Analyzing sites of OH radical attack (ring vs. side chain) in oxidation of substituted benzenes via dual stable isotope analysis (δ13C and δ2H)

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 (epsilon(C) and epsilon(H)) of various substituted benzenes. For benzene and halobenzenes that only undergo ring addition, low values of epsilon(C) (-0.7 parts per thousand to -1.0 parts per thousand) were observed compared with theoretical values (-7.2 parts per thousand to -8 parts per thousand), 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 epsilon(C) (-3.9 parts per thousand), probably due to the lower reactivity. Xylene isomers, anisole, aniline, N,N-dimethylaniline, and benzonitrile yielded normal epsilon(H) values (-2.8 parts per thousand to -29 parts per thousand) indicating the occurrence of side-chain reactions, in contrast to the inverse epsilon(H) (11.7 parts per thousand to 30 parts per thousand) observed for ring addition due to an sp(2) to sp(3) hybridization change at the reacting carbon. Inverse epsilon(H) values for toluene (14 parts per thousand) and ethylbenzene (30 parts per thousand) 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 (Delta delta H-2/Delta delta C-13) 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. (C) 2015 Elsevier B.V. All rights reserved.

Keywords

Isotope fractionation; OH radical reaction; Aromatic compounds; Competing mechanisms

Published in

Science of the Total Environment
2016, Volume: 542, number: Part A, pages: 484-494 Publisher: ELSEVIER SCIENCE BV

UKÄ Subject classification

Environmental Sciences
Theoretical Chemistry


Publication identifier

DOI: https://doi.org/10.1016/j.scitotenv.2015.10.075

Permanent link to this page (URI)

https://res.slu.se/id/publ/89949