Substituted p‐hydroquinones as a chain‐breaking antioxidant during the oxidation of styrene

Abstract

ABSTRACT:The technique based on monitoring oxygen consumption was applied to test 13 substituted p-hydroquinones (QH 2 ) as a chain-breaking antioxidant during the oxidation of styrene initiated by 2,2 -azobis(2,4-dimethylvaleronitrile) at 37 • C. The methodology originally developed to test monophenolic antioxidants was modified to fit it to specific features of oxidative transformation of QH 2 . Chain-breaking capability of QH 2 was characterized by two parameters: the rate constant k 1 for reaction of QH 2 with the peroxy radical LO 2 · : (1) QH 2 + LO 2 · → QH · + LOOH and the stoichiometric factor of inhibition, f , which shows how many kinetic chains may be terminated by one molecule of QH 2 . Rate constants k 1 × 10 5 (in M −1 s −1 ) were found to be 5.54 (nonsubstituted QH 2 ); 7.13 (Me-QH 2 ); 13.1 (Et-QH 2 ); 12.0 (tBu-QH 2 ); 15.6 (2,6-Me 2 -QH 2 ); 18.8 (2,3-Me 2 -QH 2 ); 17.1 (2,5-Me 2 -QH 2 ); 13.7 (2,6-(MeO) 2 -QH 2 ); 4.70 (2,6-Ph 2 -QH 2 ); 0.90 (2,5-Cl 2 -QH 2 ); 23.2 (Me 3 -QH 2 ); 4.40 (2,3-(MeO) 2 -5-Me-QH 2 ). Parameter f never exceeds 2; for the majority of QH 2 , f was found to be visibly less than 2, decreasing when the rate of initiation decreased. Both experimental data and results of computer kinetic simulations suggest that, contrary to monophenolic antioxidants, the main path of QH · transformation is the self-disproportionation rather than recombination of QH · with LO 2 · . Most likely, f is commonly lower than 2 because of the reaction of QH · with molecular oxygen. The latter may result in faster depletion of QH 2 and the increase in the rate of inhibited oxidation.

Intermolecular Hydrogen Bonding Modulates the Hydrogen‐Atom‐Donating Ability of Hydroquinones

“…[4] The impact of hydrogen bonding on the oxidation of hydroquinones to give hydrogen-bonded neutral semiquinones has received somewhat less attention, despite this reaction having been reported to be crucial for the cytochrome bc 1 complex, [5] a component of respiratory electron-transfer chains, and despite it being the basis of the antioxidant action of hydroquinones in apolar solvents. [6] The presence of a neutral semiquinone intermediate has also been proposed to be formed in the high affinity site (Q H ) of cytochrome bo 3 from Escherichia coli. [7] To better understand the mechanism of the enzymatically catalyzed oxidation of ubiquinol, and to inspire the synthesis of new antioxidants, the relationship between the hydrogen-atom-donating ability of hydroquinones and the hydrogen-bonding behavior of the reduced (hydroquinone) and partially oxidized (semiquinone) species should be rationalized quantitatively.…”

“…The rate constant for the reaction of 1 with peroxyl radicals (k inh ) was also determined by means of inhibited autoxidation studies, [6,13] because of the importance of ROOC radicals in causing oxidative damage in natural and synthetic materials. The data reported in Figure 2 show that 1S (S = DMSO) is five times more reactive than 1, with k inh values of (3.0 AE 0.6) 10 6 for the complexed and (5.8 AE 0.8) 10 5 m À1 s…”

Kinetics and Mechanism of Reactions of Aliphatic Stable Nitroxide Radicals in Chemical and Biological Chain Processes

“…Oxidation kinetics was studied in area of initial O2 consumption rates in temperature range of 310 -343 K with highly sensitive capillary microvolumometer according to technique (Loshadkin et al, 2002). Initial >NO • concentrations were in range of 10 -7 -10 -3 M. Experiments were carried out at Po2 = 20 or 100 kPa.…”

“…6-Hydroxy-2,2,5,7,8-pentamethylbenzochroman was used as inhibitor (InH). Kinetic modeling was performed as described in (Loshadkin et al, 2002).…”

“…This assertion confirms with distinct induction period on oxygen consumption's kinetic curve (typical one represented at Figure 3 (Pliss et al, 2012)). Rate constants of such antioxidants' reactions with peroxide radicals can be determined from inhibited oxidation's rate dependence on time according to the following equation (Loshadkin et al, 2002):…”