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

Hydrogen atom transfer (HAT) by 3O2 and HO2 • from arenols (ArOH), aryloxyls (ArO•), their tautomers (ArH), and auxiliary compounds has been investigated by means of CBS-QB3 computations. With 3O2, 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 • → ArO• + H2O2 neither afford a linear correlation nor agree with the experiment. The precise mechanism for the liquid-phase autoxidation of anthrahydroquinone (AnH2Q) appears to be not fully understood. A kinetic analysis shows that the HAT by chain-carrying HO2 • occurs with a high rate constant of ≥6 × 108 M–1 s–1 (toluene). The second propagation step pertains to a diffusion-controlled HAT by 3O2 from the 10-OH-9-anthroxyl radical. Oxanthrone (AnOH) is a more stable tautomer of AnH2Q with a ratio of 13 (298 K) in non-hydrogen-bonding (HB) solvents, but the reactivity toward 3O2/HO2 • is much lower. Combination of the computed free energies and Abrahams’ HB donating (α2 H ) and accepting (β2 H ) parameters has afforded an α2 H (HO2 •) of 0.86 and an α2 H (H2O2) of 0.50.