title: Analyzing sites of OH radical attack (ring vs. side chain) in oxidation of substituted benzenes via dual stable isotope analysis (δ(13)C and δ(2)H).
abstract: OH radicals generated by the photolysis of H2O2 can degrade aromatic contaminants by either attacking the aromatic ring to form phenolic products or oxidizing the substituent. We characterized these competing pathways by analyzing the carbon and hydrogen isotope fractionation (εC and εH) of various substituted benzenes. For benzene and halobenzenes that only undergo ring addition, low values of εC (-0.7‰ to -1.0‰) were observed compared with theoretical values (-7.2‰ to -8‰), possibly owing to masking effect caused by pre-equilibrium between the substrate and OH radical preceding the rate-limiting step. In contrast, the addition of OH radicals to nitrobenzene ring showed a higher εC (-3.9‰), probably due to the lower reactivity. Xylene isomers, anisole, aniline, N,N-dimethylaniline, and benzonitrile yielded normal εH values (-2.8‰ to -29‰) indicating the occurrence of side-chain reactions, in contrast to the inverse εH (11.7‰ to 30‰) observed for ring addition due to an sp(2) to sp(3) hybridization change at the reacting carbon. Inverse εH values for toluene (14‰) and ethylbenzene (30‰) were observed despite the formation of side-chain oxidation products, suggesting that ring addition has a larger contribution to isotope fractionation. Dual element isotope slopes (∆δ(2)H/∆δ(13)C) therefore allow identification of significant degradation pathways of aromatic compounds by photochemically induced OH radicals. Issues that should be addressed in future studies include quantitative determination of the contribution of each competing pathway to the observed isotope fractionation and characterization of physical processes preceding the reaction that could affect isotope fractionation.
authors.affiliation: Department of Isotope Biogeochemistry, Helmholtz Centre for Environmental Research-UFZ, Permoserstraße 15, 04318 Leipzig, Germany.
authors.forename: Ning
authors.initials: N
authors.lastname: Zhang
authors.affiliation: Institute of Applied Radiation Chemistry, Lodz University of Technology, Żeromskiego 116, 90-924 Łódź, Poland.
authors.forename: Inacrist
authors.initials: I
authors.lastname: Geronimo
authors.affiliation: Institute of Applied Radiation Chemistry, Lodz University of Technology, Żeromskiego 116, 90-924 Łódź, Poland.
authors.forename: Piotr
authors.initials: P
authors.lastname: Paneth
authors.affiliation: Department of Chemistry, Leibniz Institute for Tropospheric Research (TROPOS), Permoserstraße 15, 04318 Leipzig, Germany.
authors.forename: Janine
authors.initials: J
authors.lastname: Schindelka
authors.affiliation: Department of Chemistry, Leibniz Institute for Tropospheric Research (TROPOS), Permoserstraße 15, 04318 Leipzig, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Schaefer
authors.affiliation: Department of Chemistry, Leibniz Institute for Tropospheric Research (TROPOS), Permoserstraße 15, 04318 Leipzig, Germany.
authors.forename: Hartmut
authors.initials: H
authors.lastname: Herrmann
authors.affiliation: Department of Isotope Biogeochemistry, Helmholtz Centre for Environmental Research-UFZ, Permoserstraße 15, 04318 Leipzig, Germany.
authors.forename: Carsten
authors.initials: C
authors.lastname: Vogt
authors.affiliation: Department of Isotope Biogeochemistry, Helmholtz Centre for Environmental Research-UFZ, Permoserstraße 15, 04318 Leipzig, Germany. Electronic address: hans.richnow@ufz.de.
authors.forename: Hans H
authors.initials: HH
authors.lastname: Richnow
mesh_terms: D001554:Benzene / Q000737:chemistry*; D002247:Carbon Isotopes; D006861:Hydrogen Peroxide; D017665:Hydroxyl Radical / Q000737:chemistry*; D007700:Kinetics; D008956:Models, Chemical; D010084:Oxidation-Reduction; D010782:Photolysis
chemical_list: D002247:Carbon Isotopes; D017665:Hydroxyl Radical; D006861:Hydrogen Peroxide; D001554:Benzene
keywords: Aromatic compounds; Competing mechanisms; Isotope fractionation; OH radical reaction