Haber-Weiss mechanism

The degree of dissociation is very small but the diphenylcyanomethyl radical is sufficiently reactive to induce polymerization in styrene. Methyl radicals or hydrogen atoms bring about polymerization of vinyl monomers in the gas phase.Hydrogen peroxide in the presence of ferrous ions initiates polymerization in the aqueous phase or in aqueous emulsions through generation of hydroxyl radicals according to the Haber-Weiss mechanism... 

Although the feasibility of reaction 1 (an important component of the Haber-Weiss mechanism for Fe(II)-mediated decomposition of H202)... 

A major problem associated with such autoxidations is that they are largely indiscriminate, i.e. they exhibit poor chemo- and regio- selectivities. They are synthetically useful only with relatively simple substrates containing one reactive position, e.g. the oxidation of toluene to benzoic acid or p-xylene to terephthalic acid. Any catalytic oxidation has to complete with this non-catalytic pathway. Moreover, the situation is further complicated by the fact that transition metal ions also catalyze autoxidations by mediating the decomposition of trace amounts of hydroperoxides into chain-initiating radicals, via the so-called Haber-Weiss mechanism ... 

These complexes (205) were found to be good models for the reactive intermediates involved in the catalytic decomposition of alkyl hydroperoxides (Haber Weiss mechanism), and in the catalytic hydroxylation of hydrocarbons by ROOH. 

Therefore, having considered the experimental material published on this reaction, we seen no reason to assume any mechanism other than the well established Haber-Weiss mechanism, which requires a catalyst of the right oxidation/ reduction properties. 

The hydroperoxides formed are relatively stable at temperatures below 100 °C, but may be decomposed by almost any agent A, which can complex with it and give a redox reaction, reminiscent of the Haber-Weiss mechanism... 

As mentioned above, catalytic oxidation of olefins via coordination catalysis with an intermediate such as LnM (olefin) 02 seemed an attractive possibility, and Collman s group (45) tentatively invoked such catalysis in the 02-oxidation of cyclohexene to mainly 2-cyclo-hexene-1-one promoted by IrI(CO)(PPh3)2, a complex known to form a dioxygen adduct. Soon afterwards (4, 46, 47) such oxidations involving d8 systems generally were shown to exhibit the characteristics of a radical chain process, initiated by decomposition of hydroperoxides via a Haber-Weiss mechanism, for example Reactions 10 and 11. Such oxidations catalyzed by transition-metal salts such as... 

Figure 7 Scheme demonstrating the cobalt-catalyzed decomposition of hydroperoxides (Haber-Weiss mechanism). 

In this regard the metal catalyst has little effect, if any, on the process selectivity but it helps to speed up the hydroperoxide decomposition by the well established Haber-Weiss mechanism (Figure 7). 

The modified Haber-Weiss mechanism which has been proposed to account for this and other observations is... 

Induced chain reactions To illustrate an induced chain reaction, consider the oxidation of Fe(II) by hydrogen peroxide, which induces a chain decomposition reaction. The classical interpretation is that the decomposition proceeds through the Haber-Weiss mechanism," as modified by Baxendale and others, in which the chain decomposition is initiated by... 

The main role of Co is to accelerate the autoxidation reaction by decomposition of the intermediate hydroperoxide, ROOH, thus leading to the formation of alkoxyl or peroxyl radicals (Haber-Weiss mechanism) ... 

A review by Schuchardt et al. thoroughly analyses the various catalytic systems reported in the literature up to 2000, both homogeneous and heterogeneous ones, and those that use oxidants other than oxygen [e.g., HP or t-butyl hydroperoxide (f-BuOOH)[ [2c[. The mechanism involves the formation of cyclohexanol via the cyclohexyl radical and cyclohexyl hydroperoxide. According to the Haber-Weiss mechanism, cyclohexyl hydroperoxide decomposes into alkoxy and alkyloxy radicals (Section 7.2.1). Cyclohexanol is finally oxidized to cyclohexanone. A similar mechanism may occur at the a-C, affording 1,2-cyclohexanedione, which is finally cleaved to AA. Oxidation of the intermediately formed cyclohexanone to AA then occurs through a mechanism similar to that illustrated in Scheme 7.5. 

The action of the catalysts in the wet-oxidation has not been explicitly clarified. In the usual liquid-phase oxidation of hydrocarbons, the most important function of the catalyst (metal ions) is to decompose hydroperoxide (ROOH) formed during the reaction (known as Haber-Weiss mechanism) and accelerate chain propagation step by producing active radicals (ROO- and RO-) according to eq. 14.13 and 14.14. 

The general Haber-Weiss mechanism involves an intermediate reduction oxidation of the ferric-ferrous ion system. This has been accepted by a number of authors and Simon et al. (15) have shown that catalytic decomposition of hydrogen peroxide by ferric salts only takes place under conditions where the formation of ferrous ions can be demonstrated. There is, therefore, no experimental basis for a mechanism without the intermediate reduction of the ferric ions. 

Oxidation of Organic Contaminants with Hydrogen Peroxide Catalyzed hy AC Recently, there has been renewed interest in the catalytic system H2O2/AC, due to its potential application in the oxidation of organic contaminants in water and wastewater. The key factor for this application is the formation of free radicals [see reactions (6.12) and (6.13)] which are known to be very strong oxidants in the liquid phase. Carbon materials can act as electron-transfer catalysts, similar to the Haber-Weiss mechanism known from the Fenton reaction, with AC and AC" as the reduced and oxidized catalyst states [161] ... 

Kinetic studies are in agreement with a monoelectronic Haber-Weiss mechanism involving rhodium(I), rhodium(II) and rhodium(III) species, equations (222)-(225). 

To compare different polymers with respect to their chemical stability, the Fenton test is widely applied in fuel cell membrane research. In these tests, membrane samples are immersed in hydrogen peroxide solution containing a small amount of Fe " ", e.g., iron(lI)sulfate. In the presence of the metal ion, the decomposition of hydrogen peroxide is accelerated. The ongoing reactions are very complex, and several reactive intermediates are formed. Just as an example and demonstrating the catalytic nature, the following partial reactions of the so-called Haber-Weiss mechanism are highlighted, as shown in (6.28)-(6.31) [49, 50]. 

It is generally accepted that, in acid solution, hydrogen peroxide is reduced to water by ferrous ion in two one-electron steps, the Haber-Weiss mechanism (equations 61,62) (31,266,306,771,790). In con-... 

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