Degenerate branching intermediates

In the oxidation of PX and MX, formaldehyde is a degenerate branching intermediate, whereas phthalan is formed from OX ... 

The re-oxidation of lead was expected to occur rapidly at 600 K the surface of the lead oxide would then remain unchanged in the presence of oxygen. The authors concluded that, as a consequence, general hydrocarbon combustion in which formaldehyde is a degenerate branching intermediate is inhibited in the presence of PbO by the rapid removal of formaldehyde. 

Figure 6 shows the variation of peroxide concentration in methyl ethyl ketone slow combustion, and similar results, but with no peracid formed, have been found for acetone and diethyl ketone. The concentrations of the organic peroxy compounds run parallel to the rate of reaction, but the hydrogen peroxide concentration increases to a steady value. There thus seems little doubt that the degenerate branching intermediates at low temperatures are the alkyl hydroperoxides, and with methyl ethyl ketone, peracetic acid also. The tvfo types of cool flames given by methyl ethyl ketone may arise from the twin branching intermediates (1) observed in its combustion. 

It is clear that the concept of formaldehyde as the relatively stable intermediate responsible for degenerate branching is a fundamental one in elaborating the kinetics of methane oxidation. Indeed the statement has been made (64) that the combustion of methane may be regarded as the formation and oxidation of formaldehyde. 

The formaldehyde concentration built up to a maximum which was reached at the time of maximum rate, and thereafter declined. The addition of formaldehyde reduced the induction period, and when an amount was added approximately equal to that present at the maximum rate, oxidation commenced immediately at the maximum rate. Consequently, it was believed that formaldehyde was the intermediate responsible for degenerate branching [29]. 

The kinetics of the slow reaction and the cool-flame and explosion limits for the gas phase oxidation of this compound between 240 and 340 °C have been investigated, and the proposed mechanism involves hydroperoxides as the intermediates responsible for degenerate branching [91(b)] ... 

The slow combustion of methylene chloride is a degenerately branched chain reaction it proceeds by a mechanism similar to that involved in the pyrolysis of the same compound which takes place at a slightly higher temperature [153]. The primary chains are the same and several of the chlorinated hydrocarbon minor products are identical. Oxygen is only involved in the conversion of the intermediate dichloroethylene to the final products hydrogen chloride and carbon monoxide. 

The first steps towards an explanation of these extra modes of hydrocarbon oxidation were taken by Frank-Kamenetskii who proposed that the multiple cool flames observed were oscillatory and that the period of oscillation reflected the underlying chemistry. Experimental investigations continued to concentrate upon traditional measurements of pressure versus time, of induction period, and o establishing the identity of stable intermediates and reaction pathways. Interpretations of these results continued to be made on isothermal degenerate branched-chain reactions direct measurements of temperature were not made. These interpretations were very incomplete, and much better understanding has emerged from application of thermokinetic theory. 

The difference between the kinetic behavior of conventional branching and degenerate- branching reactions is conditioned by the fact that for degenerate-branching reactions the act of branching under which the multiplication of carrier chain occurs, proceeds with the participation of a mediator , that is an intermediate molecular (nonradical) prodnct. 

For a degenerate branching chain reaction, the rate of chain carrier and the intermediate product P acciunulation is expressed by the following equations ... 

For degenerate branching chain reactions, in contrast to branching ones, the steady-state concentration approach is often valid for concentrations of all active intermediates dnidt = 0 at t >rch. Taking into account negligible consumption of the initial reagent one can derive from equations (1.15), respectively... 

Degenerate branching-chain reactions. As presented in Chapter 1, for such reactions the accumulation rate of chain carriers (n) and the intermediate product (P) responsible for the degenerate branching of chains is described by the following equations [51] ... 

As to the question which of the intermediates is responsible for degenerate branching, the bulk of experimental data available lead to the following conclusions. 

Hydrocarbons are oxidized without the introduction of a radical source but this oxidation occurs with autoacceleration. This autoacceleration was explained in the framework of the theory of degenerate-branched chain reactions by the formation of an intermediate product, initiator. It was proved in 1930-50 that these products are hydroperoxides (see above). The Bach-Engler peroxide theory was thus merged with Semenov s theory of degenerate branching. Soviet scientists made the decisive contribution to the development of this area. 

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. 

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. 

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