Hydrogen peroxide reaction with transition metal ions

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+. 

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. 

A free radical is an atom, molecule, or compound that contains one or more unpaired electrons. Common radicals include the hydrogen atom and most transition metal ions, as well as oxygen, which is a biradical since its outer two electrons have parallel spins and, therefore, are unpaired. The first reported free radical reaction was presumably by Fenton (F7), although free radicals were not known to exist at the time. The classic mechanism (Fenton reaction) with ferrous iron (as well as various other transition metal ions) predicts that hydrogen peroxide is reduced at the iron center with generation of the hydroxyl free radical (FIO). 

Enthalpies, Entropies, and Gibb s Energies of Transition Metal Ion Oxidation-Reduction Reactions with Hydrogen Peroxide in Aqueous Solution (T = 298 K) [23]... 

Rate Constants of Transition Metal Ion Reactions with Hydrogen Peroxide in Aqueous Solutions... 

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. 

The best known and most nsefnl of the chemiluminescent reactions involving electron transfer is the oxidation of luminol (3.100) or its derivatives in alkaline medium. The oxidant can be hydrogen peroxide, sodium ferricyanide or hypochlorite, usually with a catalyst that can be a transition metal ion, such as Cu " Co +, Fe + and Mtf+, or haem and haemproteins, e.g. peroxidases. The reaction mode is shown in Figure 3.22.4"... 

Vitamin C (ascorbate) (Fig. 9.5) has the ability to act as a reducing agent, i.e. it will tend to reduce more reactive species. This ability to reduce Fe3+ to Fe2+may be important in promoting iron uptake in the gut. Oxidation of ascorbate by reaction with reactive oxygen species or reactive nitrogen species seems to lead to its depletion. In vitro, vitamin C can also exert pro-oxidant properties. Fe3+ can react with ascorbate to form Fe2+ and the semi-dehydroascorbate or ascorbyl radical. The latter can react with hydrogen peroxide to form Fe3+, the hydroxyl radical and a hydroxide anion. A key question with regard to the pro- or anti- oxidant effects of ascorbate may therefore be the availability of transition metal ions. Neurons main-... 

The author hopes that the chapter has shown how the reactivity, and selectivity towards oxygen transfer reactions of hydrogen peroxide can be utilized for synthetic purposes. Generally, for laboratory and industrial application, activation of hydrogen peroxide by transition metal ions is the method of choice, although in a number of cases, the transition metal route can lead to disposal and environmental problems. The reaction of hydrogen peroxide with organic compounds can, therefore, provide a viable alternative to metal ion activation. 

Galwey and Hood [160] showed that NajCOj.l.SHjOj decomposed in vacuum (360 to 410 K) to produce Na COj + l.SHjO + O.TSOj. ar-time curves were sigmoidal and the kinetics could be described by the Avrami-Erofeev equation with = 2 or 3. The activation energy was 112 8 kJ mol. The reaction rate between 313 and 343 K was significantly increased by the presence of small amounts of liquid water. This deceleratory reaction was fitted by the first-order equation (E, = 80 10 kJ mol ) and it was concluded that breakdown of hydrogen peroxide proceeded in the liquid water, possibly with trace amounts of impurity transition-metal ions acting as catalysts. 

Early transition metal ions in their highest oxidation states, such as Ti(IV), V(V), W (VI), and Mo(VI), tend to be stable toward changes in their oxidation states. Consequently, in epoxidation reactions with hydrogen peroxide or alkyl hydroperoxides they form adducts (M-OOH and M-OOR) that are the key intermediates in the... 

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