VANADIUM CATALYSIS IN BROMOPEROXIDATION REACTION

Peroxidative bromination of phenol red to its tetrabromo derivative, b romophenol blue, required vanadate in addition to H2O2 when carried ou t in the pH range of 5-7. Excess H2O2, with ratio of H2O2:vanadate of 2:1 and above, prevented the reaction. Diperoxovanadate, known to be f ormed in such reaction mixtures, was ineffective by itself and needed uncomplexed vanadate (V-v) or vanadyl (V-iv) to support bromination. B romide-assisted reduction of the excess vanadate to vanadyl appeared t o be an essential secondary reaction. In the absence of phenol red oxy gen was released, and concomitantly bromide was oxidized to a form com petent to brominate phenol red added after termination of oxygen relea se. These findings indicated participation of reactions leading to an intermediate derived from vanadyl and diperoxovanadate, previously des cribed from this laboratory (Arch. Biochem. Biophys. 316, 319-326, 199 5). Continuous bromination of phenol red occurred when glucose oxidase -glucose system was used as a source of continuous flow of H2O2. A sch eme of reactions involving peroxovanadates (mono-, di-, mu-, and bromo -) is proposed for the formation and utilization of an active brominat ing species and for the recycling of the product, mono-peroxovanadate, by H2O2, which explains the catalytic role of vanadium in the bromope roxidation reaction. (C) 1996 Academic Press, Inc.