Hydrogen peroxide decomposition reaction mechanisms

The catalysis of hydrogen peroxide decomposition by iron ions occupies a special place in redox catalysis. This was precisely the reaction for which the concept of redox cyclic reactions as the basis for this type of catalysis was formulated [10-13]. The detailed study of the steps of this process provided a series of valuable data on the mechanism of redox catalysis [14-17]. The catalytic decomposition of H202 is an important reaction in the system of processes that occur in the organism [18-22]. 

Although Fenton did not observe hydroxyl radical-mediated reactions for mixtures of Fe3+ and hydrogen peroxide, more recent work has illustrated that such systems can produce hydroxyl radical. Haber and Weiss [5] originally proposed a free radical mechanism for the Fe3+-catalyzed decomposition of hydrogen peroxide. These reactions include [3] ... 

With these processes a similar reaction mechanism as with sulphate oxidation is assumed. This opinion is verified also by the observation that the presence of substances catalysing the hydrogen peroxide decomposition (iron, copper and manganese salts and finely dispersed platinum) inhibit almost entirely the proceeding of the oxidation in the required direction. 

The formation of halohydrins can be promoted by peroxidase catalysts.465 Recently 466 it has been shown that photocatalysis reactions of hydrogen peroxide decomposition in the presence of titanium tetrachloride can produce halohydrins. The workers believe that titanium(IV) peroxide complexes are formed in situ, which act as the photocatalysts for hydrogen peroxide degradation and for the synthesis of the chlorohydrins from the olefins. The kinetics of chlorohydrin formation were studied, along with oxygen formation. The quantum yield was found to be dependent upon the olefin concentration. The mechanism is believed to involve short-lived di- or poly-meric titanium(IV) complexes. 

It is possible that some acetate radicals are formed by the direct discharge of the ions as, it will be seen shortly, is the case in non-aqueous solutions but an additional mechanism must be introduced, such as the one proposed above, to account for the influence of electrode material, catalysts for hydrogen peroxide decomposition, etc. It is significant that the anodes at which there is no Kolbe reaction consist of substances that are either themselves catalysts, or which become oxidized to compounds that are catalysts, for hydrogen peroxide decomposition. By diverting the hydroxyl radicals or the peroxide into an alternative path, viz., oxygen evolution, the efficiency of ethane formation is diminished. Under these conditions, as well as when access of acetate ions to the anode is prevented by the presence of foreign anions, the reactions mentioned above presumably do not occur, but instead peracetic acid is probably formed, thus,... 

In non-aqueous solutions the Kolbe electrosynthesis takes place with high eflSciency at platinized platinum and gold, as well as at smooth platinum, anodes increase of temperature and the presence of catalysts for hydrogen peroxide decomposition, both of which have a harmful effect in aqueous solution, have relatively little influence. The mechanism of the reaction is apparently quite different in non-aqueous solutions and aqueous solutions in the former no hydroxyl ions are present, and so neither hydroxyl radicals nor hydrogen peroxide can be formed. It is probable, therefore, that direct discharge of acetate ions occurs at a potential which is almost independent of the nature of the electrode material in a given solvent. The resulting radicals probably combine in pairs, as in aqueous solution, to form acetyl peroxide, which subsequently decomposes as already described. ... 

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. 

How can we find out whether the proposed mechanism for a particular reaction is correct In the case of hydrogen peroxide decomposition we might try to detect the pres-... 

The ready reversibility of the ferrocyanide-ferricyanide redox system makes it a potential catalyst for the decomposition of hydrogen peroxide by the mechanism of compensating oxidation-reduction reactions. Moreover, the well-known facts that in acid solution ferrocyanide is oxidized to ferricyanide, whereas in alkaline solution the reverse reduction occurs, seem a good indication that at suitable pH s both reactions might occur to give catalytic decomposition. But from the investigations to date it would appear doubtful whether any such catalysis occurs to a measurable extent, and that what seems to be ready reactions of ferro- and ferricyanides are in fact those of partial hydrolysis products of these ions in which water molecules replace the cyanide ions in the coordination shell. 

How can we find out whether the proposed mechanism for a particular reaction is correct In the case of hydrogen peroxide decomposition we might try to detect the presence of the IO ions by spectroscopic means. Evidence of their presence would support the reaction scheme. Similarly, for the hydrogen iodide reaction, detection of iodine atoms would lend support to the two-step mechanism. For example, I2 dissociates into atoms when it is irradiated with visible light Thus, we might predict that the formation of HI from H2 and I2 would speed up as the intensity of light is increased because that should increase the concentration of I atoms. Indeed, this is just what is observed. 

One of the most used systems involves use of horseradish peroxidase, a 3-diketone (mosl commonly 2,4-pentandione), and hydrogen peroxide." " " Since these enzymes contain iron(II), initiation may involve decomposition of hydrogen peroxide by a redox reaction with formation of hydroxy radicals. However, the proposed initiation mechanism- involves a catalytic cycle with enzyme activation by hydrogen peroxide and oxidation of the [3-diketone to give a species which initiates polymerization. Some influence of the enzyme on tacticity and molecular... 

A milder procedure involves the reaction of a nitrile with an alkaline solution of hydrogen peroxide.147 The strongly nucleophilic hydrogen peroxide adds to the nitrile and the resulting adduct gives the amide. There are several possible mechanisms for the subsequent decomposition of the peroxycarboximidic adduct.148... 

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