Stabilities of Highly Conjugated Radicals from Bond Homolysis Rates.

Article abstract of DOI:10.1021/ja00272a011

Rates of Benzyl C-CH3 homolysis of 4-ethylstirene (2), 1-phenyl-1-butene (3), 1,1'-diphenylethane (4), 2,2'-diphenylpropane (5), and 1-methylindene (6) were measured in a very low pressure pyrolysis apparatus.Observed rate constants were fit by the following high-pressure Arrhenius expressions with use of RRKM theory k₂/s^-1 = 10^15.3 exp(-35930/T) 1096-1186 K, k₃/s^-1 = 10^15.3 exp(-33970/T) 1030-1115 K, k₄/s^-1 = 10^15.5 exp(-34020/T) 1000-1066 K, k₅/s^-1 = 10^15.7 exp(-33060/T) 950-1054 K, k₆/s^-1 = 10^16.0 exp(-35230/T) 1031-1168 K where A factors were estimated by comparison to A factors for analogous reactions of alkylbenzenes.Differences between activation energies for reactions 2-6 and corresponding alkylbenzene homolysis reactions were used to find "extra" resonance stabilization energies (kcal mol^-1) for the following radicals: H2C=CH-Ph-CH2., 1.6; Ph-CH=CH-CH2., 5.4; (Ph)2CH., 4.5; (Ph)2C.CH3, 4.0; and In., 1.2 (4.184 J = 1 cal).These results are in agreement with predictions of Structure Resonance Theory.In addition, product mass spectra were not consistent with the results of recent shock tube studies of ethylbenzene decomposition where benzyl C-H homolysis was reported to be faster than benzyl C-C homolysis.