Copper-Catalyzed Hydroquinone Oxidation and Associated Redox Cycling of Copper under Conditions Typical of Natural Saline Waters

Abstract

A detailed kinetic model has been developed to describe the oxidation of Cu(I) by O2 and the reduction of Cu(II) by 1,4-hydroquinone (H2Q) in the presence of O2 in 0.7 M NaCl solution over a pH range of 6.5-8.0. The reaction between Cu(I) and O2 is shown to be the most important pathway in the overall oxidation of Cu(I), with the rate constant for this oxidation process increasing with an increasing pH. In 0.7 M NaCl solutions, Cu(II) is capable of catalyzing the oxidation of H2Q in the presence of O2 with the monoanion, HQ(-), the kinetically active hydroquinone form, reducing Cu(II) with an intrinsic rate constant of (5.0 ± 0.4) × 10(7) M(-1) s(-1). Acting as a chain-propagating species, the deprotonated semiquinone radical (SQ(•) (-)) generated from both the one-electron oxidation of H2Q and the one-electron reduction of 1,4-benzoquinone (BQ) also reacts rapidly with Cu(II) and Cu(I), with the same rate constant of (2.0 ± 0.5) × 10(7) M(-1) s(-1). In addition to its role in reformation of Cu(II) via continuous oxidation of Cu(I), O2 rapidly removes SQ(•) (-), resulting in the generation of O2(•) (-). Agreement between half-cell reduction potentials of different redox couples provides confirmation of the veracity of the proposed model describing the interactions of copper and quinone species in circumneutral pH saline solutions.

Reduced graphene oxide-catalyzed oxidative coupling reaction of 4-methoxyphenol in aerobic aqueous solution

“…Considering the pK a of MOP (pK a ¼ 10.2), the enhanced catalytic efficiencies of rGOs on MOP transformation can be attributed to the dissociation of MOP. According to previous studies, the ionization of phenolics can overcome the spin restriction of phenolic OH group and was favorable for the formation of phenolic radicals [31,47]. Further increasing solution pH above 10.5, interestingly, a reversed trend was observed.…”

“…MOPS (2.0 mM) was used to control the solution pH at 7.0 because it was found to have negligible effect on the oxidation reactions of MOP [31]. Under such a condition, MOP is present in solution as neutral form.…”

Mechanistic insights into iron redox transformations in the presence of natural organic matter: Impact of pH and light

“…H 2 A; pK a1 ~ 10 and pK a2 ~ 12 for hydroquinone). Although the deprotonated form of hydroquinone is more kinetically active than H 2 A (Yuan et al, 2013), the concentration of these deprotonated forms at these acidic pH would appear to be too small (at least 6 fold less than H 2 A) to contribute to the kinetics of hydroquinone-mediated Fe(III) reduction. Although some pH-dependence in Fe(III)-SRFA speciation may occur at low pH (as is evident for Fe complexes with synthetic ligands such as EDTA), little information on this possibility is available from the literature; as such, Fe(III)-SRFA speciation is assumed to be invariant with pH and, as a result, the rate constant for the reduction of Fe(III) by the hydroquinone (reaction 17) considered to be pH-independent.…”

Copper-catalyzed tandem reaction of 2-alkynylanilines with benzoquinones: efficient access to 3-indolylquinones

“…Based on these control experiments, a plausible mechanism for the formation of 3-indolyl quinones was proposed as shown Table 2 Tandem cyclization of 2-alkynyl anilines with benzoquinones 8b which involves oxidation in the presence of excess benzoquinone, or path A, 16 where Cu(II) is reduced to Cu(I) in the presence of 3-indolyl hydroquinone, nally to access the desired product 3aa.…”