Mechanism of Hydrogenolysis. I. Effect of Methyl Substituent on the Demethylation Rate of Polymethylbenzenes

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Abstract
Rates of demethylation of six different polymethylbenzenes have been measured, at 702°C and residence times ranging from 0.5 to 6.0 sec in the presence of five-fold in excess hydrogen, by using toluene as an internal reference. The rates per one equivalent methyl group were: 1.08 for p-xylene, 1.73 for o-xylene, 1.26 for 4-position of 1,2,4-trimethylbenzene, 1.87 for 2-position of 1,2,4-trimethylbenzene, 1.85 for 1-position of 1,2,4-trimethylbenzene, 1.20 for 1,3,5-trimethylbenzene, 2.22 for 1-positionof 1,2,3-trimethylbenzene, 3.25 for 2-position of 1,2,3-trimethylbenzene, 1.5 for 5-position of 1,2,3,5-tetramethylbenzene, 2.1 for 1-position of 1,2,3,5-tetramethylbenzene, and 2.8 for 2-position of 1,2,3,5-tetramethylabenzene. all the figures being expressed against the standard value of 1.00 for toluene. The observed accelerating effect of neighboring methyl groups on the demethylation rates of crowded reactant molecules has been correlated with modified superdelocalizability in z direction for radical aromatic substitution. The latter values calculated by the extended Hückel molecular orbital method were: 0.4070, 0.4116, 0.4908, 0.4169, 0.4124, 0.4116, 0.4131, 0.4133, 0.4148, 0.4204, 0.4120, and 0.4029 (toluene) in the order given above. The general parallelism between the observed and the theoretical values may be taken as a support not only of the extention of the concept of superdelocalizability to radical aromatic substitution, but also of our mechanistic interpretation on the key reaction of hydrogenolyses.