ARTICLES YOU MAY BE INTERESTED INAbstract. We evaluated and compared the hydroxyl radical-generating ability of chromium, iron, cobalt, nickel, copper, lanthanum, and cerium ions when reacted with electrogenerated hydrogen peroxide by usage the voltammetry. There was reproduced an electro-Fenton-like reaction in electrochemical cell when metal salt added. The nature of metal ions determined the appearance and increasing of current of hydroxyl radicals cathodic reduction due to individual rate of the redox cycling of metal ions with ongoing cathodic regeneration of Fenton-active metal ions oxidation state. The approximated current magnitude growth in relation to metal ions concentration was calculated for each tested metal. It expressed in μA·ml·μmol −1 and is equal to HO • -generating ability of metal ions. Results are usable for improvement of advanced oxidation processes (AOPs) through enhancement of exploitation of electro-Fenton-like processes with selecting metals that are more active. Besides, in accordance with concept of metal-induced hydroxyl radical stress in biological systems, the hydroxyl radical-generating ability of metal ions is equal to their pro-oxidant capacity.

We observed for the first time the reaction of oxidation of ferric xylenol orange chelates by hydrogen peroxide in aqueous solution. The reaction is accompanied with decoloration of the violet aqueous solution. Based on generally accepted conception, there is a process of free radical chain oxidation of indicator molecule in the solution. However, after investigating the final colorless solution by 1H NMR-spectroscopy we found the modified but not broken structure in which the initial hydrocarbon core remained mainly unchanged. We concluded that kind of reaction was an oxyfunctionalization by hydrogen peroxide versus free radical chain destruction. We argued steps of the reaction such as N-oxidation, Cope’s elimination, and certain rearrangements with possible products oligomerization. There was a need to explain the mechanism of interaction between the ferric iron ion and the hydrogen peroxide molecule and to argue the nature of intermediate reactive oxygen species. There is similarity between the ferric-catalyzed hydroperoxide xylenol orange oxidation and the peroxygenase-catalyzed biochemical oxyfunctionalization reactions. However, based on literature data and molecular orbital modeling, we proposed another mechanism of interaction between the ferric iron ion and the hydrogen peroxide molecule instead the tetravalent iron generation. Concretely, we proposed the hydrogen peroxide zwitter-ionization (isomerization to oxywater molecule) and subsequent intramolecular disproportionation with generation of a water molecule and a singlet oxygen atom as a reactive oxygen species. In this view, the iron ion oxidation state is unchanged during the reaction and remains ferric. An oxyfunctionalization of any organic substrate by hydrogen peroxide in the presence of ferric iron ions is promising approach in organic synthesis. However, the usage of organic ligands for ferric iron ions as components of catalysts is limited and requires only non-oxidizable compounds. On the other hand, one can choose an oxidation substrate as a ligand for ferric iron ions that is the formation of chelate complex of ferric catalyst with an organic substrate.Forcitation:Chumakov A.A., Kotelnikov O.A., Slizhov Yu.G. Oxidation of ferric xylenol orange chelates by hydrogen peroxide in aqueous solution: conception of oxygen singlet atoms generation from hydrogen peroxide. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2018. V. 61. N 2. P. 15-22

In the system, which combines the Fenton reaction or similar (of iron, cobalt, nickel

Hydrogen peroxide is widely used as an oxidant. The results of thermodynamic calculations indicate the impossibility of spontaneous generation of hydroxyl and hydroperoxyl radicals from hydrogen peroxide in aqueous solutions. Hydrogen peroxide spontaneously decomposes in ferrous, ferric, and cupric Fenton reaction systems. Ferric xylenol orange and ferric pyridoxine complexes are oxidized rapidly and spontaneously by this oxidant. Hydrogen peroxide in aqueous solutions spontaneously oxidizes the sulfur atoms of hyposulfite anions and benzylpenicillin molecules. Thus, a hydrogen peroxide molecule generates another intermediate that differs from hydroxyl and hydroperoxyl radicals. Theoretical modeling shows that hydrogen peroxide can participate in the proton transfer reactions. Its isomerization to oxywater zwitterion with subsequent oxywater intramolecular disproportionation is a process that is very suitable for explaining all events of hydrogen peroxide decomposition and oxidative reactivity in aqueous systems. The oxywater zwitterion is a bipolar ion in which the opposite charges are localized on neighboring oxygen atoms. This determines the displacement of electron density from the negatively charged atom to the positively charged atom. As a result, the interoxygen bond heterolytically dissociates with liberation of a water molecule and formation of an oxygen atom (oxene) in a singlet quantum state. This atom has a vacant atomic orbital. The S-oxidation of benzylpenicillin and hyposulfite occurs via targeting of electron pairs of the sulfur atoms by the vacant atomic orbitals of the singlet oxygen atoms. We substantiate an oxenemediated pathway of hydrogen peroxide disproportionation. A singlet oxygen atom interacts with a second hydrogen peroxide molecule through targeting the unshared electron pair of the oxygen atom by a vacant atomic orbital. The process may be called O-oxidation of hydrogen peroxide; it results in trioxidane (dihydrogen trioxide) formation. Hydrogen trioxide rapidly decomposes and produces water and singlet dioxygen. We have suggested a mechanism of the electron spin rotation during the singlet dioxygen quenching into the triplet quantum state. We have assumed the formation of a dimeric associate from singlet dioxygen antipodes by orbital parameter. Two simultaneous redox reactions (the electron exchange interaction) result in generation of two triplet dioxygen molecules. The first triplet molecule has +1 total electron spin, and the second one has-1 total electron spin. For Fenton reaction systems, the zwitterionization of hydrogen peroxide in Lewis acid-base complexes with metal ions is followed by intramolecular disproportionation of oxywater. The singlet oxene remains in complex with a metal ion. Ferrous iron ion changes its oxidation state to ferric due to rapid and inevitable oneelectron transfer within the iron(II)-oxene complex. The ferric-oxyl complex is known as alpha-oxygen complex. In our opinion, the classic Fenton reaction occurs through alpha-complex formation. We maintain...

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