FORMATION O F BENZENE BY THE PHOTOCHEMICAL POLYMERIZATION O F ACETYLENE AT HIGH TEMPERATURES C. H. SCHIFLETT Department of Chemistry, University of Minnesota, Minneapolis, Minnesota ROBERT LIVINGSTON
AND
Received November 3, 1933
The formation of benzene by the photochemical polymerization of acetylene has been reported by Sechi Kato ( 5 ) , who demonstrated the presence of benzene by means of its distinctive absorption spectrum. Kato claims that the formation of benzene, relative to that of cuprene, is favored by temperatures above 270°C. or by the absorption of light of short wave length. Since these results were in apparent contradiction to the results of a number of other observers (1,2, 6,8) who obtained cuprene as the only product of the photochemical reaction, it seemed worth while to repeat Kato’s experiments under slightly different conditions. The results of the present work confirm her claim that benzene is formed when acetylene at temperatures above 270°C. is irradiated with ultra-violet light. APPARATUS AND PROCEDURE
The source and purification of the acetylene were similar to those described by Lind and Livingston (6). The reaction system is represented in figure 1. The reaction vessel was a quartz tube 24.5 cm. long and 2.2 cm. in diameter; it was sealed at the lower end and connected to the line b y a graded seal a t the upper end. The upper half of this tube was wrapped with a nichrome wire coil, spaced a t about half-centimeter intervals. The back and side of the coil were covered by a loosely fitting heavy-walled asbestos shield. The light source, a vertical quartz-mercury arc, was placed opposite the shield at a distance of about 3 cm., and was run “hot” to minimize the probability of a mercury sensitized reaction. The lower part of the reaction vessel was immersed in a mixture of solid carbon dioxide and acetone. The approximate temperature of the acetylene was determined indirectly by measuring the current flowing through the heating coil. These temperature determinations were based upon a series of calibrations, in which a tube with dimensions identical with the reaction vessel was inserted in the heating coil. The upper end of this tube was not constricted but was closed by an asbestos plug through which a thermometer was inserted. The Dewar tube containing the carbon dioxide377
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acetone mixture, the asbestos shield, and the mercury arc were arranged as during reaction periods. The temperatures corresponding to current readings were measured from 250°C. to 400°C. The temperature of the irradiated gas was not uniform, but varied from the center to the ends of the heating coil by as much as 50°C. The recorded temperatures all
U r
FIG.1. THE REACTION SYSTEM
correspond to maximum values, and were measured after steady-state conditions had been attained. Before each experiment air was admitted to the reaction cell, which was then heated to burn out any non-volatile substance. After the system had been evacuated (pressure less than loM4mm.), sufficient acetylene was admitted to make the pressure approximately 1 atmosphere a t the tempera-
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ture of the experiment. The pressure was then determined with the whole system a t room temperature. The cooling bath was put in place and the heating current started, After sufficient time (about an hour) had elapsed for temperature equilibrium to be attained, the arc was started and was run from three to four and one-half hours. In some experiments an attempt was made to follow the change of pressure during the experiment; in others the manometer was closed off as soon as temperature equilibrium was reached. After each exposure the whole system was allowed to return to room temperature and the pressure was determined and corrected for any change in room temperature. RESULTS
In all of the photochemical experiments a decrease in pressure was observed and a variety of products was formed. Two of the products JTere non-volatile solids; one of these resembled cuprene, and the other was a hard dark yellow film, like a film of dried varnish. The cuprene seemed to be evenly distributed and was particularly evident in the irradiated and heated part of the tube. It was insoluble in ether and burned very readily in air. The dark yellow film occurred in the cooler part of the tube, both above the heated zone and in the bottom of the tube. When heated in vacuum it charred, but did not sublime. It was partly soluble in ether, coloring the ether dark yellow; it formed a dense and adherent coating which burned in air only with difficulty. A small quantity of a volatile liquid was always found frozen in the bottom of the tube. This liquid was yellow and viscous. When the bottom of the tube was immersed in liquid air there was no detectable residual pressure, which indicates the absence of hydrogen and methane. No other attempt was made to isolate gaseous products. While the experimental method did not allow exact measurement of the rate of change, it was demonstrated that the rate was initially at a maximum and became negligible after three or four hours. This was probably due to the absorption of the incident light by the cuprene which accumulated on the walls of the tube. The decrease in pressure varied from 3.5 per cent for a 3-hour exposure a t 270°C. to 9.3 per cent for a 4.5-hour exposure at 375°C. These changes are apparently much greater than those which would have been obtained at room temperature under otherwise similar conditions. No measurements were made at temperatures below 270°C. or above 376°C. That the reaction was truly photochemical and not thermal was demonstrated by two independent blank experiments. In one the quartz reaction vessel was replaced by a Pyrex tube of similar dimensions. All other conditions were identical with those used in the photochemical experiments; a t the end of two and one-half hours there was no measurable
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change of pressure nor visible accumulation of products, In another experiment the quartz tube was used but the arc was not started. The temperature was maintained at 350°C. for 4 hours, at the end of which time there was no evidence of any reaction having occurred. The following experiments were performed in an attempt to determine the nature of the volatile liquid products. The liquids were frozen with carbon dioxide-acetone mixture and the gas was pumped off. A capillary side tube was then immersed in liquid air and the more volatile liquids were allowed to distil over. The distillate always consisted of a few cubic millimeters of a non-viscous liquid, which was clear or faintly yellow (depending on the time allowed for distillation). After one of the experiments, which was performed at 345"C., the capillary side tube and manometer were sealed off from the reaction vessel, and the side tube was immersed in an acetone bath which was cooled with solid carbon dioxide. The vapor pressure measurements given in table 1 were made. While TABLE 1 V a p o r pressure of the distillate TEXPERATUJRE OF THE B A T H
VAPOR PRESSURE OF DISTILLATE
VAPOR PRESSURE OF PENZENE
degrees C.
mm.
mm.
-24 t o -25 -19 t o -20 -14 t o -15 -9 t o -10 - 5 . 5 t o -6
4.3 6.6 10.1 14.2 18.6
3.6 5.5 8.2 12.1 16.1
(4)
1
the vapor pressure values given are not in sufficiently close agreement with the published values for benzene to make it possible to identify the distillate as benzene, it does seem probable that the distillate is a mixture consisting chiefly of benzene. Under the conditions of the experiment the distillate appeared to melt over the range -9°C. to - 15°C. At temperature above -5°C. the volume of the liquid was very small and it had a distinct yellowish color. To obtain definite evidence of the presence (or absence) of benzene, the distillate was transferred in vacuum to a quartz absorption cell, which was 5 cm. long and was equipped with a side tube. The absorption spectrum was photographed with a Steinheil spectrograph, using a hydrogen arc as a light source (7). When the cell was at room temperature a continuous absorption set in at about 2225 A.U., with a region of partial transmission between 2140 and 2270 A.U. I n the region between 2225 A.U. and 2650 A.U. (the long wave length limit of the spectrograph), several bands were noticeable which coincided with the benzene bands recorded by
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381
Henri (3). When the side tube of the cell was maintained at -37OC. the continuous absorption vanished; a regular series of bands was noticeable between 2275 and 2603 A.U. Fourteen of these bands were identified as benzene bands. We may conclude from these results that the distillate consisted of benzene and some less volatile substance which exhibited a continuous absorption in the ultra-violet. CONCLUSIONS
It is noteworthy that the products of the photochemical polymerization of acetylene at high temperatures resemble those of the thermal polymerization (9) much more nearly than they do thoseof the low temperature photochemical reaction. At the present time any attempt to give a detailed analysis of the reaction mechanism would be premature. It seems probable that not all of the detailed analysis of Kat0 ( 5 ) will prove to be correct. Her assumption that the apparent continuum on the long wave length side of the banded absorption corresponds to a region of predissociation seems unlikely. It is quite possible that the continuous absorption was due to some impurity present in the acetylene. It is very probable that the solid condensate which is formed in the photochemical reaction is a true polymer rather than the dehydrogenated product (C7H&, assumed by Kato. In the absence of any information on the intensity of the light absorbed at the different frequencies used in Kato's experiments, it is difficult to decide whether her conclusion that light of short wave length favors benzene formation is justifiable. SUMMARY
The photochemical polymerization of acetylene at temperatures above 270°C. has been studied qualitatively. In confirmation of the results of Kato (5), benzene has been identified as one of the reaction products. REFERENCES (1) BATESA N D TAYLOR: J. Am. Chem. SOC.49, 2438 (1927). AND GAUDECHON: Compt. rend. 160, 1169 (1910). (2) BERTHELOT (3) HENRI:J. phys. radium 3, 181 (1922). (4) International Critical Tables, Vol. 111, p. 208. McGraw-Hill Book Co., New York (1928). (5) KATO:Bull. Inst. Phys. Chem. Research Tokyo 10,343 (1931). (6) LINDAND LIVINGSTON: J. Am. Chem. SOC.64,94 (1932). J. Am. Chem. SOC.66, 1042 (1933), for a description of (7) LINDAND LIVINGSTON: the details of this apparatus. (8) REINICKE:Z. angew. Chem. 41, 1144 (1928). A N D BRUNSER:Helv. Chim. Acta 13, 1125 (1930). (9) See, for example, SCHLAPFER
