Modeling the reactions of 1‐naphthylamine and 4‐methylaniline with humic acids: Spectroscopic investigations of the covalent linkages

Abstract

The covalent binding of two aromatic amines, 1‐naphthylamine and 4‐methylaniline, to substituted quinones in aqueous solutions has been studied as a model for the covalent binding of amines to humic acids. An inline compound was the only type of product identified for the reactions between 1‐naphthylamine and selected quinones. Reactions of 4‐methylaniline with the same quinones gave a 1,4‐ addition product in addition to the imine compound, although only the imine product was formed when the quinone had bulky substituents at the 2‐ and 6‐ positions. These results provide additional insight into the reactions of aromatic amines with humic acid quinones.

Covalent Binding of Aniline to Humic Substances

“…15 N NMR chemical shifts of these and other classes of nitrogen compounds relevant to this study are presented in Figure 1. The phenoloxidase enzymes (peroxidases, tyrosinases, and laccases) are known to occur extracellularly in soils (16,17), and have been considered to play a role in catalyzing the coupling of aromatic amines to humic substances (3).…”

“…Since no analyses of the spin dynamics were performed in this study, the solid state spectra presented in this manuscript will be interpreted only semiquantitatively. 15 N NMR Spectra of Unreacted Fulvic and Humic Acids. The naturally occurring nitrogens in the fulvic and humic acids were examined both to identify functional groups which might be involved in the reactions with aniline, and to determine the potential for overlap between the naturally occurring nitrogens and the labelled aniline nitrogens incorporated into the samples.…”

“…Acquisition parameters included an 18,656.7 Hz spectral window (613.7 ppm), 0.5-s acquisition time, 45° pulse angle, 2.0-s pulse delay, and τ delay of 0.1 ms. Neat formamide in a 5 mm NMR tube, assumed to be 112.4 ppm, was used as an external reference standard for all spectra. 15 N NMR chemical shifts are reported in ppm downfield of ammonia, taken as 0.0 ppm.…”

“…Quantitative 13 C NMR spectra of the unreacted fulvic and humic acid samples were recorded in DMSO-d 6 , 99.9 atom % 12 C, as previously described (23). INEPT (24) and ACOUSTIC (25) 15 N NMR spectra were recorded on the aniline-reacted fulvic and humic acids. Refocussed INEPT (proton decoupled) spectra were recorded as previously described (9).…”

“…In the case of sterically hindered quinones in aqueous solution, however, 1,2-addition by aniline is more favorable, and in some instances becomes the dominant mode of attack. As an illustration, 4-methylaniline was reported to undergo both 1,4-and 1,2-addition to 2,6-dimethyl-p-benzoquinone, resulting in a 3:1 product ratio of anilinoquinone to imine (15).…”