Hydroperoxides toward

Different initiators have varying transfer constants (Table 3-5). Further, the value of C) for a particular initiator also varies with the reactivity of the propagating radical. Thus there is a fivefold difference in C) for cumyl hydroperoxide toward poly(methyl methacrylate) radical compared to polystyryl radical. The latter is the less reactive radical see Sec. 6-3b. 

Failure of aliphatic aldehydes to be produced in high yields by application of this steam distillation method to estragole and 1-dodecene may be attributed to the increased stability of the intermediate hydroperoxides toward hydrolysis. In other words, the conversion rates of the corresponding intermediates, V to VI to VII, are so slow that the hydroperoxides are either steam distilled and/or undergo the relatively more rapid oxidative rearrangement to VIII and subsequent conversion to IX. 

Polyethylene (PE) is inherently less sensitive to oxidative attack than PP, but stabilization of PE is also mandatory for outdoor use. The stability varies with the type of polyethylene and manufacturing process. Linear low-density polyethylene (LLDPE) (1-octene comonomer) is significantly less sensitive to photooxidation than low-density polyethylene (LDPE) with comparable density and molecular weight [20, 21]. Generally, LDPE is less susceptible to photooxidation than high-density polyethylene (HDPE). The most fundamental difference between polyethylene homopolymers and polypropylene is the behavior of hydroperoxides toward photolysis. On photooxidation, hydroperoxides accumulate in PP, but decrease rapidly on UV exposure of PE. In copolymers of polyethylene with vinyl acetate, the stabihty depends on the content of vinyl acetate. The higher the content, the more the copolymers act like polyvinyl acetate, which is more susceptible to photooxidative degradation than polyethylene. 

Frlmer, Bartlett, Boschung and Jewett (32) have recently observed by means of deuterium and tritium Isotope effects that the transition states In the attack of singlet oxygen on 4-methyl-2,3 dlhydro-4H-pyrans deviate from the "least motion" path for the direct formation of either dloxetane or allyllc hydroperoxide, toward the type of transition state to be expected If perepoxlde formation were the rate determining step. Any final Judgment the perepoxlde as an Intermediate In dloxetane formation must await evidence as specific as that which has eliminated It In the competitive photo-epoxldatlons. 

There are many similar attributes of thermal-, photo- and radiafion-degradafion. They progress on the similar mechanisms based on the homolytic or heterolytic scissions and the formation of degradation initiators for loop process of oxidative degradation. The discrepancies between these three ageing ways consist of the concentration of primary free radicals and their distribution in the material. The depth of degradation is only some microns, but the diffusion of chain promoters, hydroperoxides, towards the inner layers of materials leads to a parabolic distribution of oxidation products around the symmetry axe. 

The parent indolo[2,3-fl]carbazole (1) has also been the subject of a study probing its reactivity toward oxidizing agents. One of the substrates involved, namely 85 (prepared from 1 and 2,5-dimethoxytetrahydrofuran in the presence of acid), was subjected to treatment with m-chloroperbenzoic acid, to give the dione 86 as the major product and a sensitive compound assigned the hydroxy structure 87. A cleaner reaction took place when 85 underwent oxidation with tert-butyl hydroperoxide assisted by VO(acac)2, to produce 86 exclusively in 86% yield. Likewise, A,N -dimethylindolo[2,3-fl]carbazole furnished the dione 88 on treatment with this combination of reagents (96J(X 413). 

The cobaltous acetate reduction of tert-butyl hydroperoxide in acetic acid yields mainly ter/-butanol and oxygen the metal ion stays in the +2 oxidation state because of the reactivity of Co(III) towards hydroperoxides (p. 378) °. The rate law is... 

The mechanism of NPYR formation has been studied by Coleman (37) and Bharucha et al. ( ). Coleman (37) reported that the requirement for a high temperature, the inhibitory effects of water and antioxidants, and the catalytic effect of a lipid hydroperoxide are consistent with the involvement of a free radical in the formation of NPYR. Similarly, Bharucha et al. (29) suggested that, since both NPYR and NDMA increase substantially towards the end of the frying process, N-nitros-amine formation during frying of bacon occurs essentially, if not entirely, in the fat phase after the bulk of the water is removed and therefore by a radical rather than an ionic mechanism. These authors speculated that, during the frying of... 

Younes, M. and Strubelt, O. (1990). The role of iron and glutathione in t-butyl hydroperoxide-induced damage towards isolated perfused rat livers. J. Appl. Toxicol. 10, 319-324. 

As previous investigations have shown26t37 a study of the "classical" protective mechanisms does not contribute much towards explaining the mode of action of HALS derivatives. An obvious next step therefore seemed to be a closer investigation of the action of these substances on radicals and hydroperoxides. 

At higher temperature, however, (> 100°C) an accelerated decomposition of hydroperoxide by the hindered amine II was very clearly seen. As Table I shows, the effect is observable in solvents with differing reactivities towards radicals. It is interesting to note that in the experiments made under nitrogen the loss of... 

Catalytic surface is active toward hydroperoxide and decomposes it to free radicals. The free radicals initiate the chain oxidation of RH in the liquid phase. 

This additive rH should be very reactive toward peroxyl radicals. It can be hydroperoxide ROOH or antioxidant InH. 

B) Phenols of this group slowly react with hydroperoxide and dioxygen. Respective phenoxyl radicals are relatively unreactive toward RH and ROOH, but can react with R02 giving rise to peroxides and then to free radicals. For these phenols, appropriate inhibitory mechanisms are I III and VI VIII. 

The experimental values of rate constants of R02 reactions with aromatic amines (AmH) are given in Database [52], The experimental measurement of the rate constant /c7 for aromatic amines from kinetics of oxidation faced with great difficulties. These difficulties arise due to the extremely high activity of aminyl radicals toward hydroperoxide [53-56], The reaction... 

Metal dialkyl dithiocarbamates inhibit the oxidation of hydrocarbons and polymers [25,28,30,76 79]. Like metal dithiophosphates, they are reactive toward hydroperoxides. At room temperature, the reactions of metal dialkyl dithiocarbamates with hydroperoxides occur with an induction period, during which the reaction products are formed that catalyze the breakdown of hydroperoxide [78]. At higher temperatures, the reaction is bimolecular and occurs with the rate v = k[ROOH][inhibitor]. The reaction of hydroperoxide with dialkyl dithiocarbamate is accompanied by the formation of radicals [30,76,78]. The bulk yield of radicals in the reaction of nickel diethyl dithiocarbamate with cumyl hydroperoxide is 0.2 at... 

The decomposition of hydroperoxide on the surface of Mo and MoS2 started after some induction period (see Table 20.2). This induction period is the time for the activation of the surface toward hydroperoxide decomposition. It was evidenced in special experiments that the catalyst is not dissolved in hydrocarbon and catalytic hydroperoxide decomposition occurs only heterogeneously. 

Hydroperoxides have been considered better oxidation agents with respect to H2O2 in view of their excellent thermal and chemical stability, the high selectivities towards desired products and their solubility in organic solvents [3]. However the industrial interest in the use of H2O2 for selective oxidations remains high in view of the advantages offered by this oxidant namely... 

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