Peroxides reactions with transition metals

The thermodynamic functions (AH, AS, AG(298 K)) of hydrogen peroxide reactions with transition metal ions in aqueous solutions are presented in Table 10.1. We see that AG(298K) has negative values for reactions of hydroxyl radical generation with Cu1+, Cr2+, and Fe2+ ions and for reactions of hydroperoxyl radical generation with Ce4+, Co3+, and Mn3+. 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

The values of the rate constants for the reactions of transition metal ions with hydrogen peroxide in an aqueous solution are presented in Table 10.2. 

Several studies suggest that LA and DHLA form complexes with metals (Mn2+, Cu2+, Zn2+, Cd2+, and Fe2+/Fe3+) [215-218]. However, in detailed study of the interaction of LA and DHLA with iron ions no formation of iron LA complexes was found [217]. As vicinal dithiol, DHLA must undoubtedly form metal complexes. However, the high prooxidant activity of DHLA makes these complexes, especially with transition metals, highly unstable. Indeed, it was found that the Fe2+-DHLA complex is formed only under anerobic conditions and it is rapidly converted into Fe3+ DHLA complex, which in turn decomposed into Fe2+ and LA [217]. Because of this, the Fe3+/DHLA system may initiate the formation of hydroxyl radicals in the presence of hydrogen peroxide through the Fenton reaction. Lodge et al. [218] proposed that the formation of Cu2+ DHLA complex suppressed LDL oxidation. However, these authors also found that this complex is unstable and may be prooxidative due to the intracomplex reduction of Cu2+ ion. 

Thiols are susceptible to oxidation by peroxides, molecular oxygen, and other oxidizing processes (e.g., radical-catalyzed oxidation) (Fig. 67). Because thiols easily complex with transition metals, it is believed that most thiol autoxidation reactions are metal-catalyzed (108). Autoxidation of thiols is enhanced by deprotonation of the thiol to the thiolate anion. Thiol oxidation commonly leads to disulfides, although further autoxidation to the sulfinic and, ultimately, sulfonic acid can be accomplished under basic conditions. Disulfides can be reduced back to the thiol (e.g., upon addition of a reducing agent such as dithiothreitol). Thiols are nucleophilic and will readily react with available electrophilic sites. For a more thorough discussion, see Hovorka and Schoneich (108) and Luo et al. (200). 

Metal deactivators, which form inactive chelates or insoluble reaction products with transition metals originally present in a form that promotes the decomposition of peroxides to free radicals. Examples are ethylenediaminetetraacetic acid, salicylaldehyde-diamine condensation products or metalalkyl dithiocarbamates such as of nickel or zinc. 

Oxidative degradation of the membrane is believed to occur due to attack from peroxide radicals, e.g., OH and OOH. Peroxide formation is suggested in connectitm with oxygen diffusion through the electrolyte towards the anode followed by incomplete reduction. Subsequent reaction of the peroxide with transition metal ions, e.g., Fe leads to formation of the detrimental peroxide radicals. In practice, a trace amount of transitiOTi metals may be present as impurities in the MEA, originating from corrosion of metallic components such as bipolar plates, end plates, and tubing [6]. 

The most widely used redox systems, serving for the production of free radicals, are the mixtures of diluted in the reaction environment solutions of hydroperoxides or organic peroxides with transition metal ions ... 

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