Structure–Activity Relationships for Rates of Aromatic Amine Oxidation by Manganese Dioxide

Abstract

New energetic compounds are designed to minimize their potential environmental impacts, which includes their transformation and the fate and effects of their transformation products. The nitro groups of energetic compounds are readily reduced to amines, and the resulting aromatic amines are subject to oxidation and coupling reactions. Manganese dioxide (MnO2) is a common environmental oxidant and model system for kinetic studies of aromatic amine oxidation. In this study, a training set of new and previously reported kinetic data for the oxidation of model and energetic-derived aromatic amines was assembled and subjected to correlation analysis against descriptor variables that ranged from general purpose [Hammett σ constants (σ(-)), pKas of the amines, and energies of the highest occupied molecular orbital (EHOMO)] to specific for the likely rate-limiting step [one-electron oxidation potentials (Eox)]. The selection of calculated descriptors (pKa, EHOMO, and Eox) was based on validation with experimental data. All of the correlations gave satisfactory quantitative structure-activity relationships (QSARs), but they improved with the specificity of the descriptor. The scope of correlation analysis was extended beyond MnO2 to include literature data on aromatic amine oxidation by other environmentally relevant oxidants (ozone, chlorine dioxide, and phosphate and carbonate radicals) by correlating relative rate constants (normalized to 4-chloroaniline) to EHOMO (calculated with a modest level of theory).

Oxidation potentials of phenols and anilines: correlation analysis of electrochemical and theoretical values

“…In a recent study, we showed that correlations based on k rel for aniline oxidation by MnO 2 were sufficiently comparable to justify tting QSARs using kinetic data from multiple sources. 8 In Fig. 1A, all of these dataincluding both phenols and anilines-are summarized, showing that the combined dataset for k rel correlate sufficiently well to E 1/2 from Suatoni et al to t a single QSAR.…”

“…In previous work, 8 we compared the performance of several electronic structure methods (functionals, basis sets, and solvation models) for computation of one-electron oxidation potentials for aromatic amines (E 1 ) from chemical structure theory, and a selection of those methods was used in this study, with minor modications. Only oxidation of the neutral form of the parent compounds was considered (ArOH # ArOH + + e À and ArNH 2 # ArNH 2 + + e À ).…”

“…Third, to provide an avenue for extending the coverage of substituent combinations in future work, we chose moderately-high, but accessible levels of theory, so calculations could be done for many compounds without special accommodations (such as for the larger or more exible compounds). The range of computational conditions used was chosen to include those that proved most useful in our recent work, 8 included one basis set (6- 311++G(2d,2p)), two functionals (B3LYP and M062S) and two solvation models (COSMO and COSMO-SMD). The newly calculated values of E 1 are given in Table S6 † (phenols) and Table S7 † (anilines).…”

“…The results of this calculation are shown in Fig. 1B for selected sets of E 1 calculated by Winget et al [17][18][19] and Salter-Blanc et al 8 A linear regression (not shown) performed on all the data in this correlation does not differ signicantly from the 1 : 1 line included in the gure, but the residuals are highly variable, within as well as between compounds, and therefore hard to rationalize as due to any one particular source of error.…”

Biotransformation and photolysis of 2,4-dinitroanisole, 3-nitro-1,2,4-triazol-5-one, and nitroguanidine

“…70 DNAN reduction products can be oxidized by manganese oxides and DNAN can be reduced by various iron redox systems. [71][72][73] NTO Electrochemical and Fenton's oxidation of NTO yields complete mineralization. 74,75 In terms of environmental mobility, NTO was shown to weakly sorb to soil slurries, as expected from the high solubility.…”