Redox cycling of iron: Effects of chemical composition on reaction rates with phenols and oxygen in model wine

Abstract

Wine oxidation is mediated by the redox cycling of iron between two oxidation states: the oxidation of iron(II) by oxygen and reduction of iron(III) by phenols. The effects of phenolic structure, pH, and copper on the rates of these reactions were evaluated in model wine. In the absence of a nucleophile, pyrogallol exhibited greater reactivity with iron(III) than 4-methylcatechol. However, both compounds ultimately required aid from the nucleophile benzenesulfinic acid for unrestricted reduction of iron(III) to occur, illustrating the differential structure-dependent reactivities of phenols and the importance of nucleophiles to oxidation. It was hypothesized that the rate of oxygen consumption depends on the rate at which iron(II) is recycled from iron(III), though this was not found to be the case. While the rate of iron(III) reduction by 4-methylcatechol in the presence of benzenesulfinic acid decreased at higher pH, oxygen consumption was faster with increases in pH. Furthermore, copper had no effect on the rate of iron(III) reduction but significantly increased the rate of oxygen consumption, indicating the two reactions may not occur synchronously despite being coupled through iron. Pseudo-first order rate constants for oxygen consumption were much lower than those for iron(III) reduction except when nucleophiles were absent (unlikely in wine), suggesting that iron(II) oxidation is the rate-determining reaction for the wine oxidation pathway. Therefore, the rate at which wine ages is likely limited by oxygen ingress, not chemical composition. However, the overall capacity of wine for oxidation may still depend on constituent phenols and nucleophiles, and a method to assess these factors is of interest.