Mechanism of thermal toluene autoxidation

Aerobic oxidation of toluene (PhCH3) is investigated by complementary experimental and theoretical methodologies. Whereas the reaction of the chain-carrying benzylperoxyl radicals with the substrate produces predominantly benzyl hydroperoxide, benzyl alcohol and benzaldehyde originate mainly from subsequent propagation of the hydroperoxide product. Nevertheless, a significant fraction of benzaldehyde is also produced in primary PhCH3 propagation, presumably via proton rather than hydrogen transfer. An equimolar amount of benzyl alcohol, together with benzoic acid, is additionally produced in the tertiary propagation of PhCHO with benzylperoxyl radicals. The "hot" oxy radicals generated in this step can also abstract aromatic hydrogen atoms from PhCH3, and this results in production of cresols, known inhibitors of radical-chain reactions. The very fast benzyl peroxyl-initiated co-oxidation of benzyl alcohol generates HO2* radicals, along with benzaldehyde. This reaction also causes a decrease in the overall oxidation rate, due to the fast chain-terminating reaction of HO2*with the benzylperoxyl radicals, which causes a loss of chain carriers. Moreover, due to the fast equilibrium PhCH2OOH+HO2* right harpoon over left harpoonPhCH2OO* + H2O2, and the much lower reactivity of H2O2 compared to PhCH2OOH, the fast co-oxidation of the alcohol means that HO2* gradually takes over the role of benzylperoxyl as principal chain carrier. This drastically changes the autoxidation mechanism and, among other things, causes a sharp decrease in the hydroperoxide yield.