Electrophilic reaction of the OH radical with phenol: determination of the distribution of isomeric dihydroxycyclohexadienyl radicals

Using pulse radiolysis with optical detection and high-pressure liquid chromatography techniques it is shown that the isomeric dihydroxycyclohexadienyl radicals formed in aqueous solution by reaction of OH with phenol react with quinones by electron transfer to yield semiquinones and dihydroxybenzenes. The rate constants for these electron-transfer reactions depend on the redox potential of the quinone and on the isomer structure of the dihydroxycyclohexadienyl radical; i.e., the rate constants are large (107-109M-1s-1) in the case of the ortho and para isomers and small (less than or equal to 107M-1s-1) in the case of the meta and ipso isomers. The isomeric dihydroxycyclohexadienyl radicals differ also with respect to their rates of H+-catalyzed dehydration to yield phenoxyl radical. It is thus possible to distinguish kinetically between the different dihydroxycyclohexadienyl radicals. On this basis and from the product analysis data the fractions of OH attack at the ipso, ortho, meta, and para position of phenol are 0.08, 0.48, 0.08, and 0.36, respectively. These data show the strong preference of the electrophilic OH radical for attack at the ring positions activated by the phenolic OH group