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, N 3 ) and reducing radicals (e − aq, CO 2 − , acetone ketyl radical) with 2-hydroxy-3-methoxybenzaldehyde ( o-vanillin) at different pH. Hydroxyl radicals react mostly by addition reaction forming radical adducts ( λ max=420 nm) and the oxidation is only a minor process even in the alkaline region. The reaction with azide radicals produced phenoxyl radicals ( λ max=340 nm), which are formed on fast deprotonation of solute radical cation. Using PMZ + /PMZ and ABTS − /ABTS 2− 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 ABTS 2− quantitatively. The e aq − is observed to react with o-vanillin with rate constant value of 2×10 10 dm 3 mol −1 s −1. CO 2 − 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 λ max at 350 and 390 nm at pH 4.5 and 9.7, respectively. The p K a 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.