Branching, oxidation reactions

The nature of dangerous reactions involving organic chemicals depends on the saturated, unsaturated or aromatic structures of a particular compound. Saturated hydrocarbons are hardly reactive, especially when they are linear. Branched or cyclic hydrocarbons (especially polycyclic condensed ones) are more reactive, in particular as with oxidation reactions. With ethylenic or acetylenic unsaturated compounds, the products are endothermic . 

As already noted (see Chapter 4), autoxidation is a degenerate branching chain reaction with a positive feedback via hydroperoxide the oxidation of RH produces ROOH that acts as an initiator of oxidation. The characteristic features of inhibited autoxidation, which are primarily due to this feedback, are the following [18,21,23,26,31-33] ... 

The mechanisms for the NMHCs (except DMS) required to fully characterise OH chemistry were extracted from a recently updated version of the Master Chemical Mechanism (MCM 3.0, available at http //mcm.leeds.ac.uk/MCM/). The MCM treats the degradation of 125 volatile organic compounds (VOCs) and considers oxidation by OH, NO3, and O3, as well as the chemistry of the subsequent oxidation products. These steps continue until CO2 and H2O are formed as final products of the oxidation. The MCM has been constructed using chemical kinetics data (rate coefficients, branching ratios, reaction products, absorption cross sections and quantum yields) taken from several recent evaluations and reviews or estimated according to the MCM protocol (Jenkin et al., 1997, 2003 Saunders et al., 2003). The MCM is an explicit mechanism and, as such, does not suffer from the limitations of a lumped scheme or one containing surrogate species to represent the chemistry of many species. 

These results may be explained on the basis that xylan possesses a linear chain of 1,4-linked anhydroxylose units. However, it should be pointed out that on hydrolysis of the oxidized xylan, some D-xylose is found in addition to the expected glyceraldehyde and glyoxal.I04(b) While it is possible that the oxidation reaction is not complete, another explanation is that a branched xylan chain may be present. If branching occurs on a pentose chain unit, only one free hydroxyl would remain on the unit hence, it would not be oxidized or degraded during the course of the periodate reaction. 

The sequence [Eqs. (17)—(20)] is of great importance in the oxidation reaction mechanisms of any hydrocarbon in that it provides the essential chain branching and propagating steps as well as the radical pool for fast reaction. 

Such reactions have been used to explain the three limits found in some oxidation reactions, such as those of hydrogen or of carbon monoxide with oxygen, with an "explosion peninsula between the lower and the second limit. However, the phenomenon of the explosion limit itself is not a criterion for a choice between the critical reaction rate of the thermal theory and the critical chain-branching coefficient of the isothermal-chain-reaction theory (See Ref). For exothermic reactions, the temperature rise of the reacting system due to the heat evolved accelerates the reaction rate. In view of the subsequent modification of the Arrhenius factor during the development of the reaction, the evolution of the system is quite similar to that of the branched-chain reactions, even if the system obeys a simple kinetic law. It is necessary in each individual case to determine the reaction mechanism from the whole... 

The methoxy radical may subsequently react to form formaldehyde (H atom abstraction) or methanol (H atom addition). The sequence of reactions (R15) through (R17) is strongly chain branching and serves to build up a radical pool. Once this radical pool is established, another chain-branching oxidation route becomes dominating. Methane consumption now occurs mainly by the reactions [254]... 

A prototype study for this issue was performed for the conversion of ethane to acetic acid [71] and the same group highlighted in an earlier comparative study of C3 oxidation [54] that, although initial propane activation is a difficult step, subsequent reactions associated with either excessive residence times of intermediates or with branching of reaction sequences into total oxidation may interfere with the overall selectivity to partial oxidation products. 

The first term on the right represents the initial dissociation, the second the branching chain reactions, and the third recombination. Coefficients A, B and C are functions of the rate constants and the concentration of fuel and oxidizer, but are independent of the intermediate concentration. 

Fig. 2 shows clearly that the quasi-equilibrium radical concentration sets the rate of fuel consumption and chemical heat release. It also shows the stability. Whatever the initial value of [r] it moves towards [R]e and remains there. It can only increase as [R]e increases with temperature. Thus, though the oxidation of methane is a branching chain reaction, fuel... 

In initiation of oxidation, the important role may also be played by reactive bonds in a polymer. With polyethylene which is the most simple structurally, such reactive sites may be small concentrations of double bonds or more numerous sites of branching of a main chain to side alkyls. Investigation of the oxidation reaction of different types of polyethylene has. however, revealed that the degree of polyethylene branching from 0 to 20 branches for each 1000 carbon atoms of the main chain does not affect the induction period of oxidation [13]. 

The formation of oxyl radicals in reaction of alkyl radicals with oxygen is important for the possible occurrence of branching of oxidation reaction due to oxygen atoms. This aspect is not, however, obvious from the experiments performed. It seems that oxyl radicals in HDPE are not formed in propagation reaction of alkyl radicals and oxygen but as the product of termination of two peroxyl radicals. 

I) The oxidation of DMS fluid occurs by a free radical, branching chain reaction which is initiated by oxygen attack on methyl groups (22, 23, 25, 35, 39). 

In a two-phase system similar to that used by Price, Stamatoff (29) obtained from 2,6-dichloro-4-bromophenol a branched polymer having approximately the statistical ratio of ortho and para ether linkages. When the reaction was carried out using the anhydrous salt of the phenol in the presence of highly polar aprotic solvents, such as dimethyl sulfoxide, the product was the linear poly(2,6-dichlorophenylene oxide) (Reaction 23). 

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