Antioxidant activity of 4-[alkyl(benzyl)sulfanylmethyl]-5-methyl-2,4-dihydro-3H-pyrazol-3-ones

In order to search for efficient antioxidants of the pyrazolone series, sulfur-containing derivatives of 2,4-dihydro-3H-pyrazol-3-one and 1H-pyrazole (1a–e) were studied in the model system of radical chain oxidation of 1,4-dioxane. The antioxidant activity of these compounds was determined. The rate constant for the reaction of peroxyl radicals with inhibitor 1 (k7) and stoichiometric inhibition coefficient (f) were calculated at 333 K. Sulfur-containing 2,4-dihydro-3H-pyrazol-3-ones exhibit a higher antioxidant activity fk7 = (5.7–9.2)·104 L mol−1 s−1) than 4-(pentylsulfanylmethyl)-3,5-dimethyl-1H-pyrazole fk7 ⩽ 2 · 103 L mol−1 s−1), which is possibly due to the presence of the hydroxyl group in the pyrazolone cycle in the hydroxy form. The stoichiometric coefficients of inhibition for 4-(benzylsulfanylmethyl)-5-methyl-, 5-methyl-4-(pentylsulfanylmethyl)-, and 5-methyl-4-(propylsulfanylmethyl)-2,4-dihydro-3H-pyrazol-3-ones f = 0.95–1.3) indicate the “decay” of one peroxyl radical on one inhibitor molecule. In the case of 5-methyl-4-[(2-propylsulfanyl)methyl]-2,4-dihydro-3H-pyrazol-3-one, the stoichiometric coefficient of inhibition decreases f = 0.59).