Hydroperoxides measurement

Steric effects in oxidation are found primarily in the H-atom donor as, for example, isobutane versus 2,4-dimethylpentane where both co-oxidation and added hydroperoxide measurements indicate isobutane to be 1.4 times as reactive toward either R02 radical [38,39], For a series of branched alkanes, the value of kp was more sensitive to changes in steric bulk on the adjacent carbon than on the same carbon but all values were within a factor three [39], Increasing size in f-R02 radicals has little effect on the value of kp for H-atom transfer from primary, secondary or tertiary sites [69]. However, kp for a given hydrocarbon does increase by a factor of about 5—10 as R02 is changed from a tertiary to secondary or primary ROa [78], Moreover, in the oxidation of aromatic compounds, the ratio of kp s for H-atom transfer to parent R02- and to f-Bu02-correlate with meta substituent constants which suggests that this difference in reactivity is due mainly to polar effects [86]. 

Lacoste ]., and D.J. Carlsson. 1991. A critical comparison of methods for hydroperoxide measurement in oxidized jxjlyolefins. Polym Degr Stab 32 377-386. 

The anchovy oil was stripped of tocopherols by chromatography. The relative antioxidant activity was based on inhibition of hydroperoxides measured by conjugated diene formation. 

The soybean oil was stripped of tocopherols by short-path molecular distillation. The relative potency was based on inhibition of hydroperoxides measured by conjugated diene formation at optimal concentrations of about 100 ppm for a-tocopherol, about 300 ppm for y-tocopherol. The optimal concentration for 5-tocopherol was not clearly defined and the antioxidant activity increased with concentration up to -1900 ppm. 

Jiang, T.Y., Wollard, C.S. and Wolff, S.P. (1991) Lipid hydroperoxide measurement by Fe in the presence of xylenol orange. Comparison with TBA assay and an iodometric method. Lipids 26, 853-856. 

In the flask were placed 10.0 g of the propargylic amine (see Chapter lIII-5, Exp. 1). The air in the flask was replaced with nitrogen and a solution of 0.01 mol of KO-tert.-Ci,H,3 in 10 g of THF (free from hydroperoxide) was added. The mixture was warmed at about 40 C. A weakly exothermic reaction was observed and the temperature rose to about 45°C. After 1-2 min the gel originally present, had disappeared almost completely and a brown solution had formed. The refractive index of the solution (note 1) was measured after intervals of about 2 min. After the... 

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. 

Rgure 2.3 The antioxidant activity of butyiated hydroxytoluene in the presence of exogenous iipid hydroperoxides. The oxidation of LDL was monitored by measuring the increase in absorbance at 234 nm as described in Fig. 2.2 and the lag phase (time before the phase of maximum rate of oxidation) estimated as described by Esterbauer et at. (1989). Samples of LDL were supplemented with the cortcentrations of 13-hydroperoxyoctadecanoic acid (13-HPODE) indicated and in the presence of 3 fM BHT. The lag phase in the absence of BHT for this preparation of LDL was 48 min. 

A series of reactions with gases have been selected for the rapid quantification of many of the major products from the oxidation of polyolefins. Infrared spectroscopy is used to measure absorptions after gas treatments. The gases used and the groups quantified include phosgene to convert alcohols and hydroperoxides to chloroformates, diazomethane to convert acids and peracids to their respective methyl esters, sulfur tetrafluoride to convert acids to acid fluorides and nitric oxide to convert alcohols and hydroperoxides to nitrites and nitrates respectively. 

CH Nj reactions, 382 COClj reactions, 383 free radical addition of hexafluoroacetone, 257 identification of oxidation products CHjNj to measure peracids as peresters, 385 extinction coefficients, 388-389/ iodometry to measure -OOH, 385 NO to measure alcohols and hydroperoxides, 386 residual, simplified carbonyl envelope which results from SF exposure, 386... 

The complex formation between hydroperoxides and HALS derivatives proposed for the preceding reaction was recently postulated by two different groups of investigators. First, Carlsson determined a complex formation constant for +00H and a nitroxide on the basis of ESR measurements—. Secondly, Sedlar and his coworkers were able to isolate solid HALS-hydroperoxide complexes and characterize them by IR measurements. The accelerated thermal decomposition of hydroperoxides observed by us likewise points to complex formation. It is moreover known that amines accelerate the thermal decomposition of hydroperoxides-. Thus Denisov for example made use of this effect to calculate complex formation constants for tert.-butyl hydroperoxide and pyridineitZ.. 

Complex formation constants could also be determined directly from UV spectrophotometric measurements. Addition of tert.-butyl hydroperoxide to a solution of nitroxide I in heptane at RT causes a shift of the characteristic absorption band of NO at 460 nm to lower wavelengths (Fig. 9). This displacement allows calculation of a complex equilibrium constant of 5 1 1/Mol. Addition of amine II to the same solution causes reverse shift of theC NO" absorption band. From this one can estimate a complex formation constant for amine II and +00H of 12 5 1/Mol (23 2 1/Mol was obtained for tert.-butyl hydroperoxide and 2,2,6,6-tetramethylpipe-ridine in ref. 64b). Further confirmation for an interaction between hindered amines and hydroperoxides is supplied by NMR measurements. Figure 10a shows part of the +00H spectrum in toluene-dg (concentration 0.2 Mol/1) with the signal for the hydroperoxy proton at 6.7 ppm. Addition of as little as 0.002 Mol/1 of tetra-methylpiperidine to the same solution results in a displacement and marked broadening of the band (Fig. 10b). 

Experimental set up as in section a). Alkoxy radicals were produced by photolysis of di-tert.-butyl peroxide (2 Mol/1). The build up of hydroperoxide concentration was measured by a modified version of the iodometric method used by Carlsson and Wiles—. 

Figure 7 Hydroperoxide index (HI) determined from mid-infrared spectroscopy (ratio of the integrated intensity of the 3,552 cm 1 band to the integrated intensity of the band at 2,010 cm-1) as a function of total hydroperoxide content measured by iodiometric titration.

Thermal decomposition of dialkyl peroxides, diacyl peroxides, hydroperoxides and peracids depending on the structure of the peroxidic compound occurs in a measurable rate usually above 60°C. Diacyl peroxides and peracids are considerably less stable than dialkyl peroxides and hydroperoxides. 

Finally, we have studied the metabolism of a series of PAH with decreasing IP. In these metabolic studies with Aroclor-induced rat liver microsomes, the formation of quinones was measured in the presence of NADPH or cumene hydroperoxide as cofactor. 

The last reaction occurs much rapidly than the disproportionation of two cumylperoxyl radicals and accelerates chain termination in oxidized cumene [15]. The addition of cumene hydroperoxide helps to avoid the influence of the cross termination reaction Me2PhCOO + CH302 on the oxidation of cumene and to measure the pure disproportionation of cumylperoxyl radicals [15]. 

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