Hydrogen peroxide decomposition active intermediates

The XH can be the parent hydrocarbon but is more usually an intermediate oxidation product with weaker C—H bonds, such as an aldehyde or alkene. Even so, the abstraction reaction has a large activation energy, as does the hydrogen peroxide decomposition (which is also pressure dependent), so that the branching mechanism tends to be of greater importance towards the higher temperature and pressure part of the region. 

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-... 

The alkylation of quinoline by decanoyl peroxide in acetic acid has been studied kineti-cally, and a radical chain mechanism has been proposed (Scheme 207) (72T2415). Decomposition of decanoyl peroxide yields a nonyl radical (and carbon dioxide) that attacks the quinolinium ion. Quinolinium is activated (compared with quinoline) towards attack by the nonyl radical, which has nucleophilic character. Conversely, the protonated centre has an unfavorable effect upon the propagation step, but this might be reduced by the equilibrium shown in equation (167). A kinetic study revealed that the reaction is subject to crosstermination (equation 168). The increase in the rate of decomposition of benzoyl peroxide in the phenylation of the quinolinium ion compared with quinoline is much less than for alkylation. This observation is consistent with the phenyl having less nucleophilic character than the nonyl radical, and so it is less selective. Rearomatization of the cr-complex formed by radicals generated from sources other than peroxides may take place by oxidation by metals, disproportionation, induced decomposition or hydrogen abstraction by radical intermediates. When oxidation is difficult, dimerization can take place (equation 169). 

Many compounds, especially various metallic oxides, also induce very rapid decomposition of hydrogen peroxide without themselves being permanently changed.4 In addition to the solutions of the alkali hydroxides already,mentioned, manganese dioxide, cobalt oxide, and lead oxide (massicot) are remarkably active, and as might be expected a colloidal solution of manganese dioxide 5 is also able to exert powerful catalytic influence.6 The effect in such cases may be partly a surface effect, but is also probably due in part to the intermediate formation and decomposition of unstable highly oxidised derivatives. 

The pivotal role of different protonation pathways to boimd oxygen derivatives in bioinorganic chemistry has been shown in various mechanistic studies The same concept has also been considered for the case of catalysis involving HjOj, in which the correlation between the ability to efficiently deliver the proton to the metal-bound hydroperoxide anion, and the efficiency of catalytic decomposition of hydrogen peroxide has been noted. In heme-enzyme catalysis, however, the ability to supply the necessary one or two protons for the generation of the proper active intermediate from the iron-bound peroxide/ hydroperoxide anion state is an essential and crucial aspect of the detailed structural fine-tuning provided by the enz3me active center. 

For the decomposition of hydrogen peroxide we saw that the reaction rate depends on the concentration of iodide ions even though 1 does not appear in the overall equation. We noted that I acts as a catalyst for that reaction. A catalyst is a substance that increases the rate of a reaction by lowering the activation energy. It does so by providing an alternative reaction pathway. The catalyst may react to form an intermediate with the reactant, but it is regenerated in a subsequent step so it is not consumed in the reaction. 

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