Alkylperoxy free radicals

The lipid free radical then reacts preferentially with the available oxygen in subsequent reactions to form an alkylperoxy free radical, LOO (Equation 2). 

Although primary and secondary alkyl hydroperoxides are attacked by free radicals, as in equations 8 and 9, such reactions are not chain scission reactions since the alkylperoxy radicals terminate by disproportionation without forming the new radicals needed to continue the chain (53). Overall decomposition rates are faster than the tme first-order rates if radical-induced decompositions are not suppressed. 

Alkyl orthophosphate triesters, 79 41 terteAlkyl peroxycarbamates, decomposition of, 78 486 Alkyl peroxyesters, 78 478-487 chemical properties of, 78 480 487 physical properties of, 78 480 primary and secondary, 78 485 synthesis of, 78 478-480 synthetic routes to, 78 479 tert-Alkyl peroxyesters, 78 480 84, 485 as free-radical initiators, 74 284-286 properties of, 78 481-483t uses of, 78 487 Alkylperoxy radical, 74 291 Alkyl phenol ethoxylates, 8 678, 693 ... 

At elevated temperature alkyl hydroperoxides undergo thermal decomposition to alcohols [Eqs. (9.9)—(9.11)]. This decomposition serves as a major source of free radicals in autoxidation. Because of side reactions, such as p scission of alkylperoxy radicals, this process is difficult to control. Further transformation of the alkoxy... 

Benzoic Acid. The industrial-scale oxidation of toluene to benzoic acid is carried out with cobalt catalysts.973,978,982 983 The process, a free-radical autoxidation, is significantly promoted by bromide ions.984 Under these conditions bromine atoms rather than alkylperoxy radicals serve as a regenerable source of chain-initiating free radicals. Substantial rate increase can be achieved by the addition of manga-nese(II) ions.984... 

Isomerization of Alkylperoxy Radicals. Alkylperoxy radicals can isomerize to hydroperoxyalkyl radicals (19, 25, 45, 55), which subsequently react to regenerate free radicals capable of attacking C H2 + 2. 

Hydroperoxyalkyl radicals can react in several ways. First, of course, alkylperoxy radical isomerization is reversible (Reaction —4). Secondly, several modes of decomposition (Reaction 5) occur, giving O-hetero-cycles, alkenes, and saturated and unsaturated aldehydes and ketones. (Alcohols can also be formed by the decomposition of the alkylperoxy-alkyl radicals which result from isomerization by group transfer in alkylperoxy radicals.) These modes of decomposition have been enumerated (24, 25) only examples need be given here. Each mode of decomposition gives one or more product molecules plus one free radical, acting, therefore, as a propagation step. Moreover, the radical produced... 

Han P, Bartels DM (1994) Encounters of H and D atoms with 02 in water relative diffusion and reaction rates. In Gauduel Y, Rossky P (eds) AIP conference proceedings 298. "Ultrafast reaction dynamics and solvent effects." AIP Press, New York, 72 pp Hasegawa K, Patterson LK (1978) Pulse radiolysis studies in model lipid systems formation and behavior of peroxy radicals in fatty acids. Photochem Photobiol 28 817-823 Herdener M, Heigold S, Saran M, Bauer G (2000) Target cell-derived superoxide anions cause efficiency and selectivity of intercellular induction of apoptosis. Free Rad Biol Med 29 1260-1271 Hildenbrand K, Schulte-Frohlinde D (1997) Time-resolved EPR studies on the reaction rates of peroxyl radicals of polyfacrylic acid) and of calf thymus DNA with glutathione. Re-examination of a rate constant for DNA. Int J Radiat Biol 71 377-385 Howard JA (1978) Self-reactions of alkylperoxy radicals in solution (1). In Pryor WA(ed) Organic free radicals. ACS Symp Ser 69 413-432... 

Co(acac)3 in combination with N-hydroxyphthalimide (NHPI) as cocatalyst mediates the aerobic oxidation of primary and secondary alcohols, to the corresponding carboxylic acids and ketones, respectively, e.g. Fig. 4.71 [205]. By analogy with other oxidations mediated by the Co/NHPI catalyst studied by Ishii and coworkers [206, 207], Fig. 4.71 probably involves a free radical mechanism. We attribute the promoting effect of NHPI to its ability to efficiently scavenge alkylperoxy radicals, suppressing the rate of termination by combination of al-kylperoxy radicals (see above for alkane oxidation). 

The acronym BHT stands for butylated hydroxytoluene, the common name for 2,6-di-r-butyl-4-methylphenol. It is used widely as an antioxidant in foodstuffs and food packaging. The reaction of oxygen with unsaturated fats gives, after several steps, alkylperoxy radicals (ROO ) that, upon further reaction, give smaller odiferous molecules that can ruin the palatability of foods. BHT acts as a scavenger for alkylperoxy radicals, ROO , because hydrogen abstraction by ROO- gives the stable free radical, 5-4. The hydroperoxide ROOH, which also is formed in the reaction, is much less reactive than ROO and consequently causes much less oxidation. The new radical, 5-4, which is formed from BHT, is relatively unreactive for two... 

Laboratories in Armenia [115] and France [21,116] have a well-established esr technique for the detection of alkylperoxy and hydroperoxy radicals. The procedure involves the freeze-trapping of free radicals on a liquid nitrogen cooled trap located within the cavity of an esr machine. The reaction products, including relatively long-lived RO2 radicals, are pumped continuously at low pressure from a flow reactor to the collection... 

A free radical flux is maintained in this system. The predominant member of the flux is usually the alkylperoxy radical since it is a relatively weak hydrogen abstractor and, as a result, builds up to the highest concentration [8, 10-12], Each new molecule of hydrocarbon is brought into the reaction process by undergoing hydrogen abstraction. When this reaction involves an alkylperoxy radical, the result is the first propagation step, shown in eq. (2). This produces an alkyl radical and an alkyl hydroperoxide. 

One aspect which sets oxidation apart from other reactions, e.g. hydrogenation and carbonylation is the fact that there is almost always a reaction (free radical chain autoxidation) in the absence of the catalyst (Reactions 1-3). Moreover, (transition) metal ions which readily imdergo a reversible one-electron valence change, e.g. manganese, cobalt, iron, chromium, and copper, catalyze this process by generating alkoxy and alkylperoxy radicals from RO2H (Reactions 4-6). 

In very general terms, the Co-Br-catalyzed oxidation is a particular case of the free radical chain oxidation, common for all liquid phase oxidations of hydrocarbons [8-10]. The free radical chain oxidation occurs with four types of free radicals alkyl, alkoxy, alkylperoxy, and acylperoxy radicals [11, 12]. Other key active intermediates are hydroperoxides and peracids [11,12]. The nomenclature and structures are displayed in Figure 4.2. 

The transferred Hs are primary, the free radical is of the alkylperoxy type and the activated complex has a size equal to 6. Table XIV-2.4 gives = 84 kJ mol" and... 

This oxidation model, however, ignores factors such as relativity differences between polymers and differences in oxygen permeability. Also, it does not account for the fact that the nature and character of the propagating radicals is not the same for aU polymers. Polymers show a wide variation in susceptibility to thermal autoxidation. The ease of autoxidation depends primarily on the relative C-H bond dissociation energies for the component parts of the polymer structure. Once the free-radical process is initiated, autoxidation proceeds. The abstraction of the most labile hydrogen atom in the polymer by alkylperoxy radical predominates in the propagation step. This follows this order ... 

ESR of paramagnetic free radicals can be used to check the efficacy of AOs and other stabilisers. ESR was used in the study of phenothiazines as antioxidants in PP aromatic secondary amines can retard polymer oxidation by reacting with alkylperoxy radicals [824]. Tkac [825] has described hydrogen and electron transfer reactions of AOs by ESR and has shown the efficiency of the ESR technique in elucidating the relationship between structure and reactivity of radicals formed from antioxidants possessing different H- and e-donor functional groups, including (hindered) phenols, amines, etc. 

Stable nitroxide radicals are traditionally considered as the principal species which account for the free-radical scavenging activity of HAS. Nitroxide radicals can trap alkyl, alkylperoxy and acylperoxy radicals. The probability of direct radical scaveng-... 

The first acts by removing the active free radicals which are involved in the cyclical chain reaction described in (1). Normally the alkylperoxy radical (ROO-) is the reaction species present in highest concentration and this reacts readily with phenolic and arylamine antioxidants (AH) to give a new stable free radical species which cannot continue the chemical chain reaction (2a). The absorption of oxygen and the rapid formation of hydroperoxide is therefore slowed down ... 

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