Oxidation of Sulfonamide Antibiotics of Six-Membered Heterocyclic Moiety by Ferrate(VI): Kinetics and Mechanistic Insight into SO₂ Extrusion

Abstract

This work presents ferrate(VI) (FeVIO42–, FeVI) oxidation of a wide range of sulfonamide antibiotics (SAs) containing five- and six-membered heterocyclic moieties (R) in their molecular structures. Kinetics measurements of the reactions between FeVI and SAs at different pH (6.5–10.0) give species-specific second-order rate constants, k5 and k6 of the reactions of protonated FeVI (HFeO4–) and unprotonated FeVI (FeVIO42–) with protonated SAs (HX), respectively. The values of k5 varied from (1.2 ± 0.1) × 103 to (2.2 ± 0.2) × 104 M–1 s–1, while the range of k6 was from (1.1 ± 0.1) × 102 to (1.0 ± 0.1) × 103 M–1 s–1 for different SAs. The transformation products of reaction between FeVI and sulfadiazine (SDZ, contains a six-membered R) include SO2 extrusion oxidized products (OPs) and aniline hydroxylated products. Comparatively, oxidation of sulfisoxazole (SIZ, a five-membered R) by FeVI has OPs that have no SO2 extrusion in their structures. Density functional theory calculations are performed to demonstrate SO2 extrusion in oxidation of SDZ by FeVI. The detailed mechanisms of oxidation are proposed to describe the differences in the oxidation of six- and five-membered heterocyclic moieties (R) containing SAs (i.e., SDZ versus SIZ) by FeVI.