Thermochemistry and Kinetics of Intermolecular Addition of Radicals to Toluene and Alkylaromatics

To better understand intermolecular radical additions to aromatic rings that take place in the pyrolysis of alkylaromatics at low to moderate temperatures (∼450 °C), the thermochemistry and kinetics of several reactions of this type are investigated using the CBS-QB3 level of theory. The calculated thermochemistry of the adduct radicals is significantly different from previous estimates; the average discrepancy in Gibbs free energy at 298 K is 5.3 kcal/mol. A group additivity value for aromatic π radicals was developed to facilitate rapid accurate estimates for other molecules containing this functional group; the average discrepancy in Gibbs free energy using the updated group additivity value is improved to 0.5 kcal/mol. Previous estimations of the rate coefficients of these reactions were found to be inaccurate due to the lack of important features such as loss of aromaticity in the compounds used as the training set for the estimation. Rate coefficients for addition of several different radicals to aromatic rings are reported. The reaction rates are comparable for ortho, meta, and para additions, slowest for addition to the substituted position, and insensitive to the length of alkyl chains attached to the aromatic reactant.