Hydroquinones (benzene-1,4-diols) are naturally occurring chain-breaking antioxidants, whose reactionswith peroxyl radicals yield 1,4-semiquinone radicals. Unlike the 1,2-semiquinone radicals derived fromcatechols (benzene-1,2-diols), the 1,4-semiquinone radicals do not always trap another peroxyl radical,and instead the stoichiometric factor of hydroquinones varies widely between 0 and 2 as a function ofring-substitution and reaction conditions. This variable antioxidant behavior has been attributed to thecompeting reaction of the 1,4-semiquinone radical with molecular oxygen. Herein we report the results ofexperiments and theoretical calculations focused on understanding this key reaction. Our experiments,which include detailed kinetic and mechanistic investigations by laser flash photolysis and inhibitedautoxidation studies, and our theoretical calculations, which include detailed studies of the reactions ofboth 1,4-semiquinones and 1,2-semiquinones with O
2, provide many important insights. They show thatthe reaction of O
2 with 2,5-di-
tert-butyl-1,4-semiquinone radical (used as model compound) has a rateconstant of 2.4 ± 0.9 × 10
5 M
-1 s
-1 in acetonitrile and as high as 2.0 ± 0.9 × 10
6 M
-1 s
-1 in chlorobenzene,i.e., similar to that previously reported in water at pH ~7. These results, considered alongside our theoreticalcalculations, suggest that the reaction occurs by an unusual hydrogen atom abstraction mechanism, takingplace in a two-step process consisting first of addition of O
2 to the semiquinone radical and second anintramolecular H-atom transfer concerted with elimination of hydroperoxyl to yield the quinone. Thisreaction appears to be much more facile for 1,4-semiquinones than for their 1,2-isomers.
