Hydrogen peroxide and ferrous ions

The most commonly employed reagent for the hydroxylation of aromatic compounds is that consisting of ferrous ion and hydrogen peroxide. The suggestion that hydroxyl radicals are intermediates in this reaction was first made by Haber and Weiss, who proposed the following radical-chain mechanism for the process ... 

Thus, superoxide itself is obviously too inert to be a direct initiator of lipid peroxidation. However, it may be converted into some reactive species in superoxide-dependent oxidative processes. It has been suggested that superoxide can initiate lipid peroxidation by reducing ferric into ferrous iron, which is able to catalyze the formation of free hydroxyl radicals via the Fenton reaction. The possibility of hydroxyl-initiated lipid peroxidation was considered in earlier studies. For example, Lai and Piette [8] identified hydroxyl radicals in NADPH-dependent microsomal lipid peroxidation by EPR spectroscopy using the spin-trapping agents DMPO and phenyl-tcrt-butylnitrone. They proposed that hydroxyl radicals are generated by the Fenton reaction between ferrous ions and hydrogen peroxide formed by the dismutation of superoxide. Later on, the formation of hydroxyl radicals was shown in the oxidation of NADPH catalyzed by microsomal NADPH-cytochrome P-450 reductase [9,10]. 

Treatment of olefins with sodium azide, ferrous ion, and hydrogen peroxide... 

The modification of the properties of cotton cellulosic textile products, through free radical-initiated graft copolymerization reactions with vinyl monomers, has been investigated at the Southern Laboratory for a number of years (6, 9). In this chapter, we summarize the basic mechanisms and principles involved in free radical reactions of cellulose, initiated by high energy radiation, ceric ion in acidic solution, and aqueous solutions of ferrous ion and hydrogen peroxide. Some of the properties of fibrous cotton cellulose graft copolymers are also presented. 

Wolfrom and coworkers commented on the similarity of the products from the irradiation of alditol solutions and from the action of Fenton s reagent (ferrous ions and hydrogen peroxide) thereon. 

Redox reactions, e.g., the reaction between the ferrous ion and hydrogen peroxide in solution produces hydroxyl radicals. 

The formation of hydroxyl or hydroxyl-like radicals in the reaction of ferrous ions with hydrogen peroxide (the Fenton reaction) is usually considered as a main mechanism of free radical damage. However, Qian and Buettner [172] have recently proposed that at high [02]/ [H202] ratios the formation of reactive oxygen species such as perferryl ion at the oxidation of ferrous ions by dioxygen (Reaction 46) may compete with the Fenton reaction (2) ... 

This mechanism can be illustrated by the reaction of ferrous ions with hydrogen peroxide (42), the reduction of organic peroxides by cuprous ions (63), as well as by the reduction of perchlorate ions by Ti(III) (35), V(II) (58), Eu(II) (71), The oxidation of chromous ions by bromate and nitrate ions may also be classified in this category. In the latter cases, an oxygen transfer from the ligand to the metal ion has been demonstrated (8), As analogous cases one may cite the oxidation of Cr(H20)6+2 by azide ions (15) (where it has been demonstrated that the Cr—N bond is partially retained after oxidation), and the oxidation of Cr(H20)6+2 by 0-iodo-benzoic acid (6, 8), where an iodine transfer was shown to take place. 

Apart from this mathematical aspect I think that the useful concept of critical antioxidant concentration is valid for degenerate chain branching where the effect of the presence of antioxidant on hydroperoxide decomposition is relatively minor but not when it is the predominant initiation reaction. For metals reacting with hydroperoxides the number of radicals formed may even exceed unity. Kolthoff and Medalia [/. Am. Chem. Soc. 71,3777 (1949) ] demonstrated that for the reaction of ferrous ion with hydrogen peroxide as many as six ferrous atoms can be oxidized by one molecule of hydrogen peroxide as a result of this effect. I do not think, therefore, that the critical antioxidant concentration should be applied to those cases in which the so-called antioxidant is the catalyst. 

This brings out also the difference between the ferrous and the ferric reactions starting from ferric ions and hydrogen peroxide there must be an induction period during which the reduction process, ferric —> fer-... 

The evidence for the free radical mechanisms of the reaction between ferrous and ferric ions and hydrogen peroxide is fully discussed in the article by J. H. Baxendale in this volume, and it is necessary here only to summarize and comment on those features especially relevant to hemoprotein reactions. This evidence is essentially indirect. Experiment shows very reactive intermediates to be present and extensive kinetic studies reveal competition reactions for these intermediates in that the overall order of the reaction is found to depend on the reactant concentrations. A free radical mechanism is adopted because it accounts for the chemical reactivity of the system in the oxidation of substrates (Fenton s reaction) and the initiation of the polymerization of vinyl compounds (Baxendale, Evans, and Park, 84) and it provides a set of reactions which largely account for the observed kinetics. The set of reactions which fit best the most recent experimental data is that proposed by Barb, Baxendale, George, and Hargrave (83) ... 

The e.s.r. spectra of oxovanadium ions in redox systems have been reported. The interaction of free-radicals generated using the reactions of cerium(iv) or ferrous ions with hydrogen peroxide with oxovanadium(v), produces a complex which decays in a first-order manner (k = 6-2 s at 22 °C) with the formation of vanadium(iv). The oxidation of phenetidines by bromate is catalysed by vanadium(v) and kinetic parameters involved in the interactions of various substrates with vanadium(v) have been correlated with electron configurations. The redox behaviour of oxo-3,5-disulphocatecholatovanadium(v) has been studied and the acidity dependence in the reaction with phenylethyl alcohol reported. In the... 

The graft copolymerizations were conducted in a three-neck flask fitted with a nitrogen bubbler, stirrer, and dropping funnel. In a typical reaction, dextran (M 100,000-200,000) was dissolved in distilled water under a nitrogen atmosphere with stirring for 10 minutes. The ferrous ion solution was added and 30 minutes were allowed for adsorption. Acrylamide monomer was added and stirred for 10 minutes and then 50 ml of hydrogen peroxide solution were added dropwise for 20 minutes. The variations of reaction parameters are shown in Table 4. Reaction series PA, PB, PC, PC, and PE correspond to variations of ferrous ion concentration, hydrogen peroxide concentration, reaction time, monomer concentration, and dextran substrate concentration, respectively. 

The Eenton process involves the reaction of ferrous ions with hydrogen peroxide molecules in aqueous solution to generate a series of oxidizing species of which the hydroxyl radical is the most powerful and reactive. In this way, it is widely accepted that the OH species is formed as described by Eq. 1 [5—8] ... 

Such a reaction regenerates ferrous ion showing that only low concentration of Fe " is needed in the system [86]. Moreover, the photo-Fenton process may proceed using photons of wavelength close to 500 nm in the case of mixtures of ferric ion and hydrogen peroxide. It can also be performed by solar irradiation making it a low cost process [79, 80, 87, 88]. The photocatalytic cycle may be presented as shown in Scheme 6.6. 

In positive electrospray ionization, the ions most commonly formed by the electrolysis current are ferrous ions from the oxidation of the steel tip. For more electrochemically inert contact materials, the oxidation products can be hydrogen ions and hydrogen peroxide or oxygen. In negative ESI, the reaction at the contact must be reduction and since the contact material cannot be further reduced, something in the solution must be reduced. This reduction can sometimes lead to a significant change in the composition of the sprayed solution. 

The most common water-soluble initiators are ammonium persulfate, potassium persulfate, and hydrogen peroxide. These can be made to decompose by high temperature or through redox reactions. The latter method offers versatility in choosing the temperature of polymerization with —50 to 70°C possible. A typical redox system combines a persulfate with ferrous ion ... 

Inhibition and stimulation of LOX activity occurs as a rule by a free radical mechanism. Riendeau et al. [8] showed that hydroperoxide activation of 5-LOX is product-specific and can be stimulated by 5-HPETE and hydrogen peroxide. NADPH, FAD, Fe2+ ions, and Fe3+(EDTA) complex markedly increased the formation of oxidized products while NADH and 5-HETE were inhibitory. Jones et al. [9] also demonstrated that another hydroperoxide 13(5)-hydroperoxy-9,ll( , Z)-octadecadienoic acid (13-HPOD) (formed by the oxidation of linoleic acid by soybean LOX) activated the inactive ferrous form of the enzyme. These authors suggested that 13-HPOD attached to LOX and affected its activation through the formation of a protein radical. Werz et al. [10] showed that reactive oxygen species produced by xanthine oxidase, granulocytes, or mitochondria activated 5-LOX in the Epstein Barr virus-transformed B-lymphocytes. 

Merz and Waters (1949) showed that oxidation of organic compounds by Fenton s reagent could proceed by chain as well as non-chain mechanisms, which was later confirmed by Ingles (1972). Kremer (1962) studied the effect of ferric ions on hydrogen peroxide decomposition for Fenton s reagent. It was confirmed that once ferric ions are produced the ferric-ferric system is catalytic in nature, which accounts for relatively constant concentration of ferrous ion in solutions. 

Barb, W.G., Baxendale, J.H., George, P., and Hargrave, K.R., Reactions of ferrous and ferric ions with hydrogen peroxide, Trans. Faraday Soc., 47, 591, 1951. 

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