A Theoretical and Experimental Investigation on the Oxidation of Gallic Acid by Sulfate Radical Anions

PAULA CAREGNATO; PEDRO M. DAVID GARA; GABRIELA N. BOSIO; MÓNICA C. GONZALEZ; NINO RUSSO; MARÍA DEL CARMEN MICHELINI; DANIEL O. MÁRTIRE

JOURNAL OF PHYSICAL CHEMISTRY A

ACS

Año: 2007 vol. 112 p. 1184 - 1188

1089-5639

By monitoring the decay of SO4.- after flash photolysis of aqueous solutions of S2O82- at different pH values, the kinetics of the reaction of SO4.- radicals with gallic acid and the gallate ion was investigated. The bimolecular rate constants for the reactions of the sulfate radicals with gallic acid and the gallate ion were found to be (6.3 ± 0.7) ´ 108 and (2.9 ± 0.2) ´ 109 M-1s-1, respectively. Based on the oxygen-independent second order decay kinetics and on their absorption spectra, the organic radicals formed as intermediates of these reactions were assigned to the corresponding phenoxyl radicals. DFT calculations in the gas phase and in aqueous solution support the formation of the phenoxyl radicals by H-abstraction from the phenols to the sulfate radical anion. The observed recombination of the phenoxyl radicals of gallic acid to yield substituted biphenyls and quinones is also supported by the calculations. HPLC/ MS product analysis showed formation of one of the predicted quinones.