Kinetics and mechanism of antioxidant action of polysubstituted tetrahydroquinolines in liquid‐phase oxidation reactions of organic compounds by oxygen

Abstract

The antioxidant properties of a series of biologically active polysubstituted tetrahydroquinolines containing chromanyl fragments have been experimentally studied in the liquid phase. Probed was the ability of the substrates to kinetically retard the initiator‐induced radical chain oxidation by O2 of the model compounds 1,4‐dioxane or isopropyl alcohol. The oxidation reactions were monitored at 348 K using a manometric technique, measuring under various initial conditions the time‐dependent concentration of oxygen in the gas phase above a reaction mixture containing the radical‐chain initiator azobisisobutylnitrile and the substrates under study. When tetrahydroquinolines were present, the kinetic curves of oxygen absorption exhibited characteristic induction periods (τ) wherein the reaction rate is immeasurably small. The dependence of τ on the initial concentration of the introduced antioxidants has been determined and the results interpreted within the framework of a classical scheme describing the radical‐chain oxidation of organic compounds in the presence of an inhibitor. Values of the stoichiometric coefficient of inhibition, f, which is related to the number of chain propagating radicals destroyed by each antioxidant molecule, is >>2 for all the studied antioxidants. Consistent with the existing literature, it is inferred that the original antioxidant molecule is regenerated from its radical inhibition product during reaction. Previous studies suggest that oxyperoxy radicals play an important role in the inhibitor regeneration process due to their ability to act either as an oxidizer (in chain propagation) or as a reducing agent (in regeneration of the inhibitor from its corresponding radical). In the presently studied systems containing 1,4‐dioxane, it is argued that the source of oxyperoxy radicals is 2‐hydroxy‐1,4‐dioxane. A reaction mechanism that includes the stages of inhibitor regeneration and that is quantitatively consistent with the present results has been developed. It is suggested that the high antioxidant capacity of the tetrahydroquinoline derivatives is due to their large chain breaking rate constants in conjunction with regeneration of the inhibitor during chain termination processes. The relationship of the studied chemical structures and their antioxidant capacity is discussed.

QSPR Modeling and Experimental Determination of the Antioxidant Activity of Some Polycyclic Compounds in the Radical-Chain Oxidation Reaction of Organic Substrates

“…The synthesis, the physico-chemical properties, and the antioxidant assays of compounds AO1 and AO2 ( Figure 3 ) were reported previously [ 72 ]. In the present study, we describe the kinetics of the radical chain oxidation of an organic compound in the presence of additives AO1 and AO2.…”

“…The experimental logk 7 values for compounds AO1 and AO2 were determined by the manometric method using air oxygen absorption as a model liquid-phase oxidation of 1,4-dioxane, initiated by azobis(isobutyronitrile) (AIBN). The experiments were performed according to the standard technique described earlier [ 72 , 73 , 74 , 75 , 76 , 77 , 78 ]. The model reaction was carried out in a thermostatically controlled glass reactor where the solutions of the initiator (AIBN) and the studied substances in 1,4-dioxane were loaded.…”