Substituent effects in the reaction of OH radicals with aromatics: Toluene

The relative rates for addition of .OH radical to toluene at its ortho, meta, and para sites have been shown to be in the ratios of 0.84:0.41:1. These ratios provide a quantitative basis for considering the effects of alkyl substiments in determining the site of .OH attack on other aromatics. Because .OH is a strong electrophile, the relative yields of .OH adducts provide a measure of the effect of the methyl group on the distribution of negative charge in toluene's aromatic system. Comparison of the partial rate constants observed for reaction at the different sites of toluene, biphenyl, and phenol with theoretical estimates of the distribution of charge in the valence shell indicates that factors other than charge also play some role.