3325
GAS-PHASE RADIOLYSIS OF TOLUENE
Gas-Phase Radiolysis of Toluene by Yukio Yamamoto, Setsuo Takamuku, and Hiroshi Sakurai The Radiation Laboratory, The Institute of Scientific and Industrial Research, 0sah-u University, Suita, Osaka, J a p a n (Received December 31, 1969)
The y radiolysis of toluene in the gas phase was studied at room temperature as a function of pressure (1.5-18 mm) and added NO and NzO. The G values obtained at 10.7 mm and a total dose of 1.9 X 10’9 eV are hydrogen, 0.36; methane, 0.15; ethane, 0.07; acetylene, 0.41; benzene, 0.18; ethylbenzene, 0.05; xylenes, 0.05; 2-methyldiphenylmethane (MDPM), 0.05; 3-MDPM, 0.34; 4-MDPM, 0.05; and bibenzyl, 0.03. The effect of added KO indicates that ethane, ethylbenzene, xylenes, bibenzyl, and some of the benzene are formed by radical reactions but not acetylene and MDPMs. The product formation was compared with that of the y radiolysis in the liquid phase and the mercury-sensitizedphotolysis in the gas phase which was reinvestigated in this study. The basic difference between these studies is due to the different reactive species formed by the P-bond fissioiis; in the gas-phase radiolysis C7H7+ions are formed and produce three isomers of MDPM, while in the photolysis and liquid phase radiolysis benzyl radicals, producing predominantly bibenzyl, are formed. The addition of NzO results in the considerable increase of MDPMs and bibenzyl yields, which can be attributed to the additional formation of benzyl and tolyl radicals.
Introduction Many papers have been published on the radiolysis of toluene in the liquid phase. In some detailed studies the yields of a number of products, gases and liquids including dimers, were determined, and their formation has been attributed to the reactions through excited molecules.1~2 However, little attention has been paid to the radiolysis of toluene in the gas phase3 where ionic reactions are expected to constitute a significant segment of the overall reaction. For the gas-phase radiolysis of simple alkylbenzenes such as toluene, ethylbenzene, and xylenes, of interest is the product formation by the reactions of C7H7+ ion, which is the most abundant ion in the mass spectra of these alkylbenzenes and whose structure and formation process have been extensively investigated mass spectrometri~ a l l y . ~ IJn the previous paper6 we reported that in the gas-phase radiolysis of toluene, ethylbenzene, and m-xylene the corresponding benzylated alliylbenzenes, alkyldiphenylmethanes, were formed as the main products in all cases, and their formation was suggested to be due to an ion-molecule reaction of the C7H7+ ion. In the present study of the gas-phase radiolysis of toluene, the formation of methyldiphenylmethanes (MDP31’s) by the reaction of the C7H7+ ion and that of other products were investigated in some detail. The mercury-photosensitized decomposition of gaseous toluene is included in this study for comparison with the radiolysis.
Experimental Section Materials. Toluene was obtained from Wako Pure Chemical Industrial Co. and purified by the usual method. After the extensive distillations using a 1-m column packed with stainless helices, the purity de-
termined chromatographically by flame-ionization detection was more than 99.99%. The purified toluene was dried over sodium, degassed, and stored in a highvacuum line. Nitric oxide and nitrous oxide, both obtained from Takachiho Shoji Co., were purified by several low-pressure distillations. Procedures. The irradiation cells were Pyrex cylinders of approximately 120-ml volume, and each cell was fitted with a break-seal. After the cells were evacuated at a pressure of mm for -10 hr with periodic heating by a hand torch, the toluene vapor from the storage was extensively dried by passing through a sodium mirror, degacised, and then introduced into the cells. The pressure of the samples was measured by a mercury manometer. Samples were irradiated with y rays from a 5000-Ci 6oCosource at room temperature. The dose rate t o toluene was determined by ethylene dosimetry, using a G value of 1.28 for hydrogen formation’ and correcting for the electron density of toluene relative to ethylene. The irradiations were carried out at a constant dose (1) (a) J. Hoigne and T. Gaumann, Hela. Chim. Acta, 44, 2141 (1961); (b) J. Hoigne and T. Gaumann, ibid., 46, 365 (1963); (c) J. Hoigne, W. G. Burns, W. R. Marsh, and T. Gaumann, {bid., 47, 247 (1964). (2) J. Weiss and C. H. Collins, Rad&. Res., 28, 1 (1966). (3) The G values of the radiolysis products in the gas phase including some discussion are reported by K. E. Wilabach and L. Kaplan, Advances in Chemistry Series, No. 82, American Chemical Society, Washington, D. C., 1968,p 134. (4) H.M.Grubb and S. Meyerson in “Mass Spectrometry of Organic Ions,” F. W. McLafferty, Ed., Academic Press, New York, N. Y., 1963,p 453. (5) A recent study is, for example, K. L. Rinehart, Jr., A. C. BuchhoIs, G. E. Van Lear, and H. L. Cantrill, J . Amer. Chem. Soc., 90, 2983 (1968). (6) Y. Yamamoto, 8. Takamuku, and H. Sakurai, ibid., 91, 7192 (1969). (7) R. A. Back, T. W. Woodward, and K. A. McLauchlan, Can. J. Chem., 40, 1380 (1962). The Journal of Physical Chemistry, Vol. 74, No. 18, 1970
3326
Y. YAMAMOTO, S. TAKAMUKU, AND H. SAKURAI
Table I: Yields in the
y Radiolysis and Hg-Sensitized Photolysis of Toluene Vapor, and Comparison with Reported Yields in the Radiolysis
---______
-----
Radoiylsi-
7--
25
30a
r -
-~--10.7
Products H2 CH4 CzHz CZH4 CzHe Benzene Ethylbenzene 0-Xylene m-Xylene p-X ylene 2-MDPM 3-MDPM 4-MDPM Bibenz yl
0.36
0.15 0.41 Trace
0.07 0.18
0.05'
----pressure
7.5 U value----
Liquid
0.47 0.15 0.64 0.10 0.06 0.20 0.07 0.02 0.06 0.007
0.14 0.012 0.002
0.34
7
25
25
5.5
16.0
7
0,034
0.034 n.d." n.d.
0.0003
...
0.017 0.137 0,079
0.0188 0.0012 0.0009 0.0015 0.0010
."'1
I..
...
0.097 0.023 n.d. n.d.
0,006 0.101 0.066
