Peroxides, reactions

A routine preparation by interaction of allyl chloride and sodium thiocyanate in an autoclave at 5.5 bar exploded violently at the end of the reaction. Peroxides were not present or involved and no other cause could be found, but extensive decomposition occurred when allyl isothiocyanate was heated to 250°C in glass ampoules. Exothermic polymerisation seems a likely possibility. 

Luminol derivatives produce emission of light by oxidation with oxygen and hydrogen peroxide under alkaline conditions. By utilizing this reaction, peroxides such as hydrogen peroxide and lipid hydroperoxides can be determined after HPLC separation. Metal ions [e.g., iron(II), cobalt(II), etc.] catalyzing the luminol CL reaction can also be determined. 

Having a weak O—O bond, peroxides split easily into free radicals. In addition to homolytic reactions, peroxides can participate in heterolytic reactions also, for example, they can undergo hydrolysis under the catalytic action of acids. Both homolytic and heterolytic reactions can occur simultaneously. For example, perbenzoates decompose into free radicals and simultaneously isomerize to ester [11]. The para-substituent slightly influences the rate constants of homolytic splitting of perester. The rate constant of heterolytic isomerization, by contrast, strongly depends on the nature of the para-substituent. Polar solvent accelerates the heterolytic isomerization. Isomerization reaction was proposed to proceed through the cyclic transition state [11]. 

Very poor biodiesel and biodiesel blends do not shed water as effectively as conventional diesel fuel fuel haze, gelling, and low-temperature handling problems can develop if biodiesel is contaminated with water in storage and transport. Poor double bonds present in the methyl ester compounds are active sites for oxidation and condensation reactions peroxide values can increase fuel darkening and deposit formation in storage systems can occur the addition of oxidation inhibitors to biodiesel helps improve storage stability. 

Another function of ascorbic add is to generate Fe(II) from Fe (III), which is part of the Haber-Weiss-Fenton Reaction (77). In that reaction, peroxide is reduced to the hydroxyl radical, HO, an extremely powerful oxidizing agent. It should also be noted that ascorbic acid can chelate metals 18 and will promote the carbohydrate-amine browning reaction (79-20). 

The production of vinyl chloride monomer is only a part of PVC production. Polymerization of the monomer completes the process. Commercially, it is a batch operation by one of three methods suspension, emulsion, or bulk. In all three methods, the chemical reaction is a free radical-initiated chain reaction. Peroxides or redox systems generally are used to provide the initial free radicals. 

In addition to the thermal initiation, the use of peroxides or azo components is a common and well established method to start the chain reaction. Peroxides increase the rate of the polymerization process and improve the grafting efficiency in the case of IPS. More recently, multifunctional peroxides have also been used in order to obtain products with special molecular weight distributions. 

Higher temperatures clearly promote pigment adsorption and various catalytic reactions (peroxide decomposition, triacylglycerol hydrolysis). As a consequence, pigment and peroxide levels drop with increasing temperatures, whereas free fatty acids levels increase dramatically (Figure 23) because of increased triacylglycerol hydrolysis reactions. 

Further radical reactions peroxidation and chain scission... 

Prooxidant effects were observed at higher antioxidant concentrations and at higher temperatures. Reversal of the direction of the isotope effects observed under these conditions showed that initiation by direct reaction of the antioxidant with oxygen is an important initiation reaction. Peroxide decomposition is quite slow at 90 °C and begins to contribute significantly to initiation only at the start of a second stage of more rapid, but still retarded, autoxidation. We have suggested (4) that some oxidation product of polymer or antioxidant may induce hydroperoxide decomposition. 

Edwards, J. O., cd., Inorganic Reaction Mechanisms, Vol. 13 of Progr. Inorg. Chem., Interscience, 1970 (authoritative reviews on cobalt binuclear complexes, fast reactions, peroxide reactions, redox processes, electron transfer and substitution in square r/ complexes). 

A property of superoxide to act as an oxidizing agent is much more ambiguous. Thermodynamically, an electron transfer to bare superoxide is almost impossible because the product of this reaction, peroxide dianion (02 ), is highly imstable. Therefore, the reduction of superoxide is either a proton-coupled process (Eq. (3)) or metal-assisted reaction (Eq. (4)), where the latter requires coordination of 02 and subsequent inner-sphere electron transfer. It is also possible to think in terms of hydrogen atom transfer reactions as a special sort of proton-coupled electron-transfer processes (Eq. (5)). 

What does the peroxide do to change the mechanism of the reaction Peroxides undergo homolysis of the weak 0—0 bond extremely easily to form two radicals. We said that HCl in the gas phase undergoes homolysis in preference to heterolysis other types of bond are even more susceptible to homolysis. You can see this for yourself by looking at this table of bond dissociation energies (AG for X-Y X" -i- Y ). 

Yawalkar et al. (2001) has developed a model for a three-phase reactor based on the use of a dense polymeric composite membrane containing discrete cubic zeolite particles (Fig. 4.5) for the epoxidation reaction of alkene. Catalytic particles of the same size are assumed vdth a cubic shape and uniformly dispersed across the polymer membrane cross-section. Effects of various parameters, such as peroxide and alkene concentration in liquid phase, sorption coefficient of the membrane for peroxide and alkene, membrane-catalyst distribution coefficient for peroxide and alkene and catalyst loading, have been studied. The results have been discussed in terms of a peroxide effidency defined as the ratio of flux of peroxide through the membrane utilized for alkene oxidation to the total flux of organic peroxide through the membrane. The paper aimed to show that, by using an organophilic dense membrane and the catalysts confined in the polymeric matrix, the oxidant concentration (in that reaction peroxides) can be controlled on the active site with an improvement of the peroxide efficiency and selectivity to desired products. 

Keywords photooxidation, UV absorbers (UVA), outdoor performance, weathering, accelerated weathering, kinetic chain length, photoinitiator, Norrish reactions, peroxide decomposer, hindered amine light stabilizers (HALS), photosensitizer, transition metal complex, UV stabilizer, time-controlled stabilization, reactive antioxidants, polymeric antioxidants. 

The next nine chapters, grouped under the subheading of "Initiator Chemistry, are concerned with various aspects of research on topics of current interest dealing with the energetics and detailed mechanisms of radical production. The first chapter in this section summarizes Paul Bartlett s most recent results on photooxidation and the identity of the elusive epoxidation reagent. Other chapters treat MAH reactions, peroxide and perester reactions, cyclic endoperoxides, diazenes, cage effects, and radiation. 

A variety of initiators have been used to promote this reaction. Peroxides and azo-compounds are thermally unstable and provide radicals. From these two classes, AIBN and benzoyl peroxide have been the most commonly used chemical initiators. The use of other diazo compounds (derivatives of AIBN,... 

The primary role of vitamin E is to protect the body tissues from damaging reactions (peroxidation) that arise from many normal metabolic processes and exogenous toxic agents. Specifically, vitamin E protects biological membranes such as those found in the nerves, muscles and cardiovascular system, helps to prolong the life of erythrocytes (red blood cells) and helps the body to make optimal use of vitamin A. 

---