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

This work presents ferrate(VI) (Fe^VIO₄^2-, Fe^VI) 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 Fe^VI and SAs at different pH (6.5-10.0) give species-specific second-order rate constants, k₅ and k₆ of the reactions of protonated Fe^VI (HFeO₄^-) and unprotonated Fe^VI (Fe^VIO₄^2-) 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 Fe^VI 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 Fe^VI 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 Fe^VI. 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 Fe^VI.