COVALENT BINDING OF ANILINE TO HUMIC SUBSTANCES .2. N-15 NMR-STUDIES OF NUCLEOPHILIC-ADDITION REACTIONS

Aromatic amines are known to undergo covalent binding with humic subst ances in the environment. Although previous studies have examined reac tion conditions and proposed mechanisms, there has been no direct spec troscopic evidence for the covalent binding of the amines to the funct ional groups in humic substances, In order to further elucidate the re action mechanisms, the Suwannee River and IHSS soil fulvic and humic a cids were reacted with N-15-labeled aniline at pH 6 and analyzed using N-15 NMR spectrometry. Aniline underwent nucleophilic addition reacti ons with the quinone and other carbonyl groups in the samples and beca me incorporated in the form of anilinohydroquinone, anilinoquinone, an ilide, imine, and heterocyclic nitrogen, the latter comprising 50% or more of the bound amine. The anilide and anilinohydroquinone nitrogens were determined to be susceptible to chemical exchange by ammonia. In the case of Suwannee River fulvic acid, reaction under anoxic conditi ons and pretreatment with sodium borohydride or hydroxylamine prior to reaction under oxic conditions resulted in a decrease in the proporti on of anilinohydroquinone nitrogen incorporated. The relative decrease in the incorporation of anilinohydroquinone nitrogen with respect to anilinoquinone nitrogen under anoxic conditions suggested that inter- or intramolecular redox reactions accompanied the nucleophilic additio n reactions.