Electron transfer from cresols to N-3(center dot), BrO2 center dot, ClO2 center dot, NO2 center dot and SO4-center dot radicals: correlation between rate constants and one-electron reduction potentials

The rate constants of oxidation of phenol and the cresol isomers to phenoxy l or methylphenoxyl radicals by inorganic radicals (R-.) were studied in aq ueous solutions at pH = 5.8 and 11.5 using pulse radiolysis. The oxidation was due to electron transfer. Using the Marcus theory the electron transfer data were evaluated on the basis of the energy difference, Delta E, betwee n the one-electron reduction potential of the inorganic couple and the mid- point potential of the half-cell of the phenoxyl/phenolate(phenol) couple. The standard reduction potentials of (o-CH3PhO./o-CH3PhO-) and (m-CH3PhO./m -CH3PhO-) couples (vs. NHE) were determined as 760 and 800 mV, respectively . The electron transfer was found to be diffusion limited when Delta E grea ter than or equal to 500 mV independently of the inorganic radical, type of phenol or pH. Between 0 less than or equal to Delta E less than or equal t o 500 mV the rate constants showed good correlation with Delta E. The rate constants were dependent on the isomeric position of the methyl substituent . (C) 1999 Elsevier Science Ltd. All rights reserved.