Photolysis of iiqiitd Cyclohexane
1899
Acknowledgments. The authors thank Dr. Harthmut Fuhr for his help in obtaining the chemiluminescence spectra, Elr. Harry Johnson for many interesting discussions, and Dr. A. H. Maki for help in interpreting the epr spectra. Support from the TJ. S. Public Health Service (Grant No. CA 11459) is gratefully acknowledged.
(1) F. McCapra, Quart. Rev., Chem. Soc., 485 (1966).
(2) T. Bremer and H. Friedmann, Buil. SOC.Chirn. Bels., 63, 415 (1954). (3) R. L. Bardsley and D. M. Hercules, J. Amer. Chem. SOC., 90, 4545 (1968). (4) P.H. Bolton and D. R. Kearns, J. Amer. Chem. SOC., 96, 4651 (1974). (5) H. Gilrnan and C. E. Adams, J. Amer. Chem. Soc., 47, 2816 (1925). (6) S. 0. Lawesson and N. C. Yang, J. Amer. Chem. Soc., 81, 4230 (1959). (7) M. Gornberg and W. E. Bachrnann, J. Arner. Chern. Soc., 49, 236 (1927). (8) We thank the referee for suggesting this experiment. (9) A. Carrington and A. D. McLachlan, "Introduction to Magnetic Resonance," Harper and Row, New York, N. Y., 1967. (10) L. F. Fieser and M. Fieser, "Advances in Organrc Chemistry," Reinhold, New York, N. Y., 1961, p 351.
Photolysis of iquid Cyclohexane at 1634 A and the Effect of the Addition of Car Tetrachloride arid Sulfur Hexafluoride d. Nafisi-Movaghar' and Yoshihlko Hafano* Laboratory of Physical Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo, Japan (Received September I I, 1973; Revised Manuscript Received June 3, 1974)
Photolysis of cyclohexane in a pure liquid state and in the presence of cc14 and SFe has h e n carried out a t 1634 A. The primary processes are molecular and atomic detachment of hydrogen from excited cyclohexane molecules leading to the formation of hydrogen, cyclohexene, and bicyciohexyl. The material balance between these products indicates the absence of others. Addition of CC14 ( to 10-l M ) and SF6 ( to 6 5 X POm2 M ) results in the reduction in the yields of the photoproducts from cyclohexane. This is exCC14 CCH12 plained in terms of quenching of the excited state of cyclohexane by CC14, CCH1p cC14*, and SF6, 66H12* SF6 C6H12 SF6*, with the reaction rates h = 2 X loL1and 1 X 10'' M - l sec-I. respectively. A chain reaction occurs in the photolysis of C ~ H I Z - C Cmixtures ~~ leading to the formaCeHllCC13, I, CHCI?, CZC16, and probably HC1 as additional products. tion of C ~ H ~ J C
+
-
+
+
I. Introduction The behavior of alkanes, pure and in the presence of additives, under high-energy and far-ultraviolet excitation, is still not fully understood.2-5 Cycloalkanes, especially cyclohexane, have rweivecl considerable attention among radiation chemists, photochemists, and photophysicists. Processes such as charge neutralization and transfer, radical reactions, and ion-molecule processes have been suggested to provide a framework for understanding the action of ionizing radiati0n.l However, protochemical studies with photons of energies near to, or slightly above, the ionization threshold of cyclohexane indicate the importance of the contributions of excited cyclohexane molecules in the formation of the final products, common in both radiation and photochemistry from this The recent development in the photophysics of a l k a ~ i e s especially ,~ the low yield of fluorescence emission, is In accord with the assignment of the excited cyclohexane molecule as the main precursor of the final products. This view gains further support from new information about this excited state of cyclohexane, populated under pulse radiolysis and X-ray excitation.7,s However, the reaclion sequences following the disappearance of the electronically excited species and the rela-
-
+
tive yields of the individual processes vary with the exeitation energy and the state of aggregation of the system.5c Further, polymer formationg and the effect of the olefinic products on the rate of decay of the excited state and the quantum yield of fluorescence emission have been suggested, with the conclusion that necessary corrections should be made for these effects.6.8 These complications necessitated further investigation of the photochemical processes in cyclohexane systems, which has been undertaken in the present work. Excitation was performed using the Br (1634 A) emission line the energy of which is about 2 eV below the ionization potential of cyclohexane. Carbon tetrachloride and sulfur hexafluoride, commonly used as electron scavengers in radiation chemistry, were used as additives. 11. Experimental Section
Materials. Cyclohexane (Phillips Research Grade) was used as supplied. Impurities of less than 0.01%, about half of which was 2,4-dimethylpentane, were detected by gas chromatography using a flame ionization detector . Cyclohexene and other olefins were not detected. Further, upon radiolysis of the same supply performed in this laboratory,lo the G value of hydrogen did not change by treatment The Journal of Physical Chemistry. Vol. 78. No. 19. 7974
1900
on silica gel. Carbon tetrachloride (Merck, 99.8%) and sulfur hexafluoride (Matheson, >98%) were used without further treatmient,. Chloroform (Wako Junyaku, 99%), bicyclohexyl (K and K Laboratories, >99%), cyclohexene, cyclohexyl chloride, and Iiexachloroethane (all of Tokyo Kasei, >99%) were used as standards without purification. Ethylene (Takachiho, >99.9%) was checked by gas chromatography. About 0.03% ethane impurity was detected. It was used for actinometry as supplied. Bromine (Koso Kagaku, >99.9%) was purified by vacuum distillation over phosphorus pentoxide and sealed. Llght Source and Photolysis Cell. The photolysis apparatus is illustrated in Figure 1. The electrodeless discharge lamplI was constructed from an inner 34/35 standard taper joint with an upper tube of 1.5 cm 0.d. The lower tube was joined, via a graded seal, to a quartz tube of 2 cm o.d., on one end of which a Suprasii quartz window of 1 mm thickness was attached. The full length of the lamp and the distance from -the windoiy to the ground glass were 35 and 3.3 cm, respectlveiy. 'Thle other end of the lamp was provided with a side arm. 'The lamp was evacuated to about Torr at room temperature for 2 weeks and at ca.. 300' for 1 week. After introducing 5 Torr of Ne the lamp was discharged for about 30 min and then evacuated. This cycle was repeated several times un.til the C O of~ the discharge glow remained unchanged. Finally, bromine, through phosphorus pentoxide, and 2 Torr of Ne were introduced and the lamp was sealed. In order to avoid contamination of the lamp by grease etc. during the introduction of bromine, all the tubes leading to stopcocks werc sealed. The discharge was initiated with a tesla coil and sustained by a stabilized microwave generator (Ito Chotaaipa Co.) of 2450 MHz and 100 W. During the operation of the lamp the side arm was immersed in a Dry Ice-methan,ol slush. Lamp spectra, obtained by a vacuum uv spectrometer (JASCB Model VUV-lB), consisted of an intense line at 1634 and a very weak line a t 1930 A, the relative intensity o f the latter being negligibly small. This weak emission., previously assigned as a carbon line due to possible contarnination of the lamp by vacuum grease,l* seems to be inherent in lamps containing Ne. A similar line is reported in Ne I1 emission.l3 As mentioned above, cont,amiriation of the laimp by vacuum grease was avoided in the present ,work. It was observed during the operation of the lamp that the intensity decreased gradually with time. This variation was due to the gradual collection of bromine in the side arm as the part of the lamp inside the cavity warmed, and hence, to the decrease in bromine pressure inside the lamp. This defect was rectified by keeping the lamp at a constant 200' prior to and during the operation, using heating tape. The lamp intensity under this condition remained constant (within 5%) for at least 60 min of continuous operation. The yield of hydrogen ( ~ ( H Z=)0.42) from ethylene photoll ~ light intensity was ysis was used as an a ~ t i n 0 m e t e r . The 5.3 x IO-9 einst,ein/sec. The photolyG cell was made of a Pyrex tube of 3.5 cm o.d., closed at m e e1i.d and joined to t,he outer 34/35 standard taper joint at the other end. A grease trap, situated just below the ground glass, prevented contamination of solution by vacuum grease. The cell was 3 cm in length (exclusive of the joint and the trap), 53 ml in volume, and equipped with a vacuum stop cock on one side. The cell and the lamp were fitted together uia the ground glass joint. The distance from the lamp window to the flat botThe Journal of Physicai Chemistry. Vol. 78 N o . 19 1974
J. Nafisi-Movaghar and Yoshihiko Hatano
IIU
Figure 1. Photolysis apparatus.
tom of the cell was 1.2 cm. The window was covered with solution when 10 ml of the latter was introduced into the cell. Sample Preparation and Photolysis After introducing 10 ml of cyclohexane the cell was fitted to the lamp and connected to the vacuum line. Dissolved oxygen was removed by the freeze-thaw technique. Known amounts of Cc14 and SFe were distilled into the cell and thoroughly mixed. The concentration of CC14 was checked by gas chromatography using a dimethylsulfolane column (4 m) a t room temperature, and the concentration of SFGin the liquid was calculated15 from the Ostwald absorption coefficient of 1.25. Photolysis was carried out at room temperature with stirring for 4 min. This was repeated about 10 times for cyclohexane both as a neat liquid and in the presence of additives. No appreciable change in the light intensity was observed, as confirmed by actinometry before and after each run. Product Analysis. The noncondensabie gases at -196' were collected and measured by means of a Teopler. McLeod system and identified by mass spectrometry. Hydrogen chloride was estimated photometrically16 and by material balance. All liquid products were analyzed quantitatively using Shimadzu gas chromatographs (Models GC-1C and GC5A) equipped with flame ionization detectors. The columns used were as follows: for cyclohexene, a 3 mm by 4 m, 20% dimethylsulfolane on 60-80 mesh Neosorb N P a t room temperature; for cyclohexyl chloride and chloroform, polyethylene glycol 6000, 3 mm by 2 m, 20%, on 60-80 mesh Celite 545 a t 60'; for bicyclohexyl and trichloromethylcyclohexane, a 3 mm by 3 m, 0.1% Apiezon-L grease on Neosorb NGK at 90'; for hexachloroethane, the same column, but at 45'. The gas chromatograph was calibrated and the measurement of each product was carried out relative to its standard. No sample of trichlorornethylcyclohexane was available. It was prepared17 by radiolysis of a Q;r,Hlz-CCl4 mixture (1:l). The retention time of the photoproduct
Photolysis of Liquid Cyc:lohexane
1901
TABLE 1: Phlotolysis of Cyclohexane i n the Presence and Absence of Additives Concii of additiv'sP
0 CCl*(O.1 M )
81
SFs(0.066 A!")
56
82
Rate of producl formation, pMlmin"
__________
-~
Photolysis time, minC
H?
CsHio
0.320 0.214 0.250
0.276 0.201 0.257
CI~H??
~
.
~
~
CsHi161
HCld
CeHiicCl~"
CHCII
CCI,
0.695
