Rate constants for H-atom abstraction by HOO˙ from H-donor compounds of antioxidant relevance

Abstract

The rate constants for the reaction of hydroperoxyl (or perhydroxyl) radical HOO˙ with fifteen phenols and ascorbyl palmitate were measured in acetonitrile at 37 °C by evaluating the effect that the antioxidants had on the rate of autoxidation of γ-terpinene. The HOO˙ radical represents an important reactive species that can be formed by protonation of superoxide anions (O₂˙⁻) or by fragmentation of alkylperoxyl radicals (ROO˙) formed during the autoxidation of pro-aromatic derivatives like γ-terpinene. The phenols investigated in this study include natural compounds like phenolic acids (protocatechuic, caffeic and dihydrocaffeic acids), flavonoids (3-hydroxyflavone, pinobanksin, galangin, catechin, luteolin, quercetin, 6-methoxyluteolin), 4-methylcatechol and antioxidant additives ascorbyl palmitate and the a-tocopherol analogue 2,2,5,7,8-pentamethyl-6-chromanol. The rate constants for the reaction of HOO˙ with the above compounds (k_inh) spanned from 1 × 10³ M⁻¹ s⁻¹ for the unsubstituted phenol, to 7 × 10⁴ and 9 × 10⁴ M⁻¹ s⁻¹ for 4-methylcatechol and ascorbyl palmitate, respectively. As in a typical Evans–Polanyi plot, the log(k_inh) was found to be inversely proportional to the bond dissociation enthalpy of the reactive OH. The comparison of the results with the data reported in the literature shows an unusual kinetic solvent effect that enlightens the unique behavior of HOO˙ and provides a rationale for the superior radical trapping ability of catechols and ascorbyl palmitate.