide was made commercially available in the late thirties. Phosphorus Sesquisulfide This compound became an industrially important chemica about 1909 when it was introduced into the match industry as a substitute for white phosphorus in the manufacture of the familiar "strike-any where" variety of matches. It is prac tically nontoxic and rapidly displaced the poisonous white phosphorus which had been universally used. The displacement was, indeed, accelerated by world-wide legislation forbidding the sale of white phosphorus matches in most countries. Minor quantities of phosphorus sesqui sulfide have been used in pyrotechnics, but the match industry has been the major consumer of this commodity for almost 40 years. Solutions of phosphorus sesquisulfide in organic solvents absorb oxygen from the air even at room temperatures and become turbid due to precipitation of an amorphous solid, a phosphorus oxysulfide having .the composition P4S3O4. This is a pale yellow product which melts indefi nitely between 150° and 250° C. and decomposes at a high temperature. It is very reactive with water. It has been presumed to be closely related, struc turally, to phosphorus heptasulfide, P4S7, but has been little investigated. Phosphorus sesquisulfide is a convenient starting material in the preparation of all of the higher sulfides. I t reacts with sul fur when heated, forming either phos phorus heptasulfide, P4S,0 P«S7 Yellow Color of solid Yellow Yellow Almost white Red-brown Color of melt at 300° C. Yellow η Brownish yellow Melting temperature ° C. (eorr.) 170-220» 305-310 286-290 173-174.5 a Boiling temperature ° C. 700 mm. 523 513-515 407-408 e 2.17 at 25° Density at 17 C. 2.1» 2.09 2.03 SolubUity 1:3,500 1:450 in carbon disulfide at 17° C ·. 1:1 1:20,000 1:550 in carbon disulfide at 0° (*. 1:3.7 1:1,200 1:9 in carbon disulfide at —20° C. 1:40 in benzene at 17° C. tt IV*6 is thermally unstable: it partially melts at 170°, then immediately resolidifies dun to its con version to a mixture of P4S3 and P4S7. When further heated this mixture melts at 220°.
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phorus pentasulfide, P4S5, by a similar method using special and controlled conditions has been claimed, but a more convenient method involves the photochemical reaction of phosphorus sesquisulfide with sulfur in a suitable solvent such as carbon disulfide, iodine being used as a catalyst (Helv. Chim. Acta, 18, 1161-71 11935]). Xo references to clear-cut, significant reactions of phosphorus sesquisulfide with organic reagents have been found; this is in sharp contrast with the higher sulfides which are useful mainly because of their organic reactions and derivatives. Phosphorus Pentasulfide The compound, P4S10, is often written P2S5 for convenience; this is often justifiable because, as a rule, it simplifies equations without loss of accuracy. It appears to have been first clearly described by lierzelius near the middle of the nineteenth century. It was extensively used for many years in the preparation of numerous sulfur-containing organic compounds such as mercaptans and thioacids; the procedures described are usually empirical, and a clearer understanding of some of them is due to comparatively recent work. Within the past 10 or 15 years there has occurred an avalanche of new publications, principally patents, describing modifications and variations of previously known reactions as well as several new ones. Phosphorus pentasulfide became an important commodity in the U. S. A. in the mid-twenties. The first significant industrial use was in the preparation of flotation reagents; a little later certain derivatives of the same general type were found to be valuable as ingredients of lubricants. These oil additives, alone or in combination with others, perform several useful functions. Their preparation and uses have engaged the attention of a large number of research organizations, and new reactions, new products, and new uses have been claimed at an accelerating rate. The reactions of phosphorus pentasulfide with organic acids and with alcohols are important partly because this is often a good method of introducing sulfur into organic compounds and partly because some of the intermediate organic phosphorus-sulfur derivatives have valuable properties. The reaction with alcohols has been known for about a century; it was used by Kekulé and by many others. The early interpretations of the course of the reactions were not too far from the truth, but their stage wise nature was not fully appreciated until rather recently. Phosphorus pentasulfide has recently been found to react with some hydrocarbons, specifically with terpenes such as α-pinene and with olefins. Little concerning the chemistry and structures of these reaction products has been pub 2144
lished. A review of the organic reactions of phosphorus pentasulfide inevitably leads one to the conclusion that the subject is in its infancy and that much fundamental work remains to be done. Phosphorus Heptasulfide The compound P4S7 was made avail able as a commercial product about 10 years ago. It has been used principally as a reagent in organic syntheses; it is of much less importance to industry than the two sulfides previously described. In the literature of organic chemistry there are countless references to the use of "phosphorus trisulfide" as a reagent, often accompanied by a suitable recipe for making it from red phosphorus and sulfur (A. H. Blatt, editor "Organic Syn theses," Coll. Vol. II, p. 579, New York, John Wiley and Sons, Inc., 1943). Al though the nonexistence of a compound of the formula P2S3 among the products of such a reaction was well established by Alfred Stock about 40 years ago, this fact has apparently not been noted by many organic chemists; repetition of the references and the recipe have persisted to the present day. When it became desirable to^supply this reagent in commercial quantities, "phos phorus trisulfide" was reinvestigated and the results reported by Stock and others were fully confirmed; all samples having the composition P2S3, corresponding to the trisulfide and made by thermal reaction of phosphorus with sulfur, were mixtures containing phosphorus heptasulfide as a major component and could be separated by the use of solvents into fractions of lower and higher sulfur content. The evidence indicated that a "phosphorus trisulfide" made in this way would be merely a somewhat impure phosphorus heptasulfide, so we decided to manufacture a technically pure heptasulfide. We do
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A. BATTISTA
(ALL. R I G H T S R E S E R V E D )
CHEMICAL
not know of any organic reaction in which the product has been found inferior to the mixture hitherto used. Phosphorus Oxysulfides The formation of a phosphorus oxysulfide by the direct oxidation of phos phorus sesquisulfide in organic solvents has been mentioned; this product is amorphous, melts indefinitely, and is thermally unstable; its formula is P4S3O4. A very stable phosphorus oxysulfide, tetraphosphorus tetrathio hexoxide, P4S4O6, was first prepared by Thorpe and Tutton by the reaction of phosphorus trioxide with sulfur in a sealed tube at 160° C. The reaction is violent; small quantities of the reactants should be used to avoid the danger of an explosion. When equivalent quantities of phosphorus trioxide and sulfur are used, the reaction is quantitative: P4U6 + 4 S
>• P4S4Oe
This compound was named phosphorus sulfoxide by its discoverers and has been commonly referred to by this name. It is described as a yellowish-gray mass which may be sublimed at 140° to 150° C , yielding colorless, feathery crystals. Thorpe and Tutton gave the melting point as 102° C. and the boiling point as 295°; it distills at atmospheric pressure without decomposition, condensing as a pale yellow liquid. It is soluble in some organic solvents, notably carbon disul fide, and may be recrystallized. It deli quesces in air and dissolves rapidly in water, forming an acidic solution. This phosphorus oxysulfide resembles phos phorus pentoxide in many respects more than it does the pentasulfide. Phosphorus sulfoxide is of special in terest because of its high degree of sta bility in the vapor phase; it is said to show little or no tendency to dissociate, and this property has led to extensive studies of its vapor. The structure of the vapor molecules was determined by the electron-diffraction method by Stosick (J. Am. Chem. Soc, 61, 1130-2 [1939]) who found it similar to that of phosphorus pentoxide. The preparation of this phosphorus oxy sulfide by the method of Thorpe and Tutton is obviously laborious. Phos phorus trioxide is not readily available, and it is not easy to prepare. Its reaction with sulfur is inconvenient and somewhat hazardous. We have found a reaction by which a product having properties and composition identical to that of Thorpe and Tutton may be prepared easily and safely from the readily available reagents, phosphorus pentoxide and phosphorus pentasulfide. An intimate mixture containing two mois of phosphorus pentasulfide and three mois of fresh phosphorus pentoxide be gins to react when heated to approxi mately 400° C. If the apparatus is deAND
ENGINEERING
NEWS
signed ior distillation, and if moisture is excluded, the reaction proceeds slowly at temperatures between 400° and 500°, the product distilling as rapidly as formed and condensing as a pale yellow liquid. The crude material ordinarily melts at about 100° C ; after one or two redis tillations, conveniently at reduced pres sure, it melts completely at 102° and be gins to freeze at this temperature when the liquid is slowly cooled. The boiling tem perature at 20 mm. Hg was found to be 180° to 190° C. The product prepared as described is a pale yellow solid, not unlike paraffin in consistency; it does not become com pletely solid at its freezing temperature but gradually hardens as the temperature falls to 80 °. Even at lower temperatures the solid is plastic enough to be deformed by the application of moderate pressure. These observations are similar to those recorded by Thorpe and Tutton. The new procedure for preparing this interesting compound is published here for the first time. Reactions of the Phosphorus Sulfides with Alkalies All of the phosphorus sulfides dissolve in and react with aqueous solutions of alkalies. The reactions are complex and have not been fully investigated. They are of special interest because they may furnish some insight into the structures of the sulfides. The products ultimately ob tained from the reactions include hydro gen, phosphine, hypophosphorous, phos phorous, and phosphoric acids, and hy drogen sulfide. Treadwell and Beeli published the results of a series of investigations on hydrolysis of the three common phos phorus sulfides; although they disclaimed a high degree of accuracy, their data are the only ones available and have been quoted in textbooks (cf. : Yost, 1). M., and Russell, Horace, Jr., "Systematic In organic Chemistry," 423 pages, New York, Prentice-Hall, Inc. (1944). We made a somewhat similar investi gation of the hydrolytic products· of these sulfides and obtained data which differed considerably in all cases, and radically in some, from those cited. Our results may be stated very briefly by indicating the distribution of the four atoms of phos phorus in each molecule of P4SX among the four principal phosphorus derivatives obtained: P«Sx P«S, P4S7 P«SM
Plia Η,ΡΟί H,PO, ll,!H)4 0 . 1 I. S3 1.9» 0.23 0.9Ô 0.87 2.03 4
ITnarcountod for 0.18 0.1Γ»
We conclude, therefore, that the ques tions are not finally settled and that speculations on structure and reaction mechanisms based on available data are open to doubt. More comprehensive and precise experimental work would be of great value. VOLUME
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Structure of the Phosphorus Sulfides In working with a series of related com pounds such as this we have inevitably speculated regarding their structures. The phosphorus molecule P4 is assumed to be correctly represented by considering each of the four atoms as occupying the four corners of a regular tetrahedron. In the highest member of the series, P4S10, the phosphorus atoms may be considered as arranged similarly, but with a sulfur atom interposed between each two phos phorus atoms. Furthermore, on each atom of phosphorus there is an additional sulfur atom which may be conveniently represented and designated as thiono-. These two structural representations, illus trated below, appear to be generally ac cepted.
Phosphorus
Phosphorus pentasulfide
There is greater difficulty in postulating reasonable structures for the intermediate members of the series, and we know of none which have achieved universal ac ceptance. There is ample evidence that the 60° Ρ—Ρ—Ρ angle in the phosphorus molecule produces a highly strained struc ture, since the unstrained angle X—Ρ—X, in the phosphorus halides, for example, approximates 100°. It is therefore rea sonable to suppose that the initial reac tion between phosphorus and sulfur in volves exclusively the production of a less severely strained molecule. There are three possible structures for the first sulfide of the series P4S3 based on this as sumption:
Λ\ /•
I
II
III
We would eliminate II on the basis that it obviously does not produce the least strained structure; there remain three 60° Ρ—Ρ—Ρ angles. From this point of view both I and III are more acceptable. It is interesting to observe that III, which at first glance appears highly unsymmetrical, can be drawn, as indicated by Ilia, to show a high degree of symmetry. » JULY
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>S -
\
Ilia
.S / IV
It is not inconsistent with known facts to assume that in the formation of P4S7 four thiono groups are formed as illus trated in IV. In the intermediate P4S& there would presumably be formed only two thiono groups, but we have no basis for choosing any particular pair. In speculating on the subject of hy drolysis mechanism we have assumed that the point of attack by the alkali would, most likely, be the sulfur atoms. From III the hypothetical first product of hydrolysis would therefore have struc ture V. HS—P—Ρ—Ρ P—OII ÔHSHOH SH V Further hydrolysis is indicated by VI. HOÎH
HO!H H*:OH
HS—P ! — Ρ — :P — :P—OH OH SH OH SH VI The product H2P(OH) or its isomer phosphine oxide, H3PO, shown as one of the theoretical fragments is not known. It undoubtedly is unstable and appears among the reaction products as PH 3 and H3PO2.
If it is assumed that all possible hydrolytic products are formed in the pro portions required by probability the ratios can be shown to approximate 2 mois of H 3 P0 8 :1.5 HaPCV.0.5 PH,. The reac tions are somewhat more complicated than this, as is indicated by the presence of hydrogen among the products, but the analytical results indicate ratios fairly close to those calculated. Similarly, it can easily be shown that phosphorus heptasulfide, structure IV. should produce 2 mois of H3PC>4:1 mol of H 8 P0 3 :1 mol of H3PO2. Again the ana lytical results correspond reasonably well to those calculated. Unfortunately, no conclusive decision can be made on the basis of available data as to which of the three types of structure should be selected as the most probable. All can be shown to be capable of yielding the same products in the same ratios. PRESENTED at the Symposium on Properties, Structure, and Thermodynamics of Inorganic Substances sponsored by the Division of Physi cal and Inorganic Chemistry, ACS, at Syracuse, Ν. Υ.. June 28-30, 1948.
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