Quinolide peroxide

With 2,4,6-trialkylphenols used as inhibitors, the formed phenoxyl radicals produce quinolide peroxides by the reactions with peroxyl radicals. At sufficiently high temperatures, quinolide peroxides decompose giving rise to free radicals [18,31,32,53,54] ... 

It is seen that the decomposition of quinolide peroxides shortens the induction period. 

The 2,4,6-substituted phenoxyl radicals recombine slowly and selectively react with per-oxyl radicals, producing quinolidic peroxides [57]. 

The formed quinolide peroxides become unstable at elevated temperatures and decompose into free radicals. The rate constants of decomposition for two peroxides are given below [4] ... 

The phenoxyl radical has an increased electron density in the ortho- and pura-positions and adds dioxygen similar to alkyl radicals. However, the C—00 bond is weak in this peroxyl radical and back dissociation occurs rapidly. Therefore, the formation of quinolide peroxide occurs in two steps, which was studied for the 2,4,6-tris(l,l-dimethylethyl)phenoxyl radical [100,101],... 

Radicals cancel out each other mainly in the reaction of peroxyl radicals with phenox-yls to form quinolidic peroxide (see Chapter 6). At temperatures above 420 K, quino-lidic peroxide breaks down into radicals, which diminishes the inhibitory effect. Therefore, the synergistic effect of a mixture of AmH and ArOH can be observed in the temperature interval rmin to Tmax. 

It should be taken into account that the reaction of chain propagation occurs in polymer more slowly than in the liquid phase also. The ratios of rate constants kjlkq, which are so important for inhibition (see Chapter 14), are close for polymers and model hydrocarbon compounds (see Table 19.7). The effectiveness of the inhibiting action of phenols depends not only on their reactivity, but also on the reactivity of the formed phenoxyls (see Chapter 15). Reaction 8 (In + R02 ) leads to chain termination and occurs rapidly in hydrocarbons (see Chapter 15). Since this reaction is limited by the diffusion of reactants it occurs in polymers much more slowly (see earlier). Quinolide peroxides produced in this reaction in the case of sterically hindered phenoxyls are unstable at elevated temperatures. The rate constants of their decay are described in Chapter 15. The reaction of sterically hindered phenoxyls with hydroperoxide groups occurs more slowly in the polymer matrix in comparison with hydrocarbon (see Table 19.8). 

The oxidation of PIB occurs mainly via intramolecular addition of dioxygen to double bonds of polymer. The reaction of peroxyl radical addition to the phenoxyl radical leads to the formation of quinolide peroxide (see Chapter 15). This peroxide is unstable, and its decomposition provokes the degradation of PIB. Another reaction predominates in case of aromatic diamine. 

Q 2 denotes symmetric para, rara-quinolide peroxide... 

Recombination of 2,4,6-trialkyl-substituted phenoxyl radicals with peroxyl ones results in quinolide peroxides, which decompose at relatively elevated temperatures forming free radicals that, in turn, promote the chain process [11]. 

QPi is symmetric para, /lora-quinolide peroxide, for example ... 

It was supposed that jara-quinolide hydroperoxide decomposes yielding free radicals with the same rate constants as para, /jora-quinolide peroxide have. 

When comparing the data of these figures one can clearly see the symbasis between the accumulation of phenoxyl radicals and the dynamics of quadratic steps with their participation, steps (22) and (24). Here, the growth of negative influence of steps (27), (29), (10) may be tracked at the detectable consumption of the inhibitor, when the hydroperoxide of ethylbenzene and the quinolide peroxides are accumulated in the system. 

The main negative effect is contributed by the steps of autoinitiation (steps 10,29), due to the radical decomposition of hydroperoxide and the non-symmetric quinolide peroxide (step 29), as well as by the step of exchange of phenoxyl radicals for the peroxyl ones (27). It should be noted that the role of these steps in ethylbenzene oxidation inhibited by BHT is exclusively negative at all the studied temperatures, including the one at 37°C. 

Contribution of the step of radical decomposition of quinolide peroxide increases substantially at elevated temperatures. So, in some cases the contribution of this step at 120°C may exceed that of the step of the radical decomposition of the ethylbenzene... 

And finally, the point of view set forth by V.A. Roginsky [11] seems to be quite convincing, stating that the molecular structure of BHT is elose to that of the optimal inhibitor from this class of substances. Indeed, phenoxyl radieals formed from BHT most intensively react with each other, espeeially at the high inhibitor eoneentrations. At the same time, as this reaction occurs intensively, it causes a fall in the possibility of initiating agents (quinolide peroxides) formation by reactions between the phenoxyl and the peroxyl radicals (steps 24 and 25). Of no small importance is also the relatively low aetivity of the phenoxyl radicals in the reaction with hydroperoxide, playing a crucial role in the inhibition of the oxidation process. 

Thus, the value analysis enables to structure chemically the prognosis. As a result new experiments can be planned that are described by constructing the kinetic models, to provide a more reliable prediction of the behavior of an inhibited reaction. For example, it can be recommended to study the reactions imder the conditions of lower initiation rates so that the pro-oxidant role of the inhibitor is unsuppressed. Or, alternatively, to plan experiments with the additions of hydrogen peroxide, hydroperoxide, quinolide peroxides that would reveal a wider set of steps in the base mechanism required to perform an adequate prognosis. However, as it follows from the results obtained at 120 and the reliable kinetic information about the initial reaction mechanism, the analysis of the inhibited reaction is evidently valid also for 60 °Cand37°C. 

For the reaction of the ethylbenzene oxidation inhibited by BHT the negative contribution of steps with the participation of the quinolide peroxides formed just by the transformations of ortAo-substituted phenols is characteristic. In the case of applying BHT, as a rule, a substantial negative contribution is introduced by the reaction of the phenoxyl radical with the hydroperoxide, the reverse reaction of the peroxyl radical with BHT. At first sight this result seems to be imexpected, so far as the rate constant of the reaction between the ortho-substituted phenoxyl radical and the hydroperoxide is significantly smaller than that of the similar reaction with the ortAo-nonsubstituted phenoxyl radicals. As numerical experiments have shown, the reason is that imlike the ori/ o-nonsubstituted phenols, in the case of the ortAo-substituted phenols the reaction occurs under a kinetic mode with relatively high predominance in the concentration of the phenoxyl radicals over the peroxyl ones. In other words, in the case of BHT the role of the reaction between the phenoxyl radical and the hydroperoxide increases at the expense of the concentration factor. 

Quinolide peroxide InOOR is formed in the addition of RO 2 to 2,4,6-tri-alkylphenoxyl radical. Phenoxyl with the alkoxy substituent in the para- or orthoposition react in the decomposition reaction (40) to form the allqrl radical and... 

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