Metal ions reactions with peroxy radicals

Reaction 36 may occur through a peroxy radical complex with the metal ion (2,25,182). In any event, reaction 34 followed by reaction 36 is the equivalent of a metal ion-cataly2ed hydrogen abstraction by a peroxy radical. 

A newly emerging class of reactions is oxidative substitution. An aromatic molecule reacts with a nucleophile to give a substitution product, a proton and two electrons or their equivalent. The aromatic molecule is first oxidized by a metal ion (or even an anode124)) to a radical cation intermediate which then is trapped and converted to product. Consider the reaction of alkylbenzenes with copper(II) chloride and peroxydisulfate ion. The peroxy compound converts toluene, for example, into its radical cation which is trapped in a reaction with the copper salt. Aromatiza-tion then gives products which are mostly o- and p-chlorotoluene, Scheme 15. In the... 

The dual role of transition metals as either prodegradant or stabilizer is rationalized by the observation that the free-radical chemistry which dominates depends on the concentration of the metal ion. At low concentrations Cu I/Cu II may be a pro-oxidant, whereas at high concentrations it may be a stabilizer. The explanation lies in the complexation of hydroperoxides with transition metals (Black, 1978). Thus, taking the example of Co II/Co III, the reactions in Scheme 1.71 are recognized (Black, 1978), which together give the usual reaction for the metal-catalysed bimolecular decomposition of hydroperoxides to alkyl peroxy and alkoxy radicals. 

Transition metal ions (Co, in particular) catalyze decomposition of ROOH with formation of alkoxy and peroxy radicals, as shown in reactions (4.10) and (4.11) [11, 12]. We would like to note here that the decomposition of hydroperoxides and peracids on cobalt(II) and (III) is more complex than what are shown in Eqs. (4.10) and (4.11) and involves several steps and two equivalents of Co(II) [16, 17, 18a, 19]. 

Reactions between R02 and Go(II) or Mn(II) are discussed in terms of the methods of rate constant determination, the values of the rate constants and the effect of media on rate constants. A synergistic effect of Mn(II) addition is also discussed. The effect of the media is discussed in terms of the coordination ability of the solvent with Go(II) ion as well as the proton donor property of the solvent. It is concluded that the presence of a proton donor in the coordination sphere of Co(n) is required for hydroperoxide formation but at the same time this prevents the peroxy radical from reacting with the cobalt(II) ion. As a result, a bell-shaped dependence of the rate constant on the acetic acid and water concentration is observed. Our data explain some of the controversial literature data on solvent effects for the kinetics of metal bromide catalysis. 

Since during the Purex process TBP, alkane, and aqueous nitric acid solution are in mixture or contact condition, the radiation chemical transformations depend on the composition, concentration of nitric acid, contaminant metal ions, irradiation conditions, and oxygen concentration (Triphathi and Ramanujam 2003 Katsumura 2004). Under aerated condition, the organic radicals react with oxygen forming peroxy radicals. After successive reactions a variety of alcohols, ketones, peroxides, and carbonyl compounds form. The ratio of nitration products to oxidation products is 0.8, and the ratio increases if there is no sufficient supply of O2. 

The first step in the chemical degradation mechanism is the production of hydrogen peroxide, which may be produced as a by-product of the oxygen reduction reaction on the cathode, or may be produced chemically by crossover of either hydrogen or oxygen to the opposite electrode. The hydrogen peroxide reacts with metal ion contaminants (M"+) acting as Fenton s catalysts to produce very reactive hydroperoxy and peroxy radicals, as described by equation (1.35) and equation (1.36). 

The sulfonic acid groups in the proton exchange membrane have a high affinity for many cationic species. Replacement of the protons by the contaminant cations will result in reduced conductivity of the membrane and performance loss. In addition, the membrane morphology and structure can be affected. An additional degradation issue associated with contaminant metal ions is the occurrence of Fenton s reactions to produce peroxy and hydroperoxy radicals, which in turn will chemically attack the membrane polymer structure. This mechanism is described in section 1.72.1 of this chapter and in chapter 5. The most common Fenton s metal of concern commonly found in the MEA is iron. 

The peroxysulfate radical, SO," is a key intermediate in the autoxidation of sulfite/bisulfite solutions, but is also the intermediate about which the least is known. It is formed subsequent to the one-electron oxidation of sulfite/bisulfite by the reaction of the sulfite radical with molecular oxygen (Reaction (29)). It is also generated in the metal ion-induced free radical decomposition of peroxy-monosulfate, H8O5" [107]. Its production in the oxidation of H8O5" by Ce(IV) has been confirmed by ESR spectroscopy [108], where a g factor of 2.0145 was... 

However, hydroperoxides can also be isomer-ized by such a reaction pathway. When they interact with free radicals (H-abstraction from -OOH group) or with heavy metal ions (cf. Reaction 3.64), they are again transformed into peroxy radicals. Thus, the 13-hydroperoxide of hnoleic acid isomerizes into the 9-isomer and vice versa ... 

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