Hydroperoxides, formation

For polymer containing labile hydrogen atoms, chain peroxidation can lead to hydroperoxide formation. The following reactions are expected to take place. 

Fig. 12.14. Competing cis abstraction and trans abstraction transition structures for hydroperoxide formation 2-methyl-2-butene. Adapted J. Am. Chem. Soc., 125, 1319 (2003), by permission of the American Chemical Society.

In sum, the results described have led us to postulate the following possible mechanism as explanation of the observed retardation of hydroperoxide formation by TMP derivatives The HALS studied form a complex with the hydroperoxides which is much more efficiently broken down by peroxy and/or alkoxy radicals - with formation of harmless products - than hydroperoxides alone (reaction (26)). The result is a lowering of the rate of formation of hydroperoxides. 

Figure 8 Hydroperoxide formation in LDPE granules during oven ageing at 100°C. Reprinted from Luzuriaga et al. [18]. Copyright 2006, with permission from Elsevier.

Thus, the overall reaction may be written as RH + 02 1 ROOH. The G values for hydroperoxide formation at 50°C range from -16 for 2,2,4-trimethylpentene-l to -400 for cyclohexene (Wagner, 1969). Although this temperature is somewhat lower than the temperature of decomposition of the hydroperoxide, in practice the reactions are conducted at elevated temperatures. In such cases, the radition-induced initiation either eliminates the induction period or allows the recations to proceed at somethat lower temperatutes than would be otherwise required. 

The values of enthalpies of peroxyl radical formation (A//r°) calculated from the enthalpies of hydroperoxide formation according to the thermochemical equation ... 

The free radical mechanism of hydroperoxide formation is close to that proposed by Nalbandyan. 

The kinetic curve has a sigmoidal shape and can be intuitively divided into four stages (a) induction period, (b) stage of accelerated hydroperoxide formation, (c) stage of retarded ROOH formation, and (d) stage with prevalence of ROOH decomposition after the point... 

FIGURE 4.1 Typical kinetic curve of hydroperoxide formation during hydrocarbon oxidation. 

FIGURE 11.1 The kinetic curves of cumyl hydroperoxide formation in emulsion oxidation of cumene [8] at T — 358 K, H20 RH — 3 l (v/v) 1 N Na2C03 with input of 0.015mol L 1 H202 in the moments designated by arrows (curve 1), after 8 h (curve 2), and after 4 h (curve 3). 

The duration of the inhibition period of a chain-breaking inhibitor of autoxidation is proportional to its efficiency. Indeed, with an increasing rate of chain termination, the rates of hydroperoxide formation and, hence, chain initiation decrease, which results in the lengthening of the induction period (this problem will be considered in a more detailed manner later). It should be noted that when initiated oxidation occurs as a straight chain reaction, the induction period depends on the concentration of the inhibitor, its inhibitory capacity, and the rate of initiation, but does not depend on the inhibitor efficiency. 

FIGURE 14.1 Oxidation of cyclohexanone kinetic curves of 1-naphtol decay (curves without primes) and hydroperoxide formation (curves with primes) at T — 413K(1, l ),403K(2, 2 ), and 393 K (3, 3 ). 

Although significance of cholesterol hydroperoxides formation under physiological conditions is still unknown, they are apparently very important factors in the development of many pathophysiological disorders. Thus it has been shown [87] that 13ZE-Chl8 2-OOH exists in vivo in atherosclerotic lesions and is the primary toxin of oxidized human LDL. 

Finally, another interesting use of singlet oxygen in the oxidation of dienes concerns the reactivity of allenes. Besides the formation of endoperoxides by addition to dienes and hydroperoxide formation via the ene reaction, singlet oxygen reacts with electron-rich... 

Reaction 9 Thermal Oxidative Degradation - Hydroperoxide Formation [105]... 

The complex and incompletely understood phenomena of cool flames and then-close relationship with autoignition processes is discussed in considerable detail. As the temperature of mixtures of organic vapours with air is raised, the rate of autoxidation (hydroperoxide formation) will increase, and some substances under some circumstances of heating rate, concentration and pressure will generate cool flames at up to 200° C or more below their normally determined AIT. Cool flames (peroxide decomposition processes) are normally only visible in the dark, are of low temperature and not in themselves hazardous. However, quite small changes in thermal flux, pressure, or composition may cause transition to hot flame conditions, usually after some delay, and normal ignition will then occur if the composition of the mixture is within the flammable limits. 

In addition, there is experimental evidence showing that mitochondrial cardiolipin content markedly decreases following ischemia and reperfusion injury due to cardiolipin peroxidation (Soussi et al., 1990) and that a decrease in the mitochondrial phospholipid cardiolipin occurred in aged rat hearts (Pepe, 2000). These decreases may he attrihutahle to the hydroperoxide-formation of cardiolipin after exposure to intense or repeated oxidative stress during disease state or normal aging, respectively. 

Hydroperoxide formation by the ene reaction path may lead to formation of conjugated double bonds in polyunsaturated fatty acids (see Section V.A) this reaction is concurrent with POV increase. An increase of the CDV, as measured from the absorbance at 233 nm, therefore indicates oxidation of polyunsaturated lipids. A strong correlation exists between CDV predicted from the absorbance in the 1100 to 2200 nm NIR region and CDV determined by the Ti Ia-64 AOCS official method , by UV spectrophotometry at 233 nm. The method was applied to determine CDV for oxidized soybean oil. A secondary absorption maximum of lesser intensity appears in the 260-280 mn range, and is assigned to ketone dienes . 

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