Peroxide effect 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... 

Walling, C. and Goosen, A., Mechanism of the ferric ion catalyzed decomposition of hydrogen peroxide effect of organic substrates, /. Amer. Chem. Soc., 95(9), 2987-2991, 1973. 

The reversal of orientation of HBr addition to alkenes in the presence of peroxides. A free-radical mechanism is responsible for the peroxide effect, (p. 336)... 

However, in the presence of peroxide the addition of HBr follows anti-Markovnikov s addition to give n-propyl bromide. This is often known as the peroxide effect ° and the addition proceeds by the free radical mechanism (Scheme 2.44). 

To account for this peroxide effect, Kharasch and Mayo proposed that addition can take place by two entirely different mechanisms Markovnikov addition by the ionic mechanism that we have just discussed, and anti-Markovnikov addition by a free-radical mechanism. Peroxides initiate the free-radical reaction in their absence (or if an inhibitor, p. 189, is addedX addition follows the usual ionic path. 

In the years since the discovery of the peroxide effect, dozens of reagents besides HBr have been found (mostly by Kharasch) to add to alkenes in the presence of peroxides or light. Exactly analogous free-radical mechanisms are generally accepted for these reactions, too. 

A classic example of the ease with which a mechanism can change is the now well-known peroxide effect, discovered by Kharasch in the addition of hydrogen bromide to olefins.4 In the presence of air or a trace of peroxides the usual ionic reaction becomes a free radical process with the result that the normal (Markownikoff) direction of addition becomes almost completely reversed. 

Heikkila R and Cohen G Inhibition of Biogenic Amine Uptake by Hydrogen Peroxide A Mechanism for Toxic Effects of 6-Hydroxydopamine. Science 1971 172 1257-58. 

Although coniferaldehyde model compounds reactrapidly and quantitatively with peroxide [120], the reaction in pulp fibers is much slower. Residual coniferaldehyde has been detected in peroxide-bleached mechanical pulps [87,110], and can have a profound effect on the absorption difference spectra for irradiation. Figure 3.13 shows difference spectra for ultraviolet irradiation of peroxide-bleached softwood TMP and CTMP [121]. The spectrum for CTMP has a broad absorption increase... 

Lee and Sumimoto [47] suggested that peroxide-bleached mechanical pulps yellow readily because the hydroquinones they contain are leucochromophores, which may be subsequently oxidized further to colored compounds. In particular they showed that peroxide treatment of phenolic stilbene (XIII), which was formed from phenylcoumaran (XII) by mechanochemical action, gave rise to a labile hyd-roquinone that was easily oxidized by air to an intensely colored stilbenequinone (XIV) (Figure 12.4). It should be noted that these conclusions were based on experiments with model compounds. Conversely, Zhu et al. [48] found that mechanochemical action had no significant effect on the photosensitivity of mechanical pulps. 

In the case of peroxovanadate (which may be formed in situ), vanadate plus superoxide radicals, or vanadate with added hydrogen peroxide, the mechanism appears to be increased autophosphorylation and activation of the insulin receptor, with consequent stimulation of glucose uptake. However, effects of intracellular vanadium on calcium influx as well as intracellular and intravesicular pH modification have not been ruled out as important factors in the mechanism of action of vanadium as an insulin-mimetic agent. 

The mechanism proposed for the peroxide effect involves a radical chain reaction. The initiation step (equation 9.31) produces a bromine atom, which then attaches to the less alkyl-substituted carbon atom of a carbon-carbon double bond (equation 9.32). The resulting alkyl radical then abstracts a hydrogen atom from HBr to produce the anti-Markovnikov product and regenerate a bromine atom in the second propagation step (equation 9.33). Termination steps, not shown, interrupt the chain reaction. 

A newer class of silane coupling agents is known as silyl peroxides, represented by the general formula R mR 4-n-mSI(OOR) . A typical member of this family is vinyl-tn s-(t-butylperoxy) silane. The coupling mechanism of the silyl peroxides, effected by heat only, is free-radical in nature. The conventional silanes require an eternal... 

The lipid peroxide stimulation mechanism of paraquat acute toxicity has been questioned by Shu et al. (1979), who have shown that it is possible by pre-treatment with N,ivr-diphenyl-p-phenylene-diamine, an antioxidant, to prevent the paraquat stimulation of lipid peroxidation without protecting the animals against its lethal effects. 

Based on modern concepts of reaction mechanisms, the peroxide effect arises from decomposition of a peroxide to peroxy radicals, which then react with... 

He called this the peroxide effect and proposed that the difference in regioselectivity was due to a peroxide-induced change in mechanism. The conventional electrophilic addition pathway responsible for Markovnikov addition operates in the absence of peroxides the other mechanism is a free-radical addition sequence (Mechanism 6.8). 

Fig. 6 The effect of crosslinking (degree given by content of decomposed peroxide) on mechanical properties of PHB/PCL blends. The effect of ageing is shown by testing after 2 (full line) or 14 days (dashed line) after compresson moulding.

EFFECT OF DICUMYL PEROXIDE ON MECHANICAL STABILITY TIME OF CONCENTRATE LATEX... 

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