Kinetic Solvent Effects on Phenolic Antioxidants Determined by Spectrophotometric Measurements

The effects of polar (acetonitrile and tert-butyl alcohol) and apolar (cyclohexane) solvents on the peroxyl-radical-trapping antioxidant activity of some flavonoids, catechol derivatives, hydroquinone, and monophenols have been studied. The inhibition rate constants k inh of the antioxidants have been determined by following the increase in absorbance at 234 nm of a dilute solution of linoleic acid at 50 °C containing small amounts of antioxidant and radical initiator. Despite the low concentration of linoleic acid, the peroxidation process has been confirmed to be a free radical chain reaction described by the classical kinetic laws for this process. However, in the evaluation of k inh, a careful analysis of the peroxidation curve, absorbance versus time, must be done because the final oxidation products of phenols may absorb at 234 nm. Phenols with two ortho-hydroxyls are the most active antioxidants, with inhibition rate constants in the range of (3−15) × 105 M-1·s-1 (in cyclohexane). Nevertheless, it has been observed that in tert-butyl alcohol (a strong hydrogen bond acceptor) the rate constants dramatically decline to values not detectable by the present kinetic method. In acetonitrile (a weaker hydrogen bond acceptor) instead, the phenols with two ortho-hydroxyls scavenge the peroxyl radicals with rate constants close to those in cyclohexane. From the kinetic solvent effect, the equilibrium constant of the first solvation step of hydroquinone with tert-butyl alcohol has been determined at 50 °C, K 1 = 2.5 ± 0.5 M-1. Keywords: Antioxidant; peroxyl radical; flavonoids; catechols; kinetic solvent effect; inhibition rate constant; linoleic acid peroxidation; hydrogen bonding; conjugated diene hydroperoxides