Intermediate hydrogen peroxide oxygen

Conversion of Aromatic Rings to Nonaromatic Cyclic Structures. On treatment with oxidants such as chlorine, hypochlorite anion, chlorine dioxide, oxygen, hydrogen peroxide, and peroxy acids, the aromatic nuclei in lignin typically ate converted to o- and -quinoid stmctures and oxinane derivatives of quinols. Because of thein relatively high reactivity, these stmctures often appear as transient intermediates rather than as end products. Further reactions of the intermediates lead to the formation of catechol, hydroquinone, and mono- and dicarboxyhc acids. 

A wide variety of enzymes have been used in conjunction with electrochemical techniques. The only requirement is that an electroactive product is formed during the reaction, either from the substrate or as a cofactor (i.e. NADH). In most cases, the electroactive products detected have been oxygen, hydrogen peroxide, NADH, or ferri/ferrocyanide. Some workers have used the dye intermediates used in classical colorimetric methods because these dyes are typically also electroactive. Although an electroactive product must be formed, it does not necessarily have to arise directly from the enzyme reaction of interest. Several cases of coupling enzyme reactions to produce an electroactive product have been described. The ability to use several coupled enzyme reactions extends the possible use of electrochemical techniques to essentially any enzyme system. 

Transition metal peroxides, particularly peroxo (2), alkylperoxo (7) and hydroperoxo (8) complexes, are extremely important reactive intermediates in catalytic oxidations involving molecular oxygen, hydrogen peroxide and alkyl hydroperoxides as the oxygen source. Representative peroxo complexes are listed in Table 3, and alkylperoxo and hydroperoxo complexes are listed in Table 4 together with their reactivities. 

In fact, this is the case only at the mercury cathode (Koryta et al., 1993). At other electrodes (e.g., Pt, Au, or C) only one four-electron reduction is obtained, meaning that the hydrogen peroxide intermediate is reduced at approximately the same potential as molecular oxygen. 

One of the main disadvantages of the Damjanovic s scheme is that it does not consider possible weak adsorptions and the reversible adsorption/desorption of hydrogen peroxide at the interface. The mechanism proposed by Wroblowa et al. [104] considers the adsorption/desorption equilibrium either for the oxygen reactant or the hydrogen peroxide intermediate. They also proposed the chemical decomposition of the intermediate (1(4), besides the electrochemical reduction to water through k3 (Scheme 2.5). 

It should also be noted that the platinum surface is very sensitive to the presence of species in solution and to electrode pre-treatments (anodization, pre-reduction). Damjanovic etal reported a very strong dependence of the reaction pathway on the purity of the solution. They concluded that the oxygen reduction reaction occurred without hydrogen peroxide intermediate formation on a pre-reduced platinum electrode, and therefore that the production of hydrogen peroxide was effective only on sites affected by the presence of adsorbed impurities. 

Oxidation of ascorbic acid to dehydroascorbic acid is catalysed by many enzymes (oxidoreductases), which belong to the category of ascorbic acid antivitamins. Ascorbic acid is also oxidised by atmospheric oxygen, hydrogen peroxide and various other oxidising agents. Oxidation to dehydroascorbic acid is a reversible reaction and can be carried out by various mechanisms. The loss of one electron yields a radical of ascorbic acid as an intermediate and the reaction is known as one-electron oxidation. Oxidation of ascorbic acid by the loss of two electrons yields dehydroascorbic acid, which is the first chemically stable product. 

These studies have indicated that (i) polyacetylene can act as a catalytic electrode in an aqueous electrolyte for the reduction of gaseous oxygen, hydrogen peroxide or perchloric acid and (ii) poly-3-methylthiophene can act as the catalytic electrode for the reduction of SO in Li(S02)2AlCl and permit the cathode discharge product to become rapidly oxidized without resorting to use of chlorine as an intermediate. 

A number of chemiluminescent reactions may proceed through unstable dioxetane intermediates (12,43). For example, the classical chemiluminescent reactions of lophine [484-47-9] (18), lucigenin [2315-97-7] (20), and transannular peroxide decomposition. Classical chemiluminescence from lophine (18), where R = CgH, is derived from its reaction with oxygen in aqueous alkaline dimethyl sulfoxide or by reaction with hydrogen peroxide and a cooxidant such as sodium hypochlorite or potassium ferricyanide (44). The hydroperoxide (19) has been isolated and independentiy emits light in basic ethanol (45). 

Weak to moderate chemiluminescence has been reported from a large number of other Hquid-phase oxidation reactions (1,128,136). The Hst includes reactions of carbenes with oxygen (137), phenanthrene quinone with oxygen in alkaline ethanol (138), coumarin derivatives with hydrogen peroxide in acetic acid (139), nitriles with alkaline hydrogen peroxide (140), and reactions that produce electron-accepting radicals such as HO in the presence of carbonate ions (141). In the latter, exemplified by the reaction of h on(II) with H2O2 and KHCO, the carbonate radical anion is probably a key intermediate and may account for many observations of weak chemiluminescence in oxidation reactions. 

Other reachons involving oxygen are those reachon steps in oxygen reduction which are of importance in their own right, such as the formation of hydrogen peroxide as a relahvely stable intermediate ... 

Two major pathways exist for this reaction, one bypassing hydrogen peroxide (first pathway) and the other involving intermediate peroxide formation via reaction (15.21) (second pathway). The peroxide formed is either electrochemically reduced to water via reaction (15.22) or decomposed catalytically on the electrode surface via reaction (15.23), in which case half of the oxygen consumed to form it reemerges [in both cases the overall reaction corresponds to Eq. (15.20)]. 

In the reaction following the second pathway, the 0-0 bond is not broken while the first two electrons are added it is preserved in the HjOj produced as an intermediate, and breaks in a later step, when the hydrogen peroxide is reduced or cat-alytically decomposed. An analog for this pathway does not exist in anodic oxygen evolution. 

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