Peroxide decomposition mechanism

Vanoppen et al. [88] have reported the gas-phase oxidation of zeolite-ad-sorbed cyclohexane to form cyclohexanone. The reaction rate was observed to increase in the order NaY < BaY < SrY < CaY. This was attributed to a Frei-type thermal oxidation process. The possibility that a free-radical chain process initiated by the intrazeolite formation of a peroxy radical, however, could not be completely excluded. On the other hand, liquid-phase auto-oxidation of cyclohexane, although still exhibiting the same rate effect (i.e., NaY < BaY < SrY < CaY), has been attributed to a homolytic peroxide decomposition mechanism [89]. Evidence for the homolytic peroxide decomposition mechanism was provided in part by the observation that the addition of cyclohexyl hydroperoxide dramatically enhanced the intrazeolite oxidation. In addition, decomposition of cyclohexyl hydroperoxide followed the same reactivity pattern (i.e., NaY < BaY... 

Peroxide Decomposition Mechanism. Since virtually no work has been reported which concerns only the mechanism by which zinc dialkyl di-thiophosphates act as peroxide decomposers, it is pertinent to discuss metal dialkyl dithiophosphates as a whole. The mechanism has been studied both by investigating the products and the decomposition rates of hydroperoxides in the presence of metal dithiophosphates and by measuring the efficiency of these compounds as antioxidants in hydrocarbon autoxidation systems in which hydroperoxide initiation is significant. 

Fig. 7.5. Peroxide crosslinking mechanism of an elastomer. This mode of decomposition occurs in neutral or alkaline conditions, as normally prevails during the crosslinking of millable polyurethane rubber. If an acidic phase predominates then little or no crosslinking occurs as a different peroxide decomposition mechanism takes place.

As indicated above, a second class of antioxidation is the peroxide decomposition mechanism, which has proven to be very important for elastomers. The peroxide decomposition for tris(nonyl phenyl)phosphite antioxidant (Formula 4.2) is shown in Formula 4.11. 

The structure and stability of PB samples allowed to oxidize to various extents have been examined. Oxidation leads to a considerable lowering of the threshold temperature for the main decomposition process in samples oxidized at 0-20 °C an additional early stage of degradation commencing near 100 °C is found, due to peroxide decomposition. Mechanisms have been presented. 

The mechanism and rate of hydrogen peroxide decomposition depend on many factors, including temperature, pH, presence or absence of a catalyst (7—10), such as metal ions, oxides, and hydroxides etc. Some common metal ions that actively support homogeneous catalysis of the decomposition include ferrous, ferric, cuprous, cupric, chromate, dichromate, molybdate, tungstate, and vanadate. For combinations, such as iron and... 

The mechanism of chemiluminescence is still being studied and most mechanistic interpretations should be regarded as tentative. Nevertheless, most chemiluminescent reactions can be classified into (/) peroxide decomposition, including biolurninescence and peroxyoxalate chemiluminescence (2) singlet oxygen chemiluminescence and (J) ion radical or electron-transfer chemiluminescence, which includes electrochemiluminescence. 

Decomposition of diphenoylperoxide [6109-04-2] (40) in the presence of a fluorescer such as perylene in methylene chloride at 24°C produces chemiluminescence matching the fluorescence spectmm of the fluorescer with perylene was reported to be 10 5% (135). The reaction follows pseudo-first-order kinetics with the observed rate constant increasing with fluorescer concentration according to = k [flr]. Thus the fluorescer acts as a catalyst for peroxide decomposition, with catalytic decomposition competing with spontaneous thermal decomposition. An electron-transfer mechanism has been proposed (135). 

Decomposition Hazards. The main causes of unintended decompositions of organic peroxides are heat energy from heating sources and mechanical shock, eg, impact or friction. In addition, certain contaminants, ie, metal salts, amines, acids, and bases, initiate or accelerate organic peroxide decompositions at temperatures at which the peroxide is normally stable. These reactions also Hberate heat, thus further accelerating the decomposition. Commercial products often contain diluents that desensitize neat peroxides to these hazards. Commercial organic peroxide decompositions are low order deflagrations rather than detonations (279). 

Drier Mechanism. Oxidative cross-linking may also be described as an autoxidation proceeding through four basic steps induction, peroxide formation, peroxide decomposition, and polymerization (5). The metals used as driers are categorized as active or auxiUary. However, these categories are arbitrary and a considerable amount of overlap exists between them. Drier systems generally contain two or three metals but can contain as many as five or more metals to obtain the desired drying performance. 

Fig. 3.3.4 Reaction mechanism of the coelenterazine bioluminescence showing two possible routes of peroxide decomposition, the dioxetanone pathway (upper route) and linear decomposition pathway (lower route). The Oplopborus bioluminescence takes place via the dioxetanone pathway. The light emitter is considered to be the amide-anion of coelenteramide (see Section 5.4).

The rates of thermal decomposition of diacyl peroxides (36) are dependent on the substituents R. The rates of decomposition increase in the series where R is aryl-primary alkyKsecondary alkyKtertiary alkyl. This order has been variously proposed to reflect the stability of the radical (R ) formed on (i-scission of the acyloxy radical, the nucleophilicity of R, or the steric bulk of R. For peroxides with non-concerted decomposition mechanisms, it seems unlikely that the stability of R should by itself be an important factor. 

The chemistry of the di-/-butyl and cumyl peroxides is relatively uncomplicated by induced or ionic decomposition mechanisms. However, induced decomposition of di-/-butyl peroxide has been observed in primary or secondary alcohols31" "14 (Scheme 3.37) and primary or secondary amines.312 The reaction... 

The rate of peroxide decomposition and the resultant rate of oxidation are markedly increased by the presence of ions of metals such as iron, copper, manganese, and cobalt [13]. This catalytic decomposition is based on a redox mechanism, as in Figure 15.2. Consequently, it is important to control and limit the amounts of metal impurities in raw rubber. The influence of antioxidants against these rubber poisons depends at least partially on a complex formation (chelation) of the damaging ion. In favor of this theory is the fact that simple chelating agents that have no aging-protective activity, like ethylene diamine tetracetic acid (EDTA), act as copper protectors. 

The catalysis of hydrogen peroxide decomposition by iron ions occupies a special place in redox catalysis. This was precisely the reaction for which the concept of redox cyclic reactions as the basis for this type of catalysis was formulated [10-13]. The detailed study of the steps of this process provided a series of valuable data on the mechanism of redox catalysis [14-17]. The catalytic decomposition of H202 is an important reaction in the system of processes that occur in the organism [18-22]. 

The generation of the benzoyloxyl radical relies on the thermal or photoinitiated decomposition [reaction (49)] of dibenzoyl peroxide (DBPO). An early study (Janzen et al., 1972) showed that the kinetics of the thermal reaction between DBPO and PBN in benzene to give PhCOO-PBN" could be followed by monitoring [PhCOO-PBN ] from 38°C and upwards. The reaction was first order in [DBPO] and zero order in [PBN], and the rate constants evaluated for the homolysis of the 0—0 bond in DBPO (k = 3.7 x 10-8 s-1 at 38°C) agreed well with those of other studies at higher temperatures. Thus in benzene the homolytic decomposition mechanism of DBPO seems to prevail. 

On the basis of mechanistic studies, mainly on these isolable cychc four-membered peroxides (1 and 2), two main efficient chemiexcitation mechanisms can be defined in organic peroxide decomposition (i) the unimolecular decomposition or rearrangement of high-energy compounds leading to the formation of excited-state products, exemplified here in the case of the thermal decomposition of 1,2-dioxetane (equation i)". 5,i9. 

In this part of the chapter we briefly outline the main classes of mechanisms occurring in chemiluminescent transformations of organic peroxides. The transformations involving isolated 1,2-dioxetanes and related species will not be extensively discussed, since a specific chapter is dedicated to these compounds. We therefore limit ourselves to describing the general decomposition mechanisms of these peroxides, as these are important in the context of the more complex CL systems that we will describe in the last part of this chapter. 

The experimentally observed substituent effect on the triplet and singlet quantum yields in the complete series of methyl-substituted dioxetanes, as well as the predicted C—C and 0—0 bond strength for the four-membered peroxidic rings , have led to the hypothesis that a more concerted, almost synchronized, decomposition mechanism should lead to high excitation quantum yields (as in the case of tetramethyl-l,2-dioxetane), whereas the biradical pathway presumably leads to low quantum yields (as in the case of the unsubstituted 1,2-dioxetane)" . However, it appears that this criterion of concertedness is difficult to apply generally to structurally dissimilar dioxetane derivatives. 

Unimolecular peroxide decomposition chemiluminescence, 1227-31 asynchronous concerted mechanism, 1230 biradical mechanism, 1181-2, 1227-31 concerted mechanism, 1227, 1228-9, 1230... 

The inhibition of hydrocarbon autoxidation by zinc dialkyl dithiophosphates was first studied by Kennerly and Patterson (13) and later by Larson (14). In both cases the induction period preceding oxidation of a mineral oil at 155 °C. increased appreciably by adding a zinc dialkyl dithiophosphate. In particular, Larson (14) observed that zinc salts containing secondary alkyl groups were more efficient antioxidants than those containing primary groups. In these papers the inhibition mechanism was discussed only in terms of peroxide decomposition. 

No readily acceptable mechanism has been advanced in reasonable detail to account for the decomposition of hydroperoxides by metal dialkyl dithiophosphates. Our limited results on the antioxidant efficiency of these compounds indicate that the metal plays an important role in the mechanism. So far it seems, at least for the catalytic decpmposition of cumene hydroperoxide on which practically all the work has been done, that the mechanism involves electrophilic attack and rearrangement as shown in Scheme 4. This requires, as commonly proposed, that the dithiophosphate is first converted to an active form. It does seem possible, on the other hand, that the original dithiophosphate could catalyze peroxide decomposition since nucleophilic attack could, in principle, lead to the same chain-carrying intermediate as in Scheme 4 thus,... 

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