6PPD quinone formation from 6PPD via a radical process and its competing reactions

N-(1,3-Dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) is an essential additive in tire rubber compounds to enhance their lifespan. 6PPD quinone (6PPD-Q), a known toxic chemical, is formed through the oxidation of 6PPD. The formation of 6PPD-Q is expected to proceed via radical process, as the oxidation reaction necessarily involves an oxygen atom. However, due to the challenges in detecting radical species, many studies have inferred the formation of 6PPD-Q based on the neural oxidation products. In this study, the 6PPD oxidation products including radical intermediates were analyzed using direct atmospheric pressure chemical ionization-mass spectrometry (APCI-MS). The formation of 6PPD-Q was elucidated by detecting its radical intermediates. The first ketone group in 6PPD-Q is formed through the rearrangement of (6PPD + O − H)^• radical precursor. The second one is generated by the loss of a hydrogen atom from (6PPD + 2O − H)^• radical precursor. The abundance of 6PPD-Q was significantly lower than those of other competing oxidation products. The primary competing reactions include the rearrangements of the two radical precursors into alternative species and the dissociations of 6PPD and oxidation products into nitrobenzene, formylaniline, 4-hydroxydiphenylamine, and 4-nitrodiphenylamine. The favorability of 6PPD-Q formation and its competing reactions was explained in terms of thermodynamics.