AROMATIC HYDROXYLATION CATALYZED BY FENTONS REAGENT . AN ELECTRON PARAMAGNETIC RESONANCE STUDY .I. FURANS

Unstable intermediate free radicals of furan, 2-furoic acid, 2-furfural, and 2-furfuryl alcohol, produced by Fenton's reagent (Fe(II) + H2O2), have been detected by electron paramagnetic resonance. The observed hyperfine structures were analyzed and molecular structures were assigned on the basis of Huckel MO calculations. Most of the free radicals were hydroxylated adducts: at position 5 of furan, 2-furfural, and 2-furfuryl alcohol, and at position 4 of furoic acid. The hydroxylation occurred at the position possessing minimal localization energy for radical attack and maximal free valence. A Ti(III) + H2O2 system attacked the same position of furan derivatives as Fenton's reagent, giving the same adducts, while the reagents abstract a hydrogen from different sites in the case of saturated hydrocarbons. The reactive species involved in these oxidation systems is therefore expected to be free radical in nature.