Phenolic Hydrogen Transfer by Molecular Oxygen and Hydroperoxyl Radicals. Insights into the Mechanism of the Anthraquinone Process.

Hydrogen atom transfer (HAT) by O-3(2) and HO2 center dot from arenols (ArOH), aryloxyls (ArO center dot), their tautomers (ArH), and auxiliary compounds has been investigated by means of CBS-QB3 computations. With O-3(2), excellent linear correlations have been found between the activation enthalpy and the overall reaction enthalpy. Different pathways have been discerned for HATs involving OH or CH moieties. The results for ArOH + HO2 center dot -> ArO center dot + H2O2 neither afford a linear correlation nor agree with the experiment. The precise mechanism for the liquid-phase autoxidation of anthrahydroquinone (AnH(2)Q) appears to be not fully understood. A kinetic analysis shows that the HAT by chain-carrying HO2 center dot occurs with a high rate constant of >= 6 x 10(8) M-1 s(-1) (toluene). The second propagation step pertains to a diffusion-controlled HAT by O-3(2) from the 10-OH-9-anthroxyl radical. Oxanthrone (AnOH) is a more stable tautomer of AnH(2)Q with a ratio of 13 (298 K) in non-hydrogen-bonding (HB) solvents, but the reactivity toward O-3(2)/HO2 center dot is much lower. Combination of the computed free energies and Abrahams' HB donating (alpha(H)(2)) and accepting (beta(H)(2)) parameters has afforded an alpha(H)(2)(HO2 center dot) of 0.86 and an alpha(H)(2)(H2O2) of 0.50.