Chain termination reactions oxidations

The last reaction occurs much rapidly than the disproportionation of two cumylperoxyl radicals and accelerates chain termination in oxidized cumene [15]. The addition of cumene hydroperoxide helps to avoid the influence of the cross termination reaction Me2PhCOO + CH302 on the oxidation of cumene and to measure the pure disproportionation of cumylperoxyl radicals [15]. 

Rate Constants of Chain Termination in Oxidized Hydrocarbons by the Reaction of Alkyl Radicals with Peroxyl Radicals at 293 K in RH Solution... 

So, these reactions cannot lead to effective chain termination in oxidized alcohol. The decomposition of tetroxides depends on pH and apparently proceeds homolytically as well as heterolytically in an aqueous solution. The values of the rate constants (s 1) of tetroxide decomposition at room temperature in water at different pH values are given below [38,39],... 

Disproportionation of ester peroxyl radicals occurs very rapidly. Apparently, the ester group influences the rate constant of this reaction by increasing it. The rate constants of chain termination in oxidized esters are collected in Table 9.13. 

Rate constants for the self-reactions of a number of tertiary and secondary peroxy radicals have been determined by electron spin resonance spectroscopy. The pre-exponential factors for these reactions are in the normal range for bi-molecular radical-radical reactions (109 to 1011 M"1 sec 1). Differences in the rate constants for different peroxy radicals arise primarily from differences in the activation energies of their self reactions. These activation energies can be large for some tertiary peroxy radicals (—10 kcal. per mole). The significance of these results as they relate to the mechanism of the self reactions of tertiary and secondary peroxy radicals is discussed. Rate constants for chain termination in oxidizing hydrocarbons are summarized. 

It is considerably more difficult to inhibit oxidation in the gas phase than in the liquid phase. At the high temperatures of gas-phase oxidations the rates of the chain-propagating and branching reactions are increased to a greater extent than the rates of the chain-terminating reactions. Initiation by surfaces can also constitute a serious problem. The majority of liquid-phase antioxidants which are effective at high temperatures are too involatile to be useful in the gas phase. However, inhibition can be achieved with aliphatic amines, which are generally rather ineffective inhibitors of low temperature liquid-phase oxidations. The mechanisms by which the different types of antioxidants inhibit oxidation are briefly described below. 

Reactions Limited by Rotational Diffusion in Polymer Matrix Antioxidants Reacting with Peroxyl Radicals Antioxidants Reacting with Alkyl Radicals Cyclic Chain Termination in Oxidized Polymers... 

An equimolecular amount of tertbutyl oxide and tert-butyl peroxide radicals are formed, which can initiate the polymerization of styrene consequently, considering that the chain termination reaction of polystyrene proceeds by coupling, it results that most of the polystyrene molecules will contain one or two terminal peroxide groups. These have been used for a second step polymerization, e.g. of methyl methacrylate. In this case also, it seems that the macro end radical should be much more efficient for initiating polymerization than the small tert-butyl oxide radical (47). The styrene content of the block copolymer was about 35%. 

Kovtun G, Kameneva T, Hladyi S et al (2002) Oxidation, redox disproportionation and chain termination reactions catalysed by the Pd-561 giant cluster. Adv Synth Catal 344(9) 957-964... 

The formation of peroxides is effectively a chain termination reaction, as under most conditions these peroxides can act as effective sinks for HO . In more polluted conditions (high-NO ), peroxy radicals catalyse the oxidation of NO to NO2... 

There is an interesting exception to this observation. As noted above, aldehyde oxidations tend to be very fast and to have relatively long kinetic chain lengths. Most chain terminations occur via bimolecular reactions of acylperoxy radicals (eq. (7a)) these reactions result in carbon dioxide generation and are inefficient. If one adds manganese catalyst, some of the acylperoxy radicals will be reduced to peroxy acid and Mn " will be produced. Mn can carry the chain via an analog of reaction (18), but does not participate in chain termination reactions. As a result, kinetic chain lengths and rates tend to increase. 

Termination of the radical chain reaction As the reaction proceeds, autoxidation is followed by an autoretardation stage, resulting in a standstill before the hydrocarbon is completely consumed. This autotermination is called the chain termination reaction and dominates in this final phase of the oxidation process such that degradation comes to a halt. Termination may be effected by the combination of radical species such as peroxy radicals to yield ketones and alcohols. Reaction sequence (4.14) ... 

The oxidation of hydrocarbons, reactions (1), (la), (2)—(4), is inhibited to an extent that depends on the efficiency of chain termination, reactions (5), (6), (8)—(10), on the possibility of chain transfer and regeneration, reactions (lb), (5) and (7), and on the possibility of degradation of hydroperoxides to inert products, reaction (11). Amines and phenols are known to be efficient chain breaking inhibitors, while sulphides promote reaction (11). 

Dependence of P0, on NO, Abundance in CO Oxidation One of the key aspects of tropospheric chemistry is the dependence of ozone production on the NO, abundance. We have derived relationships for Po, for the CO system in the limits of low and high NO,. Here we examine how Po, depends on the NO, abundance over the complete range of NO, levels. To do this, we will fix the rate of HO, production, Pho,. and vary the NO concentration at a fixed N02/NO ratio. Under conditions of high H02 radical abundance relative to NO, the primary chain-terminating reaction is the HO, + HO, reaction, HO2 + H02. This condition is referred to as NOx-limited. At sufficiently high NO, levels, chain termination results from the HO, + NO, reaction, OH 4- N02. This condition is called NOx-saturated. By varying the NO, concentration, we can explore the point at which the system crosses over from NO,-limited to NO,-saturated conditions. The crossover point occurs at the NO concentration where 3Poj/3[NO] = 0. The actual value of the NO concentration at this crossover point depends on the values of Pho, and the NO2/NO ratio. 

Nitrous acid is an excellent chain terminator reactions of the oxides NO and NOa may therefore be of significance in many diazo reactions, whenever a slight excess of nitrite has not been removed, in changing homolytic to heterolytic processes. 

These NO oxidation processes by peroxy radicals play very important role in the NOx removal using the NTPs. Formation of PAN is also one of chain termination reaction between peroxyacctyl radical (RC(0)02) radical and NO2. 

There are two major classes of antioxidants and they are differentiated based on their mechanism of inhibition of polymer oxidation chain-terminating or primary antioxidants and hydroperoxide-decomposing secondary antioxidants [5]. Primary or free-radical scavenging antioxidants inhibit oxidation via very rapid chain-terminating reactions. The majority of primary antioxidants are hindered phenols or secondary aryl amines. Generally, hindered phenols are nonstaining, nondiscoloring, and are available in a wide... 

The high recombination rates of benzylperoxy radicals (Table of VII. 1), resulting in the formation of benzyl alcohol and benzaldehyde ( ) [24, 25], and low stability of benzyl hydroperoxide, BH, do not permit significant selectivity in BH formation already at low extents of oxidation. Benzaldehyde, formed at decomposition of BH and in chain termination reaction ( ), is then oxidized at high rates to benzoic acid. 

High selectivity of cumene oxidation to hydroperoxide can be connected with possibility of formation of cumyl hydroperoxide, CH, in chain termination reaction [33]. 

The oxidation of ascorbic acid is greatly favored by the presence of oxygen, oxygen radicals or metal ions, especially and Cu +, and alkaline pH. In the absence of catalytic metals ascorbate is stable at neutral pH (13). The most important reducing property of ascorbic acid in biological systems is the radical chain terminating reaction (reaction 1). 

Having established that the polymerization of propylene oxide by the zinc hexacyanocobaltate complex catalyst proceeds in the absence of a chain termination reaction, it was of interest to study separately the chain propagation reaction. A non-termi-nated polymer "seed" was prepared and its rate of reaction with propylene oxide was studied. The results of a series of these seeded polymerizations carried out at 30°, 40° and 50° are given in Figvire 6, where log [M]o/[M]t is plotted against time. 

In the free troposphere with low NOx concentration, the chain termination reaction by the cross radical reaction between HO2 and CH3O2 formed in the oxidation of CH4 is important in additimi to the self-reaction of HO2. In the polluted atmosphere where cmicentrations of organic peroxy radicals (RO2) are high, their cross radical reactions with HO2 also need to be considered in the model calculation of photochemical ozone formation. Here, as a representative radical-radical reaction of RO2, the reaction of HO2 and CH3O2 is described. 

These and other decomposition reactions will be considered in Section 11.3. Chain termination in oxidized hydrocarbons in the absence of inhibitors occur via bimolecular reactions... 

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