PULSE RADIOLYTIC STUDIES OF THE REACTION OF PENTAHALOPHENOLS WITH OH RADICALS - FORMATION OF PENTAHALOPHENOXYL, DIHYDROXYPENTAHALOCYCLOHEXADIENYL, AND SEMIQUINONE RADICALS

The optical and kinetic characteristics of the transients formed by the reaction of OH. and N3.radicals with pentabromo- (PBP-OH), pentachloro- (PCP-OH), and pentafluorophenol (PFP-OH) have been examined by pulse radiolysis techniques in buffered (pH 8, phosphate) aqueous media. The principal products from the reaction of OH. with PBP-O- and PCP-O- are the corresponding pentahalophenoxyl radicals {ca. 75% (PBP-O.) and ca. 77% (PCP-O.)} the OH.adducts dihydroxypentahalocyclohexadienyl radical anions, HO-PXP-O- (approximately 18% for X = Br, approximately 8% for X = Cl), and the semiquinone radicals bromanil (ca. 7%) and chloranil (ca. 15%). Their kinetics of formation via electron transfer and OH.addition have been assessed. Oxidation of PCP-O- and PBP-O- by N3.radicals also yields the following phenoxyl species: PBP-O., k(ET) = (6.1 +/- 0.9) x 10(9) M-1 s-1; PCP-O., k(ET) = (3.3 +/- 0.6) x 10(9) M-1 s-1. The reaction of OH. with PFP-O- anion yields exclusively the dihydroxypentafluorocyclohexadienyl radical, HO-PFP-O-, k(ADD) = (4.6 +/- 0.9) x 10(9) M-1 s-1. Solvated electrons react with PBP-O- (k(e) = 2.6 x 10(10) M-1 s-1) to give a highly reactive dianionic electron adduct, which following loss of Br- and reaction with H2O yields a mixture of isomeric tetrabromophenoxyl radicals. These observations find relevance in OH. radical photooxidative degradations of these and analogous organic contaminants in homogeneous and heterogeneous aqueous media.