Sediment-associated reactions of aromatic amines.: 2.: QSAR development

The fate of aromatic amines in soils and sediments is dominated by irreversible binding through nucleophilic addition and oxidative radical coupling. Despite the common occurrence of the aromatic amine functional group in organic chemicals, the molecular properties useful for predicting reaction kinetics in natural systems have not been thoroughly investigated, Toward this goal, the sorption kinetics for a series of anilines with substituents in the ortho, meta, or para positions were measured in sediment slurries. The sorption kinetics of the substituted anilines were characterized by an initial, rapid sorption process followed by a much slower sorption process. The initial rates of sorption varied with the type and position of the substituent group. Rate constants for the initial sorption process were correlated with molecular descriptors, including dissociation constants (pK(a)'s), Hammett sigma constants, polarographic half-wave potentials (E-1/2), one-electron oxidation potentials (E-1), highest occupied molecular orbital (HOMO) energies (E-HOMO), and ionization energies (E-IE). On the basis of the strength of linear correlations and the availability of data, dissociation constants and Hammett a constants appear to be the most useful molecular descriptors for predicting reaction rates of substituted anilines in the sediment slurries. The slow rates of sorption were much less sensitive to substituents effects than the rate constants for the faster sorption process, suggesting that the slower process was not controlled by the rate of electron transfer (i.e., nucleophilic addition or radical formation) but was limited by the availability of covalent binding sites.