Kinetics and mechanism of aromatic electrophilic substitution reactions : VII. Temperature dependence of substrate selectivity of methylbenzene hydroxylation in the system H2O2-H2SO4

The relative rates of the hydroxylation of toluene,ortho-, meta-, andpara-xylenes, mesitylene, and pseudocumene are measured by the kinetic distribution method in the H2O2-H2SO4 (70.0 wt %) system at 15 to 55°C. The activation entropies correlate with the basicities of arenes and the activation enthalpies correlate with both the basicities and ionization potentials of ArH. Substrate selectivity is described with a high accuracy by the general equation with three variables (basicity, ionization potentials, and temperature) and implies the compensation effect between changes in standard entropies and enthalpies for the basicity constants of ArH. The structure of the transition state of the slow step of hydroxylation is intermediate between the structure of a charge-transfer complex and that of a σ-complex.