Second-order combination reactions of phenoxyl radicals

Abstract

Phenoxyl radicals, when produced pulse radiolytically at concentrations > 10{sup {minus}4} M, combine in second-order processes to give 2,2{prime}-, 2,4{prime}-, and 4,4{prime}-dihydroxybiphenyl as the predominant products. The ratios of these products observed under a variety of conditions, 0.73:1.73:1.00, indicate that radical combination at the ortho and para positions of the ring occurs statistically with, however, reaction at the para site favored by a factor of 2.4 over that at the ortho site. This ratio is considerably greater than the ratio of 1.55 of the unpaired spin populations at the two sites as indicated by ESR data. There is essentially no reaction at the meta position where the unpaired spin population is negative. Approximately 10% of the radicals couple at oxygen to yield 2- and 4-phenoxyphenol. The yields of these two products are, however, considerably lower than might be expected from the unpaired spin population of 0.2 on the oxygen atom of phenoxyl. It is clear that factors in addition to the unpaired spin population are important in controlling the combination processes. Under optimum conditions these five products account for {approx}90% of the phenoxyl radicals initially produced so that electronic disproportionation or coupling to form diphenyl peroxide is relatively unimportant. Multiple-pulsemore » experiments show that after products build up to concentrations comparable to that of the phenoxyl radical, secondary electron-transfer reactions lead to more complex products. In studies with {gamma}-rays at low-dose rates phenoxyl radicals predominantly react with the products so that the yields of simple combination products are very low.« less