Solvent hydrogen bonding and structural effects on the reaction of 2-halo-5-nitropyridines with parasubstituted anilines in dimethylsulfoxide/acetonitrile mixtures

Selective oxidation of aniline contaminants via a hydrogen-abstraction pathway by Bi2·15WO6 under visible light and alkaline conditions

Conversion of aniline wastes to value-added products is always a promising method to treat aniline wastewater. In this study, a selective oxidation of aniline contaminants by Bi_2·15WO₆ was carried out under visible light and alkaline conditions. Kinetic results show that the oxidation rates of aniline increase with increasing pH values under visible light. UV-vis absorption spectra and GC-MS analysis confirm that azobenzene is the primary oxidation product with aminophenol and N,N'-diphenylhydrazine as the secondary products. The analyses from Mott-Schottky, electrochemical impedance spectroscopy (EIS), transient photocurrent and photoluminescence (PL) further indicate that OH^- promotes the separation and transfer of photogenerated electron-hole pairs on the surface of Bi_2·15WO₆, thus facilitating oxidation of aniline. Quenching experiments and electron spin resonance (ESR) analysis confirm that h⁺ is the predominant specie in the Bi_2·15WO₆ system and aniline radical cation (PhNH₂^•+) is an important intermediate. The Hammett and ΔBDE_N-H plots further reveal that e^- abstraction from aniline with the formation of PhNH₂^•+, followed by H⁺ abstraction from PhNH₂^•+ with the formation of anilino radicals (PhNH^•), is the prerequisite for the formation of N,N'-diphenylhydrazine, which is then oxidized to azobenzene via the hydrogen-abstraction pathway. This work provides a cost-effective method to selectively oxidize aniline to azobenzene.

Electronic and solvent effects on kinetics of SNAr substitution reactions of substituted anilines with 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole in MeOH-Me2SO mixtures of varying composition: one reaction with two mechanistic pathways

ABSTRACT

The kinetics and mechanism of the aromatic nucleophilic substitution reactions of 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole with para-X-substituted anilines (X = OH, OMe, Me, H, F, I, Cl) were studied in MeOH-Me2SO mixtures and pure Me2SO at 25.0 °C. The second-order rate coefficients depend on the substitutent in aniline and give good Hammett and Brønsted correlations; a polar SNAr reaction is proposed for the reaction in different MeOH-Me2SO mixtures. The measured rate coefficients of the reaction demonstrated dramatic variations for aniline donor with the increasing dimethyl sulfoxide composition in MeOH-Me2SO mixtures. In this case, the Hammett and Brønsted plots are biphasic and concave upwards with a break point at 4-methylaniline. These results indicate a change in mechanism from the polar (SNAr) for less basic nucleophiles (X = 4-Cl, 4-I, 4-F, and H) to the single electron transfer (SET) for more basic nucleophiles (X = 4-OH, 4-OMe and 4-Me). The changes of the structure of the transitions states with substituents and solvent are in accordance with the results of kinetics studies. The solvation model described is well supported by the solvatochromism exhibited by aniline in the solvent mixture under investigation. These results provide an ideal framework for understanding the paramount importance of the specific molecular structure of solvent molecules in determining chemical reactivity versus solvent effects.

GRAPHICAL ABSTRACT