REACTIONS I N PHOSGENE SOLUTION. I. BY ALBERT F. 0. GERMANiV
That phosgene is an extremely active compound is attested by the fact that the great majority of references to it’ describe its use in some synthetic process. On the organic side, phosgene has been shown to be particularly reactive, and while the particular direction of its reaction with a given compound may vary much with the precise conditions of temperature, concentration, etc., in general, to use the words of Kuhnz, “the known reactions of phosgene proceed in two directions; in the first case they depend on the affinity of chlorine for atoms of hydrogen or of a metal in amines, hydrocarbons, metallo-organic compounds, and compounds containing the hydroxyl group; in the second they depend on the tendency of phosgene to be converted into carbonic acid in the action on aldehydes, ketones and acid aniides, while the carbonyl oxygen of these compounds is exchanged for chlorine.” Kuhn then describes a new type of reaction in which “phosgene is capable of lifting the bond between carbon and tertiary amine nitrogen; the valences thus freed are satisfied by means of the phosgene parts, CO and 2C‘l. This reaction has much reseniblance to hydrolysis, and may have some connection with the ionizability of phosgene in solutions.” The work of Beckmann and Junker3 throws some light on the question as to whether phosgene yiclds conducting solutions. Molecular weight determinations using phosgene as ebullioscopic liquid showed that acetic and benzoic acids dissolve in this solvent with the formation of double molecules, while the anhydrides of these acids dissolve as simple molecules. Since this behavior is recognized as a property of non-ionizing solvents, phosgene is classed with these. The dielectric constant has apparently not been determined, nor has anyone tried to electrolyze phosgene solutions, (See page 885). As a solvent for inorganic compounds, Beckmann and Junker found phosgene to be very poor. Red phosphorus, arsenic, arsenious oxide, boric oxide, antimony, bismuth, stannous chloride, selenium and its chlorides, sulfur, sodium, potassium, calcium, thiocyanates, sulfides and sulfates of the alkalies and alkaline earths, zinc chloride, ferric chloride, ferric sulfate, chromic chloride, mercuric chloride and iodide, the chlorides of copper, lead chloride and chromate and silver chloride were all found to be insoluble. On the other hand they found iodine, iodine trichloride, the chlorides of antimony and the chlorides of sulfur soluble. John Davy,4 who discovered phosgene in the course of the controversy about the nature of chlorine, made a number of observations concerning its chemical behavior; its condensation with ammonia, and its action upon potassium, tin, zinc, antimony and arsenic when these are heated with the gas, forming chlorides of the respective metals and carbon monoxide. Bibliography. Berolaheimer: J. Ind. Eng. Chem., 11, 263 (1919). 1986 (1908). Z . anorg. Chem., 55, 371 (19~71, Phil. Trans., 102, 144 (1812); hicholson’s Journal, 30, 28 (1811).
* Rer., 33, 2900 (1900). See also Hofmann: Z. angew. Chem., 21,
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ALBERT F. 0.G E R M A "
Baud1 found that phosgene readily dissolves anhydrous aluminium chloride, and that the solution yields various phosgenates. He identified three of these. Following this work, Barta12 investigated the solubility of the other halides of aluminium; he found the fluoride insoluble; the bromide was converted to chloride, with the production of carbonyl chlorobromide ; the iodide reacted violently with the phosgene, yielding free iodine and a complex aluminium compound, Besson3 made a series of observations on phosgene reactions, He found that hydrogen bromide reacted above zoo' C., to give traces of carbonyl bromide. Hydrogen iodide dissolved abundantly in phosgene cooled with ice and salt; but after a certain concentration was reached, a violent reaction occurred, yielding a quantity of iodine; using methyl chloride as a cooling bath, solution of HI proceeded regularly; but when the tube containing this solution was sealed off, and was left for several hours a t a temperature several degrees below zero, a considerable quantity of iodine separated, and the tube contained an equivalent quantity of carbon monoxide. Phosphonium bromide reacted in the cold; rapidly a t 5o'C.; carried out in a sealed tube, very high pressures resulted, and hydrogen chloride, hydrogen bromide, solid and gaseous phosphine, and carbon monoxide were formed. Phosphonium iodide reacted slowly at zero, yielding hydrogen chloride, carbon monoxide, phosphorous, and iodides of phosphorous, Phosphine was without action. Hydrogen sulfide gave carbon oxysulfide at 200'. Hydrogen selenide gave hydrogen chloride, carbon monoxide and selenium at zoo'. Selenium heated with phosgene at 230' gave carbon monoxide and selenium dichloride. The Chemical Warfare Service has measured the vapor tension of solutions of hydrogen chloride in phosgene4. I n contrast with the general insolubility of inorganic compounds in liquid phosgene, organic compounds are, as a class, readily soluble. This fact undoubtedly is partly responsible for the readiness with which organic compounds enter into reaction with it. As an acid chloride one would expect it to be reactive, and as a derivative of carbonic acid one would expect it t o be a good solvent for carbon compounds. That phosgene is fundamentally as reactive with inorganic compounds is evident from the results of numerous investigations working a t elevated temperatures, where the speed of reaction is much accelerated. Nuricsanb passing phosgene over heated metallic SUIfides, prepared carbon oxysulfide; the preparation was especially successful with cadmium sulfide at 27oOC. Chauvenet6 using the same method converted oxides and sulfides into anhydrous chlorides. Barlot and Chauvenet' extended the method t o phosphates and silicates, using temperatures ranging Compt. rend., 140, 1688 (1905). 152 (1907); 56, 49 (1907). 3Compt. rend., 122, 140 (1896). 4 Edgewood Arsenal Chemical Laboratory Report No. 223; see also W. M. Schaufelberger: Stanford University Thesis, 1920. Ber., 24, 2967 (1891). eCompt. rend., 147, 1046 (1908); 152, 87, 1250 (1911). 7 Compt. rend., 157, 1153 (1913).
* Z. anorg. Chem., 55,
REACTIONS IN PHOSGENE SOLUTION
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from 350' to 1400'. Baskervillel describes a method for opening up acid insoluble minerals based on the great reactivity of phosgene a t temperatures of 450' and above. Milligan, Baude and Boyd2,working a t Edgewood Arsenal, found they could utilize the waste phosgene in the tail gas from the condensers by passing the gas over a mixture of arsenious oxide and carbon a t temperatures above 175', the carbon acting as catalyzer, and arsenic trichloride being quantitatively formed. Delepine and W e 3 say that liquid phosgene free from chlorine, compressed in iron cylinders, will not attack iron, but will attack iron rust, forming ferric chloride, some of which dissolves in the phosgene. S~hutzenberger~ attempted to catalyze the carbon monoxide chlorine reaction with platinized asbestos; the method was not successful because the nascent phosgene or the mixed gases destroyed the catalyst, forming volatile platinum carbonyl chlorides, Hamor and Gill6 were able to synthesize the mineral phosgenite, PbCO;,. PbC12, by passing phosgene over lead hydroxide heated to 170'. Dixon's attempt t o prepare carbonyl thiocyanate6 by a reaction between metallic thiocyanates and phosgene yielded solutions of the compound, but he was unable to get the pure substance. Germann and Jersey' have reported the formation of molecular compounds between phosgene and chlorine, and between phosgene and boron trifluoride a t very low temperatures. Paterna and Mazzucchellis have measured the vapor tension of the solution formed by saturating phosgene with chlorine a t - I ~ ' C , ,from - IS'C. t o +4ooC. The present investigation was undertaken in order to clear up certain points about the behavior of liquid phosgene, with particular reference to its free chlorine content, and the activity of its combined chlorine, Phosgene is dissociated into chlorine and carbon monoxide by ultraviolet light, as has been shown by Weige~%,~ by Coehn and BeckerlO, and by Berthelot and Gaudechon;ll and also by heat, as shown by the studies of Bodenstein and Dunant12 Harak,l3 Atkinson, Heycock and Pope,14 and others. According to the measurements of Atkinson, Heycock and Pope, which are the most complete, and probably the most reliable published, phosgene is appreciably dissociated by heat a t teniperatures above 200°, the degree of dissociation reaching nearly 20% a t 400°, and 50% at 500'. This would seem to indicate that the reactivity of phosgene a t temperatures above 200' may be due to the presence of free carbon monoxide and chlorine. As a matter of fact, most of the reactions Science, 50, 443 (1919). Ind. Eng. Chem., 12, 221 (1920). a Bull. 27, 288 (1920). Compt. rend., 66, 666 (1868); Bull. ( z ) , 10, 188 (1868); Ann. chim. phys., (4), 21, 350 (1872). 6Am. Jour. Sci., (4), 47, 430 (1919). J. Chem. SOC.,83, 84 (1903). Science, 53, 582 (1921). * Gazz. chim. ital., 5 0 , I, 30 (1920). Ann. Physik., 24, 243 (1907). lo Ber., 43, 130 (1910). l1 Compt. rend., 156, 1243 (1913). l2 Z. physik. Chem., 61, 437 (1908). l8 Thesis, Berlin, (1909). 14 J. Chem. SOC., 117, 1410 (1920). 2
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described above, taking place a t elevated temperatures, may be carried out successfully by substituting for phosgene an equimolecular mixture of carbon monoxide and chlorine, as has been shown, among others, by Ribanl and by Matignon and Delepine2 , The work of Weigert3 on the influence of light on the phosgene equilibrium loses most of its value because he enclosed his gas in a glass container, instead of quartz. Coehn and Becker4, using ultraviolet light, found that phosgene is appreciably dissociated when exposed in quartz vessels; but there is no dissociation when exposed in glass vessels; these results were obtained by direct measurement of the resulting carbon monoxide, after absorbing the chlorine and unchanged phosgene in caustic potash, Berthelot and Gaudechon5 got entirely similar results, but estimated the amount of dissociation by the soiling of a mercury meniscus by chlorine resulting from the dissociation; this required less than five seconds of exposure to ultraviolet light when the gas was in quartz, eighty seconds in uviol, and in ordinary glass no soiling was perceptible after two hours of exposure. The following statement by Berthelot and Gaudechon is surprising, in view of the fact that no change in volume occurs when the chlorine formed during dissociation is absorbed by mercury (in quartz): “En presence de mercure qui fixe le chlore, la cl6composition continue et le volume se reduit peu A peu & nioitie.” Of course, the volume remains constant. The evidence seems to be ithat reactions involving phosgene a t temperatures below 200°, certainly up to IOO’, carried out in glass containers, are not complicated by the presence of chlorine. Pure mercury in contact with pure liquid or gaseous phosgene is not soiled, unless the temperature is raised to near the critical point (178”); a t this t,emperature the reaction is slow, but unmistakable. Atkinson, Heycock and Pope6 question the accuracy of the measurements of Bodenstein and Dunant’ on the thermal dissociation of phosgene because the free chlorine was determined by passing the gas through an acid solution of potassium iodide, and titrating the liberated iodine with thiosulfate; the work of Besson8 showed that pure hydriodic acid reacts rapidly with phosgene in the cold, and Delepine9 has pointed out that phosgene liberates iodine from sodium iodide solution if the concentration of the latter exceed 0.1%. The Chemical Warfare Service recommends passing phosgene gas through tubes containing a mixture of potassium iodide and sodium thiosulfate in order to free the gas from traces of chlorine. M y experience with this method indicated that either the phosgene used contained large amounts of chlorine, or that the method was not reliable. Bull. 39, 14 (1883);Compt. rend., 157, 1432 (1913).
* Compt. rend., 132,37 (1910).
Ann. Physik, 24,243 (1907). 4Ber. 43, 130 (1910). Compt. rend. 156, 1243 (1913). 6 J. Chem. SOC.117, 1410(1920). 7 Z.physik. Chem. 61,437 (1908). * Compt. rend 122, 140 (1896). Bdl. 27,283 (1920). 3
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As a test, several grams of potassiuni iodide were sealed in a tube with a quantity of liquid phosgene, and the tube placed in a case where only diffused light reached it. After only a few days, the solution acquired the pink color of dilute permanganate solution, the color of dissolved iodine; and now, after eighteen months, the tube contains a quantity of large crystals of iodine, and the color is a deep claret, The reaction evidently does not depend on the presence of free chlorine. To dissipate a lingering doubt on this score, however, the following experiment was tried. A generous sample of potassium was distilled into a carefully dried, evacuated tube, and the side tube, serving as retort, was sealed off. The sample, thus prepared, consisted of two principal portions: one where the violet vapors had deposited as a violet sublimate on the cool walls of the receiver, and the other where the silver white liquid metal had flowed down the tube, and frozen along the way, A carefully fractionated sample of phosgene was condensed in the tube, and it was sealed off. After twelve months of repose in the dark, no change had taken place in the brilliant luster of the metal, which I interpreted as evidence of the entire absence of chlorine. For, although dry chlorine does not react with potassium, Cowperl found that, when dry chlorine was admitted to bright potassium, the metal “at first remained bright, but slowly became covered by a film of a rich purple color. This is no doubt the subchloride described by Rose2.” At the end of twelve months, the tube was brought out and exposed to bright August sunshine for a few hours. During this brief period those portions of the bright potassium not covered with liquid phosgene, and some parts of the surface adhering to the glass, became covered with a film of a rich purple color, evidently the same substance described by Cowper in his experiments with chlorine. However, that part of the potassium covered by liquid phosgene remained bright, as well as some of the surface adhering to the glass. Since it was shown in two independent investigations3 that those wavelengths capable of decomposing phosgene are screened off by ordinary glass, the effect of free chlorine in producing the reaction may be eliminated. If we assume that certain wavelengths capable of traversing glass act catalytically at the surface of the potassium to bring about the reaction, all the phenomena observed can be explained, if we assume, in addition, that phosgene absorbs the active wavelengths. There is only partial absorption in the case of phosgene vapor (under less than two atmospheres pressure at room temperature) ; but liquid phosgene absorbs them completely when presented in a thick enough layer. The formation of the purple film on the surface of the metal next the glass occurs only where the metal had become separated from the glass sufficiently for a film of phosgene to penetrate between them, A knowledge of the absorption spectrum of liquid phosgene would undoubtedly help to clear up the phenomena observed, but no information seems J. Chem. Soc., 43, 153 (1883). Pogg. Ann., 120, 15 (1863). * Coehn and Becker: Ber. 43, 130 (1910); Berthelot and Gaudechon: Compt. rend. 156, I243 (1913).
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t o be available along this line. At any rate, there seems to be no doubt that phosgene gas passed over solid potassium iodide yields a product, which while it may be chlorine free, certainly contains free carbon monoxide. The use of powdered antimony is to be preferred for the removal of free chlorine. The behavior of the metals towards liquid phosgene is, in general, the same as that of potassium. I n diffused light zinc, copper, and aluminium, in the form of bright foil, when exposed to the action of liquid phosgene, remain bright. It was to be expected that aluminium would dissolve, because aluminium chloride is soluble in liquid phosgene. The explanation a t once suggested is that the protecting oxide film is sufficiently resistant to withstand the action of the solvent. To determine this point, I tried the action of amalgamated aluminium. This involved special procedure, for the sample was so active, when amalgamated, that corrosion was well advanced before it could be dried, enclosed in a tube, the tube sealed to the apparatus, and evacuated. Carefully cleaned aluminium foil was covered with distilled water in a test tube, and a few drops of mercuric chloride added. As soon as the surface was coated with mercury, it was washed rapidly with distilled water, and then carbon tetrachloride was poured in until all the water was displaced. The amalgamated metal was. then transferred to the trial tube along with enough carbon tetrachloride to cover it, the tube was sealed to the apparatus, the carbon tetrachloride was boiled away by evacuating with the water-suction pump, and, after thorough rinsing with phosgene, a sample of liquid phosgene was introduced by distillation, and the tube sealed off. Prepared in this way, solution proceeded rapidly, carbon monoxide was evolved, and soon only a droplet of mercury remained. Amalgamated copper and amalgamated zinc, on the other hand, were not attacked-their chlorides. are insoluble. The effect of sunlight on the bright foil-not amalgamated- was startling; there was only slight surface corrosion of the zinc and copper, as was the case with potassium; but the aluminium reacted rapidly, showing lively effervescence due to evolution of carbon monoxide, which stopped when removed from the direct sunlight. As the reaction proceeded, the solution developed a yellowish brown color (due to iron as chloride), which affected the rate of reaction, so that after a time effervescence practically ceased; this effect seems to have been caused by the absorption of the active wavelengths by the colored solution. The thickness of liquid phosgene that had to be traversed by light in this experiment was about one fourth as great as in the experiment with potassium, which may account for the greater activity of the light in the present experiment. Iron present as impurity in the aluminium also reacted, yielding a yellowish brown solution, a darker deposit on the walls of the tube, and a dark brown residue. This residue, when heated carefully in the neighborhood of 2 0 0 ° , suddenly swells up, and yields a quantity of phosgene; when the temperature is raised still higher, the product fuses to the glass, and appears dark red by transmitted light , which seems to justify the conclusion that the
REACTIONS I N PHOSGENE SOLUTION
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substance is anhydrous ferric chloride, which must have been present as a slightly soluble phosgenate. The formation of ferric chloride from iron and phosgene was unexpected, since the experiments of Delepine and Villel on this subject were negative. Whether the presence of aluminium favored the reaction, or whether sunlight was a determining factor, has not been determined: nor has any study been made of the system ferric chloride-phosgene. Magnesium is inert towards liquid phosgene; chlorine dissolved in phosgene does not attack magnesium; this is in harmony with the action of liquid chlorine on magnesium2. Since aluminium dissolves readily in liquid chlorine, it is not surprising that a solution of chlorine in liquid phosgene attacks aluminium so energetically that, unless the heat resulting from the reaction is withdrawn, the tube explodes violently. The great chemical activity of aluminium chloride as displayed in a great variety of ways in both organic and inorganic chemistry, suggested that the solution of it in phosgene might have unusual properties. The first experiment carried out in this direction showed that this was actually the case, and opened up a wide field for investigation. Metallic potassium, exposed to the action of a solution of aluminium chloride in phosgene, immediately began to react; a gas was formed, and the metal was slowly corroded, forming an insoluble compound, which, in the light of another investigation carried out in this laboratory, appears to be the phosgenate of a slightly soluble double salt of potassium chloride with aluminium chlorides. I had hoped, before making the trial, that it might be possible to displace aluminium from the chloride by a metathetic reaction with potassium, since the reaction would be exothermic: 3KfAICL +3KCl+A1+154 Cal. The heat of solntion of aluminium chloride in phosgene has not been determined, but the thermal effect is not great. The fact that potassium chloride is insoluble seemed to give the reaction formulated above plausibility. That metathetic reactions of this type may take place in non-aqueous solvents has been shown, for example, by Kraus4 and by Bergstromb. It is true that ammonia resembles water in that it forms conducting solutions, while phosgene probably does not; but Kahlenberga found that zinc displaces hydrogen from a perfectly dry solution of hydrogen chloride in benzene, which conducts no better than pure benzene, he says. I can find no record of the dielectric constant of phosgene. Using Thwing’s formula which connects the dielectric constant with the chemical constitution7 Bull. 27, 288 (1920). Gautier and Charpy: Compt. rend., 113, 597 (1891); Beckmann: Z. anorg. Chem., 51, gg (1906). Cowper: J. Chem. Soc., 43, 153 (1883). Germann and Gagos: Science, 58, 309 (1923). J. Am. Chem. Soc., 44, 1224 (1922). Ibid., 45, 2788 (1923). 6 J. Phys. Chem., 6,J (1902). Thwing: Z.physik. Chem., 14, 286 (1894).
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D
K=-(alK1+a~Kz+a3Ka M
. . .'.)
where D is the density of the substance, M its molecular weight, K1, Kz, K3 etc:, specific constants depending on the element or atomic group involved, and al, a2, as the number of each occurring in the formula, the calculated dielectric constant varies according to whether we look at phosgene as a characteristic carbonyl compound, which gives a value of the same order of magnitude as the experimental value for acetyl chloride, or as a simple carbon compound, which would place it in the class with benzene. I n the first case, the value is 24.1 a t 15', in the second case 3.6. There is no reason why th.e whole question should not be settled experimentally.
Conclusion A review of the reactions of phosgene in the inorganic field indicates that phosgene is particularly reactive a t temperatures where it is appreciably dissociated into chlorine and carbon monoxide. At lower temperatures phosgene reacts readily in many cases, especially where the reacting substance or a product of the reaction is soluble. Metals react more or less readily in the presence of aluminium chloride, depending on the solubility of the double salt of the chloride of the metal with aluminium chloride. Light does not decompose phosgene in glass containers, but it may act catalytically in promoting certain reactions with phosgene. This work was carried out in collaboration with the Chemical Warfare Service, with phosgene supplied from the Edgewood Arsenal. Stanford University, California.
