Some Unimolecular Fissions Production of Free Radicals

Some Unimolecular Fissions Production of Free Radicals 

The data on the unimolecmlar rates of decomposition of stable molecules into free radicals which are summarized in liable XI.5 arc probably quantitatively the least reliable of all the data given on unimolecular reactions. The reason for this is that the rate constants given have had to be inferred from an over-all observed rate for a set of complex reactions. This is an inherent difficulty with reactions that involve the production of free radicals, which, being active intermediates, of necessity disappear by secondary reactions. 

In the decomposition of toluene, for example, the products are found to be principally H2 CH4 bibcnzyl, ( 0112)2 and presumably benzene, II. The mechanism proposed is 

Daniels and coworkers have shown [J. Aw. Chem. Soc.y 72, 424 (1950) J. Chem. Phys.f 7, 756 (1939)] that the deeomj)osition of CjIT Br proceeds via chains starting and stopping at the walls, so that, although the reaction is not terribly sensitive to surfacc/volume ratio, the walls always play a role in it. These results were demonstrated by means of radioactive tracers. This and results of other workers (to be discussed) show the difficulty of demonstrating experimentally the presence of catalyzed reactions or radical chains. 

Other values have been reported [J. Murawski et al., J. Chem. Phys.. 19,698 (1951)], with log A = 14.7 and = 36 Kcal/mole. 

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