VACUUM-ULTRAVIOLET PHOTOLYSIS OF THE C4HsISOMERS
4037
Vacuum-Ultraviolet Photolysis of the C,H, Isomers. I.
1,3-Butadiene
by Richard D. Doepker Department of Chemistry, University of Miami, Coral Cables, Florida
33184
(Received April IO, 1068)
The photolysis of l,&butadiene was investigoated using xenon (1470 8)and krypton (1236 8)resonance radiation and was compared with the 2200-2600-A photolysis. Collisional deactivation of the excited 1,3-butadiene molecule observed in the far-ultraviolet photolysis is @sent in the vacuum-ultraviolet region. The distribution of major products formed in the 1470- and 1236-A photolysis, acetylene, ethylene, and vinylacetylene, showed neither appreciable energy nor pressure dependence. The cleavage of the central carbon-carbon bond to produce two vinyl radicals was demonstrated by means of H2Sas a free-radical interceptor. It is suggested that vinylacetylene is formed through hydrogen atom elimination, as opposed to a molecular hydrogen elimination. In the krypton photolysis ionization is expected, but no products could be identified as originating exclusively from an ion-molecule path. Speculation as to the formation of ethane, ethyl radicals, and propylene via an ion-molecule reaction path is discussed. During the relatively short history of vacuum-ultraviolet photochemistry much interest has been generated in its ability to demonstrate the modes of decomposition of neutral excited molecules.'z2 I n general, almost all of the investigations have been devoted to simple hydrocarbons, monoolefins, cyclic hydrocarbons, and acetylene. Of this group, the photolysis of p r ~ p y l e n e and ~ , ~ acetylene5 hold special interest to this investigation. Further, a recent study of the photolysis (2062 A) of propynea has helped fill a void in the knowledge of the decomposition of unsaturated hydrocarbons. The primary and secondary processes occurring in the mercury-photosensitized' and the direct photolysis (2000-2500 8) of 1,3-butadiene8pghave recently been reviewed by Srinivasan. l o Later, the direct photolysis (2000-2500 A) of 1,3-butadiene-l,1,4,4-& was reported by Haller and Srinivasan. l 1 The molecular processes forming acetylene, ethylene, hydrogen, and vinylacetylene were investigated, and the partially labeled products were analyzed in order to explore the molecular rearrangements leading to their formation. It was expected by the author that these same processes might also occur in the vacuum-ultraviolet region.
Experimental Section Materials. 1,a-Butadiene (Matheson Co.) was purified by gas chromatography, using an F & M Model 500 chromatograph equipped with a 6-ft activated-alumina column maintained at 100". An impurity of 0.003% trans-Bbutene, but no other Cq or lower compounds, was disclosed by gas chromatographic analysis. Assayed reagent grade inert gases and oxygen were obtained from Air Reduction Co., Inc. and were used without further purification. Nitric oxide (Matheson Co.) was purified by low-temperature distillation, while H2S and COZ (Matheson) were used without further purification. Irradiation. The light source was an L-shaped, air-
cooled, electrodeless discharge lamp containing either krypton or xenon at a pressure of approximately 0.5 mm. It was operated from a 10-100-W microwave power generator at 2450 MHz (Raytheon Model PGM-10x1). A 1-2-mm (thickness) LiF window was attached to the xenon lamp with Torr-Seal (Varian Associates). A 1-mm CaF2 window was used with the krypton resonance lamp to eliminate the 1165-A resonance line.12 The cold fingers of the lamps were maintained at - 160' for the xenon and - 195" for the krypton in order to remove water vapor,13the presence of which would lead to a rather strong emission at wavelengths above 1500 8. The reaction vessel was either a 1-1. spherical Pyrex bulb or a 155-ml Pyrex cylinder attached directly to the lamp by means of Apiezon W wax. A film was deposited on the lamp window during photolysis which drastically reduced the intensity of the light entering the reaction vessel. It was found that after every three or four experiments it was necessary to disassemble the reaction vessel and remove this film with carbon tetrachloride and acetone. (1) J. R. McNesby and H. Okabe, Advan. Photochem., 3, 157 (1964). (2) J. R. McNesby, Actions Chim. Biol. Radiations, 9, 36 (1966). (3) E. Tschuikow-Roux, J . Phys. Chem., 71, 2355 (1967). (4) R. Gordon, Jr., R. D. Doepker, and P. Ausloos, J . Chem. Phys., 44,3733 (1966). (5) L. F. Stief, V. J. DeCarlo, and R. J. Mataloni, ibid., 42, 3113 (1965). (6) A. Galli, P. Harteck, and R. R. Reeves, Jr., J . Phys. Chem., 71, 2719 (1967). (7) J. Collin and F. P. Lossing, Can. J. Chem., 35, 778 (1957). (8) R. Srinivasan, J . Amer. Chem. Soc., 82, 5063 (1960). (9) I. Haller and R. Srinivasan, J . Chem. Phys., 40, 1992 (1964). (10) R. Srinivasan, Advan. Photochem., 4, 113 (1966). (11) I. Haller and R. Srinivasan, J . Amer. Chem. Soc., 88, 3694 (1966). (12) A. H. Laufer, J. A. Pirog, and J. R. McNesby, J . Opt. SOC. Amer., 55, 64 (1965). (13) H. Okabe, ibid., 54, 478 (1964). Volume 78, Number 18 November 1068
RICHARD D. DOEPKER
4038
2. Varying the concentration of NO from 5 to 50% of the C4H6 pressure had no effect on the relative yields of the different prbducts. However, when O2 was similarly varied, product distribution was affected. There was an observed increase in the yield of propylene and a decrease in vinylacetylene relative to acetylene. 3. Hydrogen yields were determined for a 1:l Ci":C4D6 mixture at 1470 and 1236 A. With 10% NO the krypton photolysis led to a total hydrogen yield of 70% of the acetylene, while in the xenon photolysis total hydrogen was but 21% (the total diene pressure was 15 torr). I n both cases, only a trace of HD was found, while the ratio Hz:DZwas determined to be 2: 1. These results can only be considered approximate. 4. Traces of methylacetylene and allene were observed in the vacuum-ultraviolet photolysis of C4H6-N0 mixtures but were in all cases less than 1% of the acetylene yield. In the unscavenged system, neither was observed, except at 40 torr where a total of 2% was found. Trace amounts were also found in the C4&H2S mixtures in the 2200-2600-~photolysis and in the 40 torr C4H6-H2S mixture with 1470-A irradiation. 5. Small amounts (less than 4%) of l12-butadiene were observed in the C4He-NO mixtures with 1470-A irradiation. Since this small yield was subject to large error due to an overlap of its chromatographic retention time with respect to the tail of the parent 1,3-butadiene1it was not included in Tables 1-111. 6. trans-2-Butene and cis-2-butene were also found as products in the photolysis of the C4He-H2S mixtures. trans-2-Butene was present in 11%yield, while cis-2-butene was present in a 20% yield in the xenon photolysis of C4H6-H2S (45%) mixture. Approximately 2% total butenes were found in unscavenged experiments. 7. If complete absorption can be assumed in the 1-1. reaction vessel (path length approximately 10 cm) with pressures of 0.3-41 torr and if the lamp intensity is nearly constant, a relative quantum yield for acetylene
The light source used for the 2200-2600-A irradiations was a Hanovia Utility ultraviolet quartz medium-pressure lamp. The reaction cells were the same as used in the vacuum-ultraviolet studies except the vacuum-ultraviolet lamp was replaced by a Vycor window. After photolysis, an aliquot of the irradiated material was introduced into an F & M Model 700 dual flame ionization gas chromatograph provided with a 25-ft1 30% squalane column or a 6-ft activated-alumina column. Identification of the products was made through their retention times on both the squalane and alumina columns. The squalane column gave a very satisfactory analysis of the products of carbon number 5 or less, while the usefulness of alumina was primarily for the higher molecular weight products. Neither column gave a satisfactory analysis for 1butyne in the presence of 1,3-butadiene1 so this potential product was not determined. In three experiments hydrogen was distilled off at -210", collected, and determined with a CEC 21-103 C mass spectrometer.
Results No actinometry was carried out at 1470 8,1236 A, or for the far-ultraviolet region. Therefore, only relative yields of products are reported in the tables. When NO was present, a variation in the conversion from 0.01 to 2% did not, within experimental error, produce any change in the relative yields of the products. Even when NO was not added, product distribution was unaffected as long as the conversions were kept below 1%. It was therefore standard procedure to keep conversions below 0.3% when a scavenger was present and below 0.05% when a scavenger was not present. I n addition to the results presented in the tables, the following observations should be reported. 1. Whenever NO or 0 2 was present, no products above C4 were found in the 2200-2600-A studies, while a single Cg product was observed in the vacuum-ultraviolet photolysis. Positive identification was not made, and it was a minor product never exceeding 5% of the acetylene yield.
Table I: Photolysis (1470 A) of 1,3-C& in the Presence and Absence of Additives 10.0
42.0
0.3
0.03 None
None
(NO)
2.2 100.0 53.0 4.9 0.5 87.9
3.2 100.0 68.0 3.4 0.1 97.0
a 100.0 43.0
