gamma-Radiolysis and pulse radiolysis of aqueous 4-chloroanisole

Using pulse radiolysis and steady state gamma-radiolysis in combination with product analysis by HPLC the radiolytic degradation mechanism of 4-chloroanisole (4-CIAn) has been elucidated. Pulse radiolysis experiments show that OH radicals react in neutral aqueous N2O saturated solutions with 4-CIAn by addition to all aromatic ring positions to yield hydroxycyclohexadlenyl radicals (OH-adducts), k (OH + 4-ClAn) = 6.5 x 10(9) dm(3) mol(-1) s(-1). Those OH-adducts formed on ipso positions of the molecule subsequently undergo HCl or CH3OH elimination forming methoxyphenoxyl- and chlorophenoxyl radicals. Their yield corresponds to similar 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, 2k = 1 x 10(9) dm(3) mol(-1) s(-1). The presence of oxygen leads to its addition on the hydroxpcyclohexadienyl radicals, k(OH-adduct + O-2) = 3.2 x 10(8) dm(3) mol(-1) s(-1). In airfree solution the reaction of H-atom with the substrate, k(H + 4-ClAn) = 1.2 x 10(9) dm(3) mol(-1) s(-1), results in H-adducts which decay in bimolecular reactions, 2k = 8.2 x 10(8) dm(2) mol(-1) s(-1). The rate constant for the reaction of the solvated electrons has been determined to k (e(aq)(-) + 4-CIAn) = 2 x 10(9) dm(2) mol(-1) s(-1). The absorption spectra of H- and OH-adducts were measured in the range of 280-450 nm. The products analysed by HPLC after T-radiolysis in dependence of dose (100-600 Gy) are given for N2O-, 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 are present, additionally anisole could be detected. Under both reaction conditions the amount of identified products is about 20% of decomposed 4-CIAn. The reaction of e(eq)(-) 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 similar to 50%. The efficient dechlorination (similar to 66% of the decomposed 4-CIAn) 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. Copyright (C) 1996 Elsevier Science Ltd.