title: Aqueous oxidation of sulfonamide antibiotics: aromatic nucleophilic substitution of an aniline radical cation.
abstract: Sulfonamide antibiotics are an important class of organic micropollutants in the aquatic environment. For several, sulfur dioxide extrusion products have been previously reported upon photochemical or dark oxidation. Using quantum chemical modeling calculations and transient absorption spectroscopy, it is shown that single-electron oxidation from sulfadiazine produces the corresponding aniline radical cation. Density functional theory calculations indicate that this intermediate can exist in four protonation states. One species exhibits a low barrier for an intramolecular nucleophilic attack at the para position of the oxidized aniline ring, in which a pyrimidine nitrogen acts as a nucleophile. This attack can lead to a rearranged structure, which exhibits the same connectivity as the SO2 -extruded oxidation product that was previously observed in the aquatic environment and characterized by NMR spectroscopy. We report a detailed reaction mechanism for this intramolecular aromatic nucleophilic substitution, and we discuss the possibility of this reaction pathway for other sulfonamide drugs.
authors.affiliation: Environmental Chemistry Modeling Laboratory, Swiss Federal Institute of Technology Lausanne, 1015 Lausanne, Switzerland.
authors.forename: Peter R
authors.initials: PR
authors.lastname: Tentscher
authors.forename: Soren N
authors.initials: SN
authors.lastname: Eustis
authors.forename: Kristopher
authors.initials: K
authors.lastname: McNeill
authors.forename: J Samuel
authors.initials: JS
authors.lastname: Arey
mesh_terms: D000814:Aniline Compounds / Q000737:chemistry*; D000900:Anti-Bacterial Agents / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D004579:Electron Transport; D005609:Free Radicals / Q000737:chemistry*; D010084:Oxidation-Reduction; D013449:Sulfonamides / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry
chemical_list: D000814:Aniline Compounds; D000900:Anti-Bacterial Agents; D002412:Cations; D005609:Free Radicals; D013449:Sulfonamides; D014867:Water; C023650:aniline
keywords: electron transfer; nucleophilic substitution; radical ions; reaction mechanisms; sulfonamides