Controlling Factors in the Rates of Oxidation of Anilines and Phenols by Triplet Methylene Blue in Aqueous Solution

Abstract

Anilines and phenols are structurally similar compound classes that both are susceptible to oxidation by excited state triplet sensitizers but undergo oxidation by different mechanisms. To gain an understanding of the factors that control the rate of oxidation of anilines and phenols by triplet excited states, a kinetic study was performed on the oxidation of substituted anilines and phenols by methylene blue. The rate constants of one-electron transfer from anilines to triplet state methylene blue and their dependence on the reaction free energy are well fit to a Sandros-Boltzmann model. The observed rate constants are also well modeled when aniline oxidation potentials derived computationally are used. For phenols, the proton-coupled electron transfer rate constants were found to correlate primarily with O-H bond dissociation free energy and secondarily with phenol pKa. Rate constants for phenols could be modeled using computed bond dissociation free energies. These results provide a basis for predicting aniline and phenol oxidation rates, which could be valuable, for example, in assessing the likely persistence and fate of aniline- and phenol-based aqueous environmental pollutants.

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

“…Solvation energies for the parent and oxidized compounds were approximated using both the COSMO and COSMO-SMD methods. Other recent studies have performed similar calculations, 17,18,20,[22][23][24][25][26] and the calculations here, which make use of large triple zeta basis sets, are expected to be well converged. All of the calculations were done using NWChem.…”

“…1B, should have signicant disadvantages. 21,22 This has led recent studies to calibrate E 1 's using potentials measured by methods such as pulse radiolysis, [22][23][24][25][26] which should provide a more accurate estimate of potentials for reversible, one-electron oxidation of phenols/anilines. 27 However, these data are less common, more complex to measure, and not necessarily more closely matched to the processes that are controlling solution-phase oxidation kinetics.…”

Laser flash photolysis study of the photoinduced oxidation of 4-(dimethylamino)benzonitrile (DMABN)

“…[10][11][12][13] Upon photoirradiation, the chromophoric components of DOM are promoted to their excited singlet states and subsequently 48 excited triplet states ( 3 DOM*), which are likely to be the key reactive intermediates in the photosensitized transformation of aromatic amines in surface waters. 8,14,15 The initial reaction step in the photosensitized transformation is assumed to be an electron transfer from an aromatic amine to 3 DOM*. 16 This can be inferred from the abundant studies on the photoreduction of excited triplet 52 carbonyls by amines, 17 arguments on the energetics of the redox reactions between substituted anilines and excited triplet states of aromatic ketones in aqueous solution, 14 and recent results on the quenching of the excited triplet state of methylene blue by substituted anilines.…”

“…16 This can be inferred from the abundant studies on the photoreduction of excited triplet 52 carbonyls by amines, 17 arguments on the energetics of the redox reactions between substituted anilines and excited triplet states of aromatic ketones in aqueous solution, 14 and recent results on the quenching of the excited triplet state of methylene blue by substituted anilines. 15 Formation of a radical cation of the aromatic amine, a primary product of such an electron transfer reaction, is expected.…”

In silico environmental chemical science: properties and processes from statistical and computational modelling

“…The electrochemical measurements can be confounded by nonideal behaviour, such as irreversibility, which are not included in the theoretical calculations, so there is a mismatch between these two variables that might result in less accurate calibrations. 116,117 Alternatively, E 1 measured by pulse radiolysis, 114,115,[117][118][119] is a better estimate of reversible redox potentials, and therefore is better matched to potentials calculated from computational chemistry. However, E 1 from pulse radiolysis is not necessarily more closely matched to the processes that are controlling solution-phase oxidation kinetics, so they may not provide the most useful, or even the most accurate, structure-activity relationships for oxidation reactions of environmental interest.…”