Branching chains Degenerate

In the later stages of ketone oxidation, free radicals are formed from a-ketohydroperoxide. In cyclohexanone, a-ketohydroperoxide decomposes by a firsit-order reaction [164] with a rate coefficient fei = 5.9 X 107 exp(—20,400/RT) s-1. Ketone takes part in the formation of free radicals from hydroperoxide (see below). 

Two peroxides are formed in the oxidation of methyl ethyl ketone [165], an a-ketohydroperoxide and a peroxide denoted as X. Both 

Rate coefficients of the forward and back reactions and K(=ktlkb) for addition of hydroperoxides to cyclohexanone in CC14 [167] 

In methyl ethyl ketone oxidation, free radicals are formed not only from peroxides but also from diacetyl, another intermediate product, which decomposes to radicals by a unimolecular reaction with a rate coefficient [166] of 

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At first, the question of the relative importance of ROOH versus aldehydes as intermediates was much debated however, recent work indicates that the hydroperoxide step dominates. Aldehydes are quite important as fuels in the cool-flame region, but they do not lead to the important degenerate chain branching step as readily. The RO compounds form ROH species, which play no role with respect to the branching of concern. 

In developing oxidation processes a major source of free radical formation was found to be degenerate chain branching. Among the products derived from the branching were intermediate peroxides ROOH. Formation of radicals from the hydroperoxides proceeded not only by monomolecular breakdown of hydroperoxides ... 

Mechanism III. Amines may interact with important molecule intermediates formed during the oxidation of the fuel—e.g., peroxides. If this occurred by a nonchain process, degenerate chain branching would be stopped, and there would be effective inhibition, provided that the initiation reaction between the fuel and oxygen was slow. 

The slow combustion reactions of acetone, methyl ethyl ketone, and diethyl ketone possess most of the features of hydrocarbon oxidation, but their mechanisms are simpler since the confusing effects of olefin formation are unimportant. Specifically, the low temperature combustion of acetone is simpler than that of propane, and the intermediate responsible for degenerate chain branching is methyl hydroperoxide. The Arrhenius parameters for its unimolecular decomposition can be derived by the theory previously developed by Knox. Analytical studies of the slow combustion of methyl ethyl ketone and diethyl ketone show many similarities to that of acetone. The reactions of methyl radicals with oxygen are considered in relation to their thermochemistry. Competition between them provides a simple explanation of the negative temperature coefficient and of cool flames. 

With these parameters, the half-life of a typical hydroperoxide is about 1 second at 330°C. and about 10 seconds at 290°C. These short lifetimes permit the hydroperoxides to act as secondary initiation sources to increase the rate of hydrocarbon decomposition. This is the effect that has been described by Semenov and his co-workers as degenerate chain branching. 

In recent years Emanuel, Neiman, and their respective schools have greatly contributed to the theory of antioxidant action by studying the phenomenon of the critical antioxidant concentration in terms of a degenerate branched chain reaction. The critical antioxidant concentration, a well-established feature of phenolic antioxidants, is one below which autoxidation is autocatalytic and above which it proceeds at a slow and steady rate. Since the theory allowed not only a satisfactory explanation of the critical antioxidant concentration itself but elucidation of many refinements, such as the greater than expected activity of multifunctional phenolic antioxidants (21), we wondered whether catalyst-inhibitor conversion could be fitted into its framework. If degenerate chain branching is assumed to be the result of... 

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. 

Formaldehyde is a product of the combustion of all hydrocarbons. Studies of the reactions of formaldehyde are important in leading to a better understanding of the mechanism of hydrocarbon oxidation. Its role in the low temperature region is variable but minor, and depends on the individual hydrocarbon and conditions. In sufficient quantities it appears able to suppress cool flames. In hydrocarbon oxidation above 400° C. formaldehyde is an important intermediate responsible for degenerate-chain branching. 

Provided (f - g) > 0, the chain carrier concentration, and hence, the reaction velocity will increase exponentially with time. However, (/ - g) may be small enough so that t, corresponding to the induction period, r, may be very long. If (f — g) < 0, a true explosion never develops. A slow change from — to + values of (/ - g) has been observed for hydrocarbon-oxygen systems. These phenomena are sometimes referred to as degenerate chain-branching explosions or cool flames (44, ) ... 

Degenerate chain branching may occur between various radicals produced in the autoxidation sequence, and involves bi-radical termination reactions. 

This delayed decomposition of the metastable peroxides has been termed degenerate chain branching by Semenoff and has been used by him to account for the ignition limits of hydrocarbon oxidations, in particular for the long induction periods preceding the cool flames. 

Niclause, Combe, and Letort - also demonstrated, between 1952 and 1955, that, in the slow thermal oxidation of gaseous ethanal and propanal at about 100-150°C., a process of degenerated chain branching appears during the course of the reaction, that is caused by the secondary formation of free radicals from the peracid, a feature which explains the aqto-... 

Degenerate chain branching may occur by the decomposition of 2-propyl hydroperoxide, as... 

Alkylperoxy radicals are direct precursors of hydroperoxides the key intermediates causing a degenerate chain-branching thus, transformations of alkylperoxy species determine the overall kinetic features of the process. 

The relative importance of different channels of further transformations of peroxy radicals determines the overall rate of reaction and selectivities to certain products. Reactions with any H-containing substance (first of all—with parent alkane) finally lead to degenerate chain-branching and exponential growth of the reaction rate... 

The rate constant of the reaction of cobalt(III) acetate with benzaldehyde in the absence of dioxygen was determined in independent experiments. It turned out to be virtually the same as the rate constant of the chain initiation in the oxidation reaction in the presence of O2- However, the contribution of chain initiation to the radical formation is insignificant in the developed oxidation process. The radicals are mainly formed in the reactions of the intermediates in the process of degenerate chain branching. These reactions are also catalyzed by transition metal ions. Especially well studied is the acceleration of radical decomposition of intermediately formed hydroperoxides (see, e.g., [10]). 

Very frequently, the phenomena described in the previous section are observed qualitatively but with considerably less sharpness, in the kind of autocatalysis associated with degenerate chain branching. Here, the active center involved in the chain branching step is not an active center at all but a relatively unstable intermediate product which, upon its decomposition or reaction provides active centers at a rate considerably faster than that of the original initiation. Thus the autocatalytic behavior can really be ascribed to a secondary initiation brought about by an intermediate product. This phenomenon happens frequently in the oxidation of hydrocarbons RH. At low temperatures, it is called autoxidation and it is autocatalytic because of the further decomposition into free radicals of hydroperoxides ROOH which are first produced in the oxidation (see p. 101). 

At higher temperatures, aldehydes play the role of degenerate chain branching agents and the case chosen for illustration will again be the oxidation of methane (see Section 5.4). The primary initiation in a mixture of methane and oxygen is due to a very slow step ... 

According to the reaction scheme shown above, the hydroperoxide formed in the chain propagation stage with the participation of an initial organic substrate is responsible for the degenerate chain branching. 

Liquid-phase oxidation of benzaldehyde belongs to the class of degenerate branching-chain process. In the kinetic model under consideration the degenerate chain branching takes place due to the catalytic decomposition of the formed peroxyacid (step 4). This reaction is also specific, because the nonradical reaction of the formed peroxyacid with the initial benzaldehyde (step 5) is the main channel for the further transformation of the peroxyacid. Here the following reviews should be mentioned [2,5,6] that provide additional information about the mechanism of liquid-phase oxidation of aldehydes. 

The characteristic time intervals mentioned above, selected from the kinetic trajectories of the value contributions, correlate with the kinetics of phenoxyl radical accumulation. It follows fiom Figure 7.8 drat completion of the first time interval (ti) corresponds to establishing die quasi-stationary mode of pora-methylphenoxyl radical accumulation. Over the time interval t2 - h the growth in the concentration of phenoxyl radicals is observed as a consequence of increasing the role of degenerate chain branching steps. Finally, mounting to the maximum value, the concentration of phenoxyl radicals decreases because of inhibitor consumption. 

Thus, the set of reactions (Equations 3.78-3.84) leads to degenerate chain branching. Moreover, the nitrous acid formed in reactions (Equations 3.75 and 3.81) increase the initiation rate via the reaction ... 

Increase in oxidation rate caused by radical chain reactions with degenerated chain branches,... 

Zolotova NV, Denisov ET. Mechanism of propagation and degenerate chain branching in the oxidation of polypropylene and polyethylene. J Polym Sci Polym Chem Ed 1971 9 3311-20. 

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