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PHOTOLYSIS OF CYCLOHEXANE ARD CYCLOHEXENE VAPORS ceived fellowship support from the Shell Companies Foundation, Inc. The authors extend thanks to the following: Dr. C. C. Sweeley, now of Michigan State
University, for mass spectral determinations; Dr. Eleanor J. Fendler, Chemistry Department, for nmr studies; The University of Pittsburgh computer center.
Radical Intermediates in the Vacuum Ultraviolet Photolysis of Cyclohexane and Cyclohexene Vapors1 by M. D. Sevilla Atomics International, A Division of North American Rockwell Corporation, Canoga Park, California 91304
and R. A. Holroyd Chemistry Department of Brookhaven National Laboratory, Upton, New York (Receiced December 8 , 1060)
11073
The radical intermediates in the vacuum uv photolysis of cyclohexane and cyclohexene vapor have been identified. Two methods were used for radical detection: the 14CHa-radicalsampling method and esr identification of the radicals trapped on a cold finger a t 77°K. Results of the radical sampling technique show that a t 1470 A the principal radical intermediates in cyclohexane a t 10 Torr are methyl, allyl, and cyclohexyl. Similar results were obtained for cyclohesane-dlz. Increasing the photon energy results in an increase in the yields of methyl, allyl, and other fragment radicals. Although the spectrum of the allyl radical predominates, the esr data are shown to be coiisistent with the radical sampling data. I n cyclohexene bhe main radical intermediates at 1470 d are methyl, allyl, cyclohesyl, and 3-cyclohesenyl. The mechanism of formation of radicals in these hydrocarbons is discussed.
Introduction Studies of the vacuum uv photolysis of hydrocarbons have shown that the most important primary processes involve the molecular elimination of smaller molecules.2*8 However, there is also evidence for radical formation proce~ses.~Studies of the yields and nature of radical intermediates are useful in elucidating the process by which these radicals are formed. This report deals with the detection of radical intermediates in vacuum uv photolysis by two methods: esr spectroscopy and 14CH8-radicalsampling.5~~Yields of radical intermediates in the vapor phase photolysis of cyclohexane and cyclohexene are reported. The present results show that allyl radicals are more important than indicated by an earlier study of cyclohexane4 in which HzS was used as a radical scavenger.
Experimental Section The photolysis arcs were prepared in the manner prescribed by McNesby and Okabe.2 The arcs were baked under vacuum and gettered prior to filling with inert gas. A sapphire window was joined by a graded seal to the xenon arc and a MgF2 window was sealed
wit,hepoxy to the krypton arc. Both arcs were equipped with 29/42 joints for attachment to the photolysis vessels. The arcs were powered by a microwave diathermy unit (Raytheon). For detection of radicals by esr spectroscopy a flow system similar to that of Smith and Tole’ was used. Hydrocarbon vapors (Phillips Research Grade) passed by the window of the photolysis arc and were subsequently collected on a cold finger, cooled to 77”K, located 4 cm from the window. After several minutes of photolysis, the system was pressurized with nitrogen and the cold finger was quickly immersed in liquid nitrogen and transferred to the variable temperature (1) This work was supported by the Research Division of the U. S. Atomic Energy Commission. (2) J. R. McNesby and H. Okabe, Advan. Photochem., 3, 157 (1964). (3) L. W. Sieck in “Fundamental Processes in Radiation Chemistry,” P. Ausloos, Ed., John Wiley & Sons, New York, N. Y.,1968, p 136 ff, (4) P.Ausloos, R. E. Rebbert, and 5. G. Lias, J . Phys. Chem., 72, 3904 (1968). (5) R. A. Holroyd and G. W. Klein, I n t . J . Appl. Radiat. Isotop., 15, 633 (1964). (6) R. A. Holroyd, J . Amer. Chem. Soc., 91,2208 (1969). (7) D.R. Smith and J. C. Tole, Can. J . Chem., 45,779 (1967).
The Journal of Physical Chemistry, Vol. 74* No. 18’? 1070
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M. D. SEVILLAAND R. A. HOLROYD
Table I: Radical Yields in the 1470-A Photolysis of Cyclohexane
3.4 0.03 0.040 0.012 0.031 0.004 0.016 0.045 a
10 0.03 0.044 0.012 0.034 0.003 0.022 0.035
Results obtained with HzS scavenger (ref 5).
20 0.03 0.045 0.012 0.034 0.005 0.028 0.023
79 0.03 0.044 0.010 0.034 0.008 0,049 0.038
7.7"
20 0.03 0.069 0.010 0.042 0.005 0.032 0.016
.,.
0.039 0.016
