Hydrogen peroxide, decomposition demonstrations

In this connection it is important to recall the dependency of the degradation rate of hydrogen peroxide, r(-H202), on the absorbed photon flow Op (Gmelin, 1966). In addition, the quantum yield of H2O2 photolysis is a discrete function of Op (Lunak and Sedlak, 1992, Gmelin, 1966). It was demonstrated that the rate of hydrogen peroxide decomposition at low incident photon flows (Op <10 photons s ) can be described by Eq. 6-11 and at Op above this value Eq. 6-12 is valid. 

A saturated solution of potassium or sodium iodide catalyses hydrogen peroxide decomposition effectively. This reaction can be used to create an attractive demonstration producing a jet of coloured foam that looks like gigantic toothpaste. 

Inproved contacting in laboratory trickle beds by the addition of a fine diluent to the catalyst bed have now been reported by several workers for a variety of reactions. Koros [3 ] showed that for the hydrogen peroxide decomposition in methanol, the introduction of 50 mesh sand into the interstices of the bed inproved the contacting efficiency over a wide range of liquid velocities as shown in Figure 2.2.1. The effect was demonstrated by taking a bed of 3-2 mm extrudates in a 2.54 mm diameter reactor... 

The introduction of chlorinated porphyrins (10) allowed for hydrogen peroxide to be used as terminal oxidant [62], These catalysts, discovered by Mansuy and coworkers, were demonstrated to resist decomposition, and efficient epoxidations of olefins were achieved when they were used together with imidazole or imidazo-lium carboxylates as additives, (Table 6.6, Entries 1 and 2). 

The catalytic activity of PCSs results from their semiconductor properties. The first studies in this field date from 1959—1961. Thus, we have demonstrated catalytic activity of products of the thermal transformation of PAN in the decomposition reactions of hydrogen peroxide, hydrazine hydrate, and formic acid270, 271. There is an indication of catalytic activity of poly(aminoquinone) in the reactions of the hydrogen peroxide decomposition272. ... 

Concerning the mode of formation of ES, we prefer the concept that the substrate in a monolayer is chemisorbed to the active center of the enzyme protein, just as the experimental evidence pertaining to surface catalysis by inorganic catalysts indicates that in these reactions chemisorbed, not physically adsorbed, reactants are involved. Such a concept is supported by the demonstration of spectroscopically defined unstable intermediate compounds between enzyme and substrate in the decomposition by catalase of ethyl hydroperoxide,11 and in the interaction between peroxidase and hydrogen peroxide.18 Recently Chance18 determined by direct photoelectric measurements the dissociation con-... 

Thus, antioxidant effects of nitrite in cured meats appear to be due to the formation of NO. Kanner et al. (1991) also demonstrated antioxidant effects of NO in systems where reactive hydroxyl radicals ( OH) are produced by the iron-catalyzed decomposition of hydrogen peroxide (Fenton reaction). Hydroxyl radical formation was measured as the rate of benzoate hydtoxylation to salicylic acid. Benzoate hydtoxylation catalyzed by cysteine-Fe +, ascorbate - EDTA-Fe, or Fe was significantly decreased by flushing of the reaction mixture with NO. They proposed that NO liganded to ferrous complexes reacted with H2O2 to form nitrous acid, hydroxyl ion, and ferric iron complexes, preventing generation of hydroxyl radicals. 

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. 

Ealy, "Catalytic Decomposition of Hydrogen Peroxide Foam Production," Chemical Demonstrations, A Sourcebook for Teachers, Vol. 1 (American Chemical Society, Washington, DC, 1988), pp. 101-102. Oxygen gas is catalytically produced in the presence of detergent resulting in the formation of a large quantity of foam. 

At the end of the eighteenth and the beginning of the nineteenth century the influence of metals and oxides on the decomposition of several substances was studied by many scientists. It was noticed that contact with different substances gives very different products. An example is the decomposition of alcohol in the presence of copper or iron, carbon and an inflammable gas is produced. In the presence of pumice stone decomposition into ethene and water was observed. In other words, selectivity was demonstrated. Many other important milestones can be mentioned we limit ourselves to a few. Thenard investigated the dissociation of ammonia in contact with metals. In 1813 he found that the dissociation occurs over various metals, provided they are hot. Later he systematically studied the dissociation of hydrogen peroxide. He concluded that some of the solids studied... 

The subsequent reaction of the peroxo complexes with bromide gives oxidized bromine species (see Scheme 2). HOBr, Br2, and Br3 rapidly equilibrate, and Br3 is the predominant spectrophotometrically observed intermediate (Xmax 267 nm e = 36,100 M-1 cm-1) in the absence of an organic substrate. Tribromide is stabilized with respect to HOBr, Br2 and decomposition products by high bromide and acid concentrations (34). HOBr is reduced by excess hydrogen peroxide to yield bromide, water, and dioxygen, of which dioxygen can be measured. In the presence of TMB, the oxidized species is rapidly consumed in the bro-mination of TMB to BrTMB. Quantitation of BrTMB demonstrates that bromination is stoichiometric with respect to the concentration of H202 added. Thus TMB is a rapid, quantitative trap for the oxidized bromine species (33). 

A series of iron(III) complexes, c/s-[Fe(tetraamine)(OH)2]2+, where tetra-amine is a tetradentate amine such as 7V,jV -bis(2-picolyl)ethylenediamine, bound electrostatically to poly-L-glutamate or dextran sulfate, has been studied as catalysts for the decomposition of hydrogen peroxide into water and oxygen.72 The catalyst performance varies with the mode of binding and nature of the support, thus demonstrating the importance of the local environment around the catalytically active site. 

There is some difficulty with the energetics of unimolecular hydroperoxide decomposition. The endothermicity for the reaction ROOH RO + OH is of the order of 50 kcal., whereas the observed activation energy is as low as 30 kcal. The question is, therefore, bound to arise To what extent is decomposition trace metal--catalyzed It can be demonstrated that ferrous phthalocyanine, even at concentrations below lO M, is a most powerful activator of hydroperoxides—e.g., in the oxidation of quercetin, rhamnetin, or 8-carotene. The action of ferrous phthalocyanine is in principle similar to that of ferrous ion with hydrogen peroxide, already discussed. It may be described as reduction activation. 

Bellussi et al. demonstrated that hydrolysis of Ti-O-Si bond takes place with the formation of Ti-OH and Si-OH groups [8]. This means that the structure of active site, that is, isolated Ti atoms by long chains of -O-Si-O-Si-0-, which gives high selectivity for the formation of epoxides is destroyed by water. The decomposition of active site must occur in the initial stage. Therefore, the conversion in presence of hydrogen peroxide slightly increase with reaction time. 

It has recently been demonstrated that this reaction could be used for the epoxidation of a broader class of enones and excellent enantioselectivities were obtained provided the enone was substituted at the 3-position [48,49,50]. However, essentially no asymmetric induction was observed with cyclic enones [51]. Further practical improvements in this reaction have recently been made. Roberts found that the use of anhydrous urea-hydrogen peroxide with DBU in THF as a two-phase system (polymer and organic phase) resulted in rapid epoxidation with high levels of asymmetric induction [52]. This modification solves many of the problems associated with the original procedure (oxidant decomposition, long reaction times, work up) and provides a very practical oxidation system (Scheme 17). 

Nowadays, it has been demonstrated that the reaction is indeed structure sensitive with a multielectron transfer process that involves several steps and the possible existence of several adsorption intermediates [93-96]. The main advantage that we have with the new procedures with respect to cleanliness is that we have well-ordered surfaces to study a complex mechanism such as the oxygen electroreduction reaction [96-99]. In aqueous solutions, the four-electron oxygen reduction appears to occur by two overall pathways a direct four-electron reduction and a peroxide pathway. The latter pathway involves hydrogen peroxide as an intermediate and can undergo either further reduction or decomposition in acid solutions to yield water as the final product. This type of generic model of a reaction has been extensively studied since the early 1960s by different authors [100-108]. 

This idea has been attractive because of the similarity of such a scheme to an enzyme reaction, the catalyst representing the enzyme and the intermediate compound being analogous to the enzyme-substrate complex. The viewpoint has been taken by Spitalsky and co-workers (1) who have provided much evidence in support of it. The formation of various peroxide intermediates in many catalytic systems, e.g., chromate and molybdate, is easily demonstrated and to take a less obvious case, the compound FeH02++ recently shown to be present in mixtures of ferric ion and hydrogen peroxide (2), might be considered as an intermediate in the catalytic decomposition which occurs in this system. 

---