Home – Home – Jornals – Chemistry for Sustainable Development 2017 number 5

Fenton reaction systems are widely used for oxidative activation of hydroperoxides in monooxygen oxidative functionalisation of organic compounds i.e. the introduction of an oxygen atom into the composition of organic substrates. The nature of intermediates that are direct oxidants until now remains a subject of hypotheses and discussions. Catalysts for Fenton oxidation are diverse, differing by the nature of elements (d,f,p elements) directly reacting with hydrogen peroxide species, its ligand surroundings, and the phase state. Literature explains the classic Fenton reaction in Fe²⁺/H₂O₂ systems generating either as a free hydroxyl radical, or iron (IV)-oxo cation; however, the both concepts were not verified. A common representation of hydrogen peroxide oxidation in Fe³⁺/H₂O₂ systems is not supported due to electrochemical criteria. The formation idea of the oxoiron (IV) species in Fe³⁺/H₂O₂ systems assumes the active participation of the ligand surroundings of Fe³⁺ ion as a second electron donor; therefore, it is limited by the nature of ligands, however, having obtained the widest spread in biochemistry when interpreting fermentative activity of Fe³⁺, in which protoporphyrin IX species are Fe³⁺ ion ligands. The key idea in copper Fenton chemistry that is hydrogen peroxide oxidation in Cu²⁺/H₂O₂ systems is even more unsupported from the standpoint of electrochemistry. Pathways for free hydroxyl radical generation are most often substantiated for systems based on other metals with variable valences. Interaction mechanisms of p elements with hydroperoxides are absolutely unclear. The concept of universal priorities of polarization and dissociation of hydroperoxides during their oxidative activation and decomposition by Fenton catalysts is proposed to the scientific community as a hypothesis. The initial transformation of Fe(II) dihydroperoxo species into a complex of Fe²⁺ ion with a molecule of oxywater (^-O-⁺OH₂)) that dissociates to form a complex of Fe²⁺ ion with the oxygen atom (iron (II)-oxene) in ¹D-singlet quantum state is assumed for the classic Fenton reaction. Afterwards, [Fe³⁺O^•-]²⁺ α-oxygen complex that is argumented as the major intermediate in Fe²⁺/H₂O₂ systems is formed resulting from the fast and inevitable intracomplex electron transfer. An opportunity for transformation of α-oxygen complex into intermediates for subsequent intermediates, such as oxoiron (IV) species, crypto-hydroxyl, and free hydroxyl radicals is demonstrated. A high probability for the invariability of oxidation degree of Fe³⁺ with the prevalence of [Fe³⁺O⁰(¹D)]³⁺ intermediate is substantiated for Fe³⁺/H₂O₂ systems, among other things, biochemical. The successful use of interpretation towards various catalysts, among other things, based on p elements is illustrated. Molecular oxygen (dioxygen) in the ¹∆_g singlet quantum state (¹O₂) that differs from the main (³Σ_g^-, ³O₂) triplet state is produced in Fenton degradation (disproportionation) of hydrogen peroxide. Singlet dioxygen is of preparative value in dioxygen alkene and alkadiene functionalisation processes, such as synthesis of hydroperoxides and cyclic peroxides. The life time of ¹O₂ generated in aqueous solutions of H₂O₂ is several microseconds. The ¹O₂→3O₂ quenching overcomes a ban for electron spin reversal via a yet unknown mechanism. The (¹O₂)₂ associate is formed from antipodes on the orbital moment, as supposed by us. Resulting from two simultaneous redox reactions, two ³O₂ species are formed. They are antipodes on spin moments of the unpaired electrons, the total spin of which is +1 and -1.