Effect of pH on the autoxidation kinetics of vanillin

Summary

The autoxidation kinetics of vanillin (4-hydroxy-3-methoxy benzaldehyde) have been investigated in aqueous media between pH 5.0 and 13.5. The results obtained clearly show two distinct reaction mechanisms are operative. At pH 7.3 the reaction orders with respect to vanillin concentration and oxygen partial pressure are 1.50 and 0.5 respectively, indicative of a free-radical chain process. A bimolecular ionic pathway is indicated at pH 11.0 and above since the reaction orders with respect to both vanillin and oxygen are 1.00. The formation of a hydroperoxide intermediate is believed to be the first step in both pathways. The energy of activation was found to be 14.2 kcal/mole at pH 7.3 and 13.6 kcal/mole at pH 11.0.