Redox reactions of 2-hydroxy-3-methoxybenzaldehyde (o-vanillin) in aqueous solution

Abstract

Pulse radiolysis technique has been employed to study the reactions of oxidizing (OH, N3) and reducing radicals (e−aq, CO2−, acetone ketyl radical) with 2-hydroxy-3-methoxybenzaldehyde (o-vanillin) at different pH. Hydroxyl radicals react mostly by addition reaction forming radical adducts (λmax=420 nm) and the oxidation is only a minor process even in the alkaline region. The reaction with azide radicals produced phenoxyl radicals (λmax=340 nm), which are formed on fast deprotonation of solute radical cation. Using PMZ+/PMZ and ABTS−/ABTS2− as the reference couple, different methods are employed to determine the one-electron reduction potential of o-vanillin and the average value is estimated to be 1.076±0.004 V vs. NHE at pH 6. The phenoxyl radicals of o-vanillin were able to oxidize ABTS2− quantitatively. The eaq− is observed to react with o-vanillin with rate constant value of 2×1010 dm3 mol−1 s−1. CO2− and acetone ketyl radical are also observed to react with o-vanillin by electron transfer mechanism and showed the formation of transient absorption bands with λmax at 350 and 390 nm at pH 4.5 and 9.7, respectively. The pKa of the one-electron reduced species was determined to be 8.1. The results indicate that the aldehydic group is the most preferred site for electron addition.