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

This work presents ferrate(VI) ((FeO42-)-O-VI, 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, k(5) and k(6) of the reactions of protonated Fe-VI (HFeO4-) and unprotonated Fe-VI ((FeO42-)-O-VI) with protonated SAs (FIX), respectively. The values of k5 varied from (1.2 +/- 0.1) x 10(3) to (2.2 +/- 0.2) x 10(4) M-1 s(-1) while the range of k(6) was from (1.1 +/- 0.1) x 10(2) to (1.0 +/- 0.1) x 10(3) 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.