Reduction of Nitrosobenzenes and N-Hydroxylanilines by Fe(II) Species: Elucidation of the Reaction Mechanism

Abstract

Although there has been a substantial effort toward understanding the reduction of nitroaromatics in Fe(II)-treated ferric oxide systems, little has been done to gain insight into the factors controlling the transformation of their reaction intermediates, nitrosobenzenes and N-hydroxylanilines, in such systems. Nitrosobenzenes, the first intermediates, were reduced by Fe(II) solutions as well as by Fe(II)-treated goethite suspensions at pH 6.6. Experimental observations indicate a reactivitytrend in which the presence of electron-withdrawing groups in the para position increased the rate of reduction of the nitrosobenzenes. N-Hydroxylanilines, the second intermediates, were reduced in Fe(II)-treated goethite suspensions but were not reduced by Fe(II)aq. Their reactivity trend indicates that electron-withdrawing groups in the para position decreased their rate of reduction. The bond dissociation enthalpy of the N-O linkage was the most useful molecular descriptor for predicting the rates of reduction of N-hydroxylanilines in Fe(II)-treated goethite suspensions, suggesting that the cleavage of the N-O bond is the rate-determining step for reduction. The rate of reduction of p-cyano-N-hydroxylaniline showed a linear relationship against the concentration of surface-associated Fe(II) in hematite, goethite, and lepidocrocite suspensions, while having a relatively low sensitivity toward changes in pH within the near-neutral range in hematite suspensions.