γ-radiolysis and pulse radiolysis of aqueous 4-chloroanisole

Abstract

Using pulse radiolysis and steady state γ-radiolysis in combination with product analysis by HPLC the radiolytic degradation mechanism of 4-chloroanisole (4-ClAn) has been elucidated. Pulse radiolysis experiments show that OH radicals react in neutral aqueous N 2O saturated solutions with 4-ClAn by addition to all aromatic ring positions to yield hydroxycyclohexadienyl radicals (OH-adducts), k(OH + 4-ClAn) = 6.5 × 10 9dm 3mol −1s −1. Those OH-adducts formed on ipso positions of the molecule subsequently undergo HCl or CH 3OH elimination forming methoxyphenoxyl- and chlorophenoxyl radicals. Their yield corresponds to ∼ 20% of the OH-radicals, whereby the distribution is roughly 3:1 in favor of the methoxyphenoxyl radicals, which reflects the stronger ortho-, para-directing activity of the methoxy group. The OH-adducts decay second order, 2 k = 1 × 10 9dm 3mol −1s −1. The presence of oxygen leads to its addition on the hydroxycyclohexadienyl radicals, k(OH-adduct + O 2) = 3.2 × 10 8dm 3mol −1s −1. In airfree solution the reaction of H-atom with the substrate, k(H + 4-ClAn) = 1.2 × 10 9dm 3mol −1s −1, results in H-adducts which decay in bimolecular reactions, 2 k = 8.2 × 10 8dm 3mol −1s −1. The rate constant for the reaction of the solvated electrons has been determined to k(e aq − + 4-ClAn) = 2 × 10 9dm 3mol −1s −1. The absorption spectra of H- and OH-adducts were measured in the range of 280–450 nm. The products analysed by HPLC after γ-radiolysis in dependence of dose (100–600 Gy) are given for N 2O-, air-, oxygen- and argon saturated neutral aqueous solutions. In conditions favoring the OH radical oxidation 4-chlorophenol, 4-methoxyphenol, 5-chloro-2-methoxyphenol and 2-chloro-5-methoxyphenol were determined as final products. In the presence of Ar, where about equal amounts of OH and e aq − are present, additionally anisole could be detected. Under both reaction conditions the amount of identified products is about 20% of decomposed 4-ClAn. The reaction of e aq − leads to reductive dechlorination which corresponds quantitatively to the degradation of the substrate. In the presence of air or solutions saturated with pure oxygen predominantly hydroquinone, 4-chlorophenol and muconic acids are formed and the material balance is ∼50%. The efficient dechlorination (∼60% of the decomposed 4-ClAn) as well as ring fragmentation products as intermediates en route to complete mineralization in oxygenated solution indicate that high energy radiation is a promising method for degradation of halogenated aromatic compounds in water. Variation of dose rates from 79 Gy min −1 to 266 Gy min −1 did not show any influence on the product distribution.