3952
G. B. KISTIAKOWSKY AND THEODORE A. WALTER
Photolysis of Ketene by 2139-A Radiation by G. B. Kistiakowsky and Theodore A. Walter Cibbs Chemical Laboratory, Harvard University, Cambridge, Maesachusetts 02138 (Received January 18, 1068)
The photolysis of ketene by the radiation of mainly 2139-w wavelength was investigated over a wide range of ketene pressures and radiation intensities in a static system. The effects of small additions of oxygen and of large excess of n-butane were also studied. The quantum yield of CO formation in ketene alone is about 2 and is nearly or completely pressure independent. The other main product is ethylene, whose quantum yield is about 0.8. Smaller yields of Hz, C2H2, and CzHs were also obtained, and their dependence on pressure and radiation intensity was studied. Also several Ca and C4 hydrocarbons are formed whose yields increase with the degree of decomposition, so that they are attributed to reactions with the primary products of photolysis. A polymer film is slowly formed on the irradiated surfaces of the cells. The experimental results are consistent with the conclusion that the primary reaction is the decomposition of ketene into CO and CH2. About 70% of the methylene formed reacts with ketene forming CO and C2H4. It is suggested that a small portion of the latter is so “hot” that it dehydrogenates into CZHZand Hz. It is further proposed that the remainder (about 30%) of methylene abstracts a hydrogen atom from ketene forming CH, and CHCO radicals. These then form the ethane, about 60% of the acetylene, and about 17% of ethylene at the highest radiation intensities used. At lower intensities, especially, some first-order reactions of these radicals probably occur which might lead to the formation of the polymer. With an excess of n-butane the main reaction (about 70%) appears to be an insertion of methylene, forming pentanes. Also CHI, CzHs, and butenes are observed which are attributed to an abstraction reaction by methylene to form CH, and C4He radicals. When oxygen is added to ketene alone or to mixtures of ketene and n-butane, all products which have been attributed to free; radical reactions disappear. These results make it probable, although they do not prove it, that the 2139-A radiation causes the formation of two kinds of methylene in about a 7 : 3 ratio, which behave kinetically as the singlet and triplet states of methylene formed by near-ultraviolet radiation but possess higher excess energy.
Introduction I n the nonvacuum ultraviolet region, ketene has two distinct absorption regions.’ I n the near-ultraviolet region is observed a weak n+n* transition, the extinction coefficients being below 10 l./mol cm, while below 2400 A is a strong transition, probably of the v n * type with extinction coefficients as highZ as 3.0 X lo3 l./mol cm. Photochemical work with ketene, which has been largely limited to the near-ultraviolet region, has demonstrated the formation of methylene in its singlet and triplet states, the ratio of these increasing with decreasing Only a few experiments with short wavelength radiation have been reported.8 This paper attempts to fill this gap and makes it probable that the primary photochemical reaction in the a electronic state, as that in the n state, is a dissociation into CO and CH2.
Experimental Section Use was made of a conventional glass vacuum system. Either a 1 or a 10 cm long cylindrical photolysis cell was used, depending on the partial pressure of ketene. Owing to the high extinction coefficients, the absorption of the 2000-2200-A radiation is nonuniform throughout Ihe cells. They were equipped therefore with glassenclosed, magnetically driven stirrers to reduce stratification of the gases during photolysis. The cells were made of 6-cm i.d. Pyrex cylinders with cemented, The Journal of Physical Chemistry
polished quartz plates whose transmission was better than 90% a t 2000 8. The cemented joints were shielded from radiation by annular metal diaphragms with 5-cm diameter openings. The light sources used were a zinc sparkgproduced by a resonant circuit (8000 V, 2 kW, and 1.7 X lo6 cps) and a 25-W Phillips zinc spectral lamp No. 26-288, a very much weaker source of the 2139-8 radiation. A short focal length quartz lens used with the spark produced an approximately parallel light beam. I n most experiments with the spark an interference filter with a transmission maximum a t 2120-2140 8 and a band half-width of 200 8 was interposed. Radiation intensity was further reduced when desired by interposing fine metal screens. (1) W. C. Price. J. P. Teenan. and A. D. Walsh. J . Chem. SOC..920 (1951); R. N. ‘Dixon and G: H. Kirby, Trans. Faraday Soc., 62,
1406 (1966). (2) K. H. Sauer, Ph.D. Thesis, Harvard University, 1957, p 168. (3) H. M. Frey, Progr. Reaction Kinetics, 2, 131 (1964); W. B. DeMore and 8. W. Benson, Advan. Photochem., 2, 219 (1964). (4) 8. Ho, I. Unger, and W. A. Noyes, Jr., J . Amer. Chem. SOC., 87, 2297 (1965). (5) H. M. Frey, Chem. Commun., 260 (1965). (6) F. H. Dorer and B. 5. Rabinovitch, J . Phys. Chem., 69, 1964 (1965). (7) B. A. DeGraff and G. B. KistiakouTsky, ibid., 71, 3984 (1967). (8) H. Gesser and E. W. R.Steacie, Can. J . Chem., 34, 113 (1956). (9) G . R.Harrison, R. C. Lord, and J. R.Loofbourow, “Practical Spectroscopy,” Prentice-Hall, Ino., Englewood Cliffs, N. J., 1948, pp 192-196, 441.
PHOTOLYSIS OF KETENEBY 2139-A RADIATION Upon photolysis the reaction mixture was pumped by a Toepler pump through an efficient trap cooled by
solid nitrogen. The noncondensable fraction was measured in a micro gas buret and was transferred for analysis to a Perkin-Elmer Model 810 gas chromatograph equipped with an Aerograph sampling valve, hotwire and flame ionization detectors, and a chart recorder with a disk integration device. This fraction was put through a 15-ft column filled with Molecular Sieve 5A, preceded by a 12-ft Perkin-Elmer column R to minimize surge peaks on the recorder due to the injection of the sample. Fully resolved peaks of Hz, 02,Nz, CH4, and CO were obtained in that order, and, therefore, small air leakage through the sampling valve did not disturb the analysis. To minimize secondary reactions with “stable” products during photolysis, the latter was usually carried out to only a very low degree of decomposition (about 4%), and, therefore, the condensable gas fraction was very largely unreacted ketene. Upon evaporation this fraction was placed in contact with a mixture of Ascarite and Dehydrite beads to remove unreacted ketene and was then measured in a micro gas buret. This proved to be a not wholly satisfactory procedure because fresh samples of the reagent gave off some water vapor and samples exposed to ketene many times did not remove it quantitatively. Ketene and water did not interfere with chromatographic analysis, as shown by extensive tests. However, they interfered with volumetric determination of the condensable fraction and, therefore, such information is available only in a small number of runs. Aliquots of the condensable fraction were analyzed on a 30-ft Perkin-Elmer column E (33% dimethylsulfolane on 60-80 mesh Chromosorb) at 34” and on an 8-ft column filled with 1.4% didecylphthalate on 60-80 mesh Alcoa F-1 activated alumina at 76”. This was necessary because the first column did not resolve CzH4 and CzHs peaks and the latter gave irreproducible acetylene peaks. In the experiments with excess n-butane, aliquots of the entire photolyzed sample were analyzed on the alumina column described above. The sensitivity of the chromatograph was calibrated with known gas mixtures. In the experiments with excess n-butane it was assumed that the sensitivity of the flame detector to hydrocarbons was proportional to the number of carbon atoms in the molecule of those (heavier) product hydrocarbons which were not present in the known gas mixtures used for calibration. Ketene was synthesized by the pyrolysis of acetic anhydride,’O was purified by fractional distillation, and was stored frozen in liquid nitrogen. Various samples of ketene contained about O . O l ~ o of hydrocarbon impurities, mainly methane and ethylene. A correction for these has been made in the data shown below. The Phillips research grade n-butane contained about 0.07% impurities, mainly trans-butene-2 or 1-butene, as
3953 determined by the analysis of several samples; a correction for these impurities has also been applied to the data shown below.
Results Satisfactory volumetric determinations of the condensable product fraction were made in eight runs in which ketene pressure ranged from 2.7 to 100 torr; the radiation intensity was varied by a factor of about 10 on the high end of the available range; and the reaction was carried out to about 4% decomposition. The ratio of the volumetric yields of CO and the condensable hydrocarbon fraction was 2.0, individual values scattering from 1.8 to 2.2. Upon more extensive photolysis the volumetric ratio of the two fractions was found to increase above 2, and the relative yields of some trace products, namely, butene-1, allene, propylene, and propane, became much larger. Such a trend in their yields clearly identifies these hg .rocarbons as products of secondary reactions of “primary” products. After a typical 4% photolysis, the yield of propylene was about 1% of the condensable product fraction and those of others were less than0.5%. To conserve space these products are not shown in the tables. A slow formation of a polymer film only on the irradiated portion of the front windows of the cells was observed. It was more pronounced at higher ketene pressures. The polymer probably contains carbonyl groups, since it has a weak diffuse absorption spectrum observable below 4300 A; it was not analyzed. The effective wavelength region of radiation was determined by interposing the 1-em cell between the filter and the 10-cm cell. The short cell was filled with ketene to 40 torr of pressure; the long cell was filled to 400 torr. In this arrangement virtually the entire radiation below 2200 A and very little of longer wavelengths should be absorbed in the first Upon irradiation the yields of CO in the two cells were in a 33 : 1 ratio. When the first cell contained 20 torr of ketene, the ratio was 15. Considering the relative intensities of zinc emission lines” and the transmission of the filter used, it becomes clear that the photolysis, even at high ketene pressures, is predominantly due to the 2139-A zinc line, with small contributions arising from the 2025-, 2062-, and 2211-A lines. The quantum yield of ketene decomposition was roughly determined using ammonia as an actinometer. When ammonia was used at 100 and 400 torr in the 10-cm cell and when it was assumedI2that the quantum yields of ammonia decomposition were 0.26 and 0.19, (10) A. D. Jenkins, J. Chem. Soc., 2563 (1952). (11) A. N. Zaidel’, V. K. Prokof’ev, and S. M. Raisku, “Tables of Spectrum Lines,” Pergamon Press Inc., New York, N. Y., 1961, p 515. (12) W. A. Noyes, Jr., and P. A. Leighton, “The Photochemistry of Gases,” Reinhold Publishing Corp., New York, N. Y., 1941, pp 374,375. Volume 72*Number 12 November 1968
3954
G. B. KISTIAKOWSKY AND THEODORE A. WALTER
respectively, the absorbed radiation intensity was found to be 4.1 X 1016 photons/sec at both pressures. When it was assumed that ketene absorbs the same number of photons as ammonia, the quantum yields of CO production were then found to be 2.5 (at 2.1 torr), 2.2 (at 20 torr), and 2.5 (at 50 torr), which numbers, because of the crudity of the comparison, are quite consistent with a rounded value of 2. Carbon monoxide yields after a fixed exposure to radiation were measured in a series of consecutive experiments in which ketene pressure was raised stepwise from 10 to 500 torr. The yields were found to decrease by a factor of 0.73, which may suggest a slight pressure dependence of the quantum yield. However, the trend may also be due to a gradual buildup of the polymer on the cell window, because the ketene pressures were not randomized in this series of experiments and the cell was cleaned only afterwards. The volatile products of ketene photolysis were found to be CO, H2, and a trace (
