Peroxidation chain mechanisms

Oxidation to CO of biodiesel results in the formation of hydroperoxides. The formation of a hydroperoxide follows a well-known peroxidation chain mechanism. Oxidative lipid modifications occur through lipid peroxidation mechanisms in which free radicals and reactive oxygen species abstract a methylene hydrogen atom from polyunsaturated fatty acids, producing a carbon-centered lipid radical. Spontaneous rearrangement of the 1,4-pentadiene yields a conjugated diene, which reacts with molecular oxygen to form a lipid peroxyl radical. 

It was seen in Chapter 3 that hydrocarbon polymers, of which natural rubber is a naturally occurring example, degrade both abiotically and biotically by a peroxidation chain mechanism. This process is accelerated by the introduction of photosensitive... 

Chemical initiation generates organic radicals, usually by decomposition of a2o (11) or peroxide compounds (12), to form radicals which then react with chlorine to initiate the radical-chain chlorination reaction (see Initiators). Chlorination of methane yields all four possible chlorinated derivatives methyl chloride, methylene chloride, chloroform, and carbon tetrachloride (13). The reaction proceeds by a radical-chain mechanism, as shown in equations 1 through. Chain initiation... 

The anti-Markownikoff addition of hydrogen bromide to alkenes was one of the earliest free-radical reactions to be put on a firm mechanistic basis. In the presence of a suitable initiator, such as a peroxide, a radical-chain mechanism becomes competitive with the ionic mechanism for addition of hydrogen bromide ... 

The most commonly employed reagent for the hydroxylation of aromatic compounds is that consisting of ferrous ion and hydrogen peroxide. The suggestion that hydroxyl radicals are intermediates in this reaction was first made by Haber and Weiss, who proposed the following radical-chain mechanism for the process ... 

Organotin(IV) compounds could be involved in other biological processes occurring in cells, specifically in peroxide oxidation of lipids. The latter process is very important from the viewpoint of physiology, and it follows a radical chain mechanism. ... 

Organic peroxides and hydroperoxides decompose in part by a self-induced radical chain mechanism whereby radicals released in spontaneous decomposition attack other molecules of the peroxide.The attacking radical combines with one part of the peroxide molecule and simultaneously releases another radical. The net result is the wastage of a molecule of peroxide since the number of primary radicals available for initiation is unchanged. The velocity constant ka we require refers to the spontaneous decomposition only and not to the total decomposition rate which includes the contribution of the chain, or induced, decomposition. Induced decomposition usually is indicated by deviation of the decomposition process from first-order kinetics and by a dependence of the rate on the solvent, especially when it consists of a polymerizable monomer. The constant kd may be separately evaluated through kinetic measurements carried out in the presence of inhibitors which destroy the radical chain carriers. The aliphatic azo-bis-nitriles offer a real advantage over benzoyl peroxide in that they are not susceptible to induced decomposition. 

The reductive elimination of a variety of )3-substituted sulfones for the preparation of di-and tri-substituted olefins (e.g. 75 to 76) and the use of allyl sulfones as synthetic equivalents of the allyl dianion CH=CH—CHj , has prompted considerable interest in the [1,3]rearrangements of allylic sulfones ". Kocienski has thus reported that while epoxidation of allylic sulfone 74 with MCPBA in CH2CI2 at room temperature afforded the expected product 75, epoxidation in the presence of two equivalents of NaHCOj afforded the isomeric j ,y-epoxysulfone 77. Similar results were obtained with other a-mono- or di-substituted sulfones. On the other hand, the reaction of y-substituted allylic sulfones results in the isomerization of the double bond, only. The following addition-elimination free radical chain mechanism has been suggested (equations 45, 46). In a closely related and simultaneously published investigation, Whitham and coworkers reported the 1,3-rearrangement of a number of acyclic and cyclic allylic p-tolyl sulfones on treatment with either benzoyl peroxide in CCI4 under reflux or with... 

Bieber reported that the reaction of bromoacetates is greatly enhanced by catalytic amounts of benzoyl peroxide or peracids and gives satisfactory yields with aromatic aldehydes. A radical chain mechanism, initiated by electron abstraction from the organometallic Reformatsky reagent, is proposed (Scheme 8.27).233 However, an alternative process of reacting aldehydes with 2,3-dichloro-l-propene and indium in water followed by ozonolysis provided the Reformatsky product in practical yields.234 An electrochemical Reformatsky reaction in an aqueous medium and in the absence of metal mediator has also been reported.235... 

When peroxide initiation is used, a chain mechanism may still obtain. The initiation reactions are shown in Eqs. (21)-(23). Propagation steps (18)... 

Organometallic formation may result from a chain mechanism [Eqs. (21)-(23) and (18)—(20)] and/or radical displacement [Eqs. (21)-(23), alone]. The reaction of 13C-labeled mercuric cyclohexanoate with cyclohexylcarbonyl peroxide (1 1) gave mainly unlabeled organomercu-rial, which was derived from radical displacement (122). Decarboxylation by a chain mechanism was reported for the syntheses of organomercuric carboxylates of straight chain alkyls [R = Me(CH2) , n - 0-8, 10, or 15 (123-131)], branched alkyls [R = Me2CH(CH2) , n = 0 or 2 (132) or Me3C(CH2) , n = 0-2 (133)], substituted alkyls [R = cyclopentylmethyl... 

Although peroxide initiated reactions can produce preparative yields of organomercury salts, a mixture of products was usually obtained due to chain mechanism/radical displacement competition and to the prevalence of side reactions [Eqs. (24), (25), and (109)]. An alternative to the... 

Mercuric carboxylates, which decarboxylate by a chain mechanism when initiated by peroxides, also decarboxylate under UV irradiation (123,128,129,131-140,142,144-146,153-155). In addition, decarboxylation was observed for mercuric benzoate and mercuric a-naphthoate (123). Side reactions [Eqs. (24), (25), (109)] observed in peroxide initiated reactions also occurred on UV irradiation, and mercurous salt formation [Eq.(24)] was more extensive under the latter conditions. Decarboxylation giving methylmercuric acetate occurred on irradiation of mercuric acetate in aqueous solution and is considered to be of environmental significance (156,157). Stepwise decarboxylation giving (CF3)2Hg occurred on irradiation of solid mercuric trifluoroacetate at -196° C (158), but, at 20° C, trifluoromethyl radicals diffused from the solid and dimerized (158). No other diorganomercurial has been formed by radical decarboxylation, and the reaction is not preparatively competitive with the thermal decarboxylation synthesis of (CF3)2Hg (26,27) (Section III,A). 

Generated from diacetyl peroxide, methyl radicals attack 2-methylfuran at position 5 preferentially if both 2- and 5-positions are occupied as in 2,5-dimethylfuran there is still little or no attack at the 3(4)-position. If there is a choice of 2(5)-positions, as in 3-methylfuran, then that adjacent to the methyl substituent is selected.249 These orientation rules are very like those for electrophilic substitution, but are predicted for radical attack by calculations of superdelocalizability (Sr) by the simple HMO method. Radical bromination by IV-bromsuccinimide follows theory less closely, presumably because it does not occur through a pure radical-chain mechanism.249... 

The chain mechanism of hydrogen peroxide decay under catalytic action of transition metal ions was postulated F. Haber and J. Weiss [62]... 

The nonsaturated esters with tt-C=C bonds and without activated a-C—H bonds (esters of acrylic acid (CH2=CHCOOR) and esters of vinyl alcohols (RC(0)0CH=CH2)) are oxidized by the chain mechanism with chain propagation via the addition of peroxyl radicals to the double bond. Oligomeric peroxides are formed as primary products of this chain reaction. The kinetic scheme includes the following steps in the presence of initiator I and at p02 sufficient to support [02] > 10 4 mol L-1 in the liquid phase [49]. 

Diacyl peroxides exhibit behavior somewhat like that of the dialkyl peroxides. Benzoyl, peroxide decomposes into radicals partly by a unimolecular and partly by a chain mechanism. In at least the case of benzoyl peroxide itself, if not for similar compounds, the first step is the production of benzoyloxy radicals rather than phenyl radicals and... 

Since the analogous peroxides usually decompose by a free radical mechanism, it is noteworthy that this hydroxamic acid is not sensitive to the action of free radicals from anisoyl peroxide. A radical chain mechanism like that shown below can therefore be ruled out for this compound. 

Metal ion catalyzed autoxidation reactions of glutathione were found to be very similar to that of cysteine (76,77). In a systematic study, catalytic activity was found with Cu(II), Fe(II) and to a much lesser extent with Cu(I) and Ni(I). The reaction produces hydrogen peroxide, the amount of which strongly depends on the presence of various chelating molecules. It was noted that the catalysis requires some sort of complex formation between the catalyst and substrate. The formation of a radical intermediate was not ruled out, but a radical initiated chain mechanism was not necessary for the interpretation of the results (76). 

The anodic oxidation of 1,3-diketones in the presence of olefins in an oxygen atmosphere gave the extremely stable cyclic peroxides 80 in good yield [101,102] (Scheme 40). A catalytic amount of electricity was sufiicient for the reaction and an electro-intiated radical chain mechanism was suggested. 

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... 

The results given in this paper show that aliphatic amines do not catalyze the decomposition of peroxides, and compared with their effect at the start of reaction, they have much less effect on the later stages of oxidation, although they appear to retard the decomposition of peracetic acid. The reactions of radicals with aliphatic amines indicate that an important mode of inhibition is most probably by stabilization of free radicals by amine molecules early in the chain mechanism, possibly radicals formed from the initiation reaction between the fuel and oxygen. For inhibition to be effective, the amine radical must not take any further part in the chain reaction set up in the fuel-oxygen system. The fate of the inhibitor molecules is being elucidated at present. 

The alkylation of quinoline by decanoyl peroxide in acetic acid has been studied kineti-cally, and a radical chain mechanism has been proposed (Scheme 207) (72T2415). Decomposition of decanoyl peroxide yields a nonyl radical (and carbon dioxide) that attacks the quinolinium ion. Quinolinium is activated (compared with quinoline) towards attack by the nonyl radical, which has nucleophilic character. Conversely, the protonated centre has an unfavorable effect upon the propagation step, but this might be reduced by the equilibrium shown in equation (167). A kinetic study revealed that the reaction is subject to crosstermination (equation 168). The increase in the rate of decomposition of benzoyl peroxide in the phenylation of the quinolinium ion compared with quinoline is much less than for alkylation. This observation is consistent with the phenyl having less nucleophilic character than the nonyl radical, and so it is less selective. Rearomatization of the cr-complex formed by radicals generated from sources other than peroxides may take place by oxidation by metals, disproportionation, induced decomposition or hydrogen abstraction by radical intermediates. When oxidation is difficult, dimerization can take place (equation 169). 

The effect of air and the acceleration by light of the addition of PhSH to styrene was observed in 1928, and in 1934 the formation of the PhS- radical from dissociation of PhSSPh (equation 9) and from reactions of PhSH was proposed. " The use of peroxides to catalyze the reaction was reported by Jones and Reid in 1938, and in the same year the radical chain mechanism for this process was proposed (equation 10). ... 

Kharasch and Mayo in 1933," in the first of many papers on the subject, showed that the addition of HBr to allyl bromide in the presence of light and air occurs rapidly to yield 1,3-dibromopropane, whereas in the absence of air and with purified reagents, the reaction is slow and 1,2-dibromopropane is formed. The latter reaction is the normal addition occurring by an ionic pathway giving the Markovnikov orientation. In 1933 the mechanism of the abnormal process ( anti-Markovnikov addition) was not discussed, and it was only in 1937 that the free radical chain mechanism for this process was proposed by Kharasch and his co-workers. "" The mechanism was extended to propene, for which the role of peroxides in promoting the reaction was demonstrated (equations 30, 31). This mechanism was also proposed... 

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