OXIDATION OF ANILINE AND OTHER PRIMARY AROMATIC-AMINES BY MANGANESE-DIOXIDE

This investigation evaluated the redox reaction between a manganese dioxide, δ-MnO2, and anilines and other aromatic reductants in aqueous suspensions at pH values ranging from 3.7 to 6.5. The reaction with manganese dioxide may represent a pathway for transformation of aniline and other primary aromatic amines in acidic mineralogical and soil/water environments in the absence of oxygen and substantial microbial activity. The reaction rate with aniline is pH-dependent, increasing with decreasing pH, and first order with respect to δ-MnO2 and organic solute. Aniline and p-toluidine are demonstrated to be 2-equiv reductants, as is believed to be the case for the other aromatic solutes considered in this study, including the substituted anilines, and hydroquinone and catechol and their alkyl substituents. Ring-bound nitrogen-containing aromatic solutes (methylimidazole, quinoline, and 5,5-dimethylhydantoin) were unreactive with manganese dioxide at pH 6.4. The order of the reactivity of para-substituted anilines was methoxy ≫ methyl > chloro > carboxy ≫ nitro; the relative reactivity of these compounds correlated with the solute's half-wave potential and Hammett constant. The principal oxidation products of aniline and p-toluidine with manganese dioxide at pH 4 were azobenzene and 4,4′-dimethylazobenzene, respectively, which agreed with a postulated oxidative-coupling reaction mechanism. The abiotic redox reactions of primary aromatic amines and azo compounds may result in various respective oxidative-coupling and reductive-decoupling reactions. These processes may be significant with regard to the persistence and transformation of these classes of organic contaminants in environmental systems. © 1990, American Chemical Society. All rights reserved.