Reaction of OH radicals with nitrophenols in aqueous solution

Abstract

The reaction of OH radicals with 2-, 3- and 4-nitrophenol in aqueous solution was investigated using pulse radiolysis with conductivity detection, steady state radiolysis, e.s.r. and product analysis. The yield of nitrite was measured and the formation of semiquinone radicals from 2- and 4-nitrophenol but not from 3-nitrophenol was confirmed. The yield of nitrite from 2- and 4-nitrophenol is less by a factor of two than from the corresponding phenolates and that of nitrite from 3-phenolate is eight times lower than that from 4-phenolate. These and further results are interpreted in terms of preferential addition of the electrophilic OH radical to ring positions activated by the ortho-para directing OH or O - groups. OH addition leads predominantly to the 1,2-dihydroxy-4-nitrocylohexadienyl radical and to the 1,4-dihydroxy-4-nitrocyclohexadienyl radical. The latter eliminates NHO 2 ( k = 2.3 x10 3 s -1) to yield parasemiquinone radical. The 1.2-dihydroxy-4-nitro-cyclohexadienyl radical, the p K a of which is ≤4, does not observably eliminate OH - to yield 4-nitrophenoxyl radical since 2-hydroxy-4-nitrophenol is formed as product. It is further shown that the nitro-group stabilizes anion free radical states of benzene derivatives more strongly than the cyano-group. This effect also leads to a much smaller elimination rate of OH - from OH adducts of nitrophenolate as compared to those from OH adducts of phenolates containing other electron withdrawing groups (-CN, -CHO, -COCH 3).