Production of hydroperoxides

A very serious problem was to clear up the formation of hydroperoxides as the primary product of the oxidation of a linear aliphatic hydrocarbon. Paraffins can be oxidized by dioxygen at an elevated temperature (more than 400 K). In addition, the formed secondary hydroperoxides are easily decomposed. As a result, the products of hydroperoxide decomposition are formed at low conversion of hydrocarbon. The question of the role of hydroperoxide among the products of hydrocarbon oxidation has been specially studied on the basis of decane oxidation [82]. The kinetics of the formation of hydroperoxide and other products of oxidation in oxidized decane at 413 K was studied. In addition, the kinetics of hydroperoxide decomposition in the oxidized decane was also studied. The comparison of the rates of hydroperoxide decomposition and formation other products (alcohol, ketones, and acids) proved that practically all these products were formed due to hydroperoxide decomposition. Small amounts of alcohols and ketones were found to be formed in parallel with ROOH. Their formation was explained on the basis of the disproportionation of peroxide radicals in parallel with the reaction R02 + RH. 

PRODUCTS OF HYDROPEROXIDE DECOMPOSITION 1.4.1 Hydroperoxides as the Intermediates of Hydrocarbon Oxidation... 

The reaction of ions with peroxyl radicals appears also in the composition of the oxidation products, especially at the early stages of oxidation. For example, the only primary oxidation product of cyclohexane autoxidation is hydroperoxide the other products, in particular, alcohol and ketone, appear later as the decomposition products of hydroperoxide. In the presence of stearates of metals such as cobalt, iron, and manganese, all three products (ROOH, ROH, and ketone) appear immediately with the beginning of oxidation, and in the initial period (when ROOH decomposition is insignificant) they are formed in parallel with a constant rate [5,6]. The ratio of the rates of their formation is determined by the catalyst. The reason for this behavior is evidently related to the fast reaction of R02 with the... 

The increase in the amount of catalyst introduced in oxidized cumene (353 K) increases the oxidation rate, decreases the amount of the formed hydroperoxide, and increases the yield of the products of hydroperoxide decomposition methylphenyl ethanol and acetophenone. Similar mechanism was proposed for catalysis by copper phthalocyanine in cumene oxidation [254],... 

Emulsion oxidation of alkylaromatic compounds appeared to be more efficient for the production of hydroperoxides. The first paper devoted to emulsion oxidation of cumene appeared in 1950 [1], The kinetics of emulsion oxidation of cumene was intensely studied by Kucher et al. [2-16], Autoxidation of cumene in the bulk and emulsion occurs with an induction period and autoacceleration. The simple addition of water inhibits the reaction [6], However, the addition of an aqueous solution of Na2C03 or NaOH in combination with vigorous agitation of this system accelerates the oxidation process [1-17]. The addition of an aqueous phase accelerates the oxidation and withdrawal of water retards it [6]. The addition of surfactants such as salts of fatty acids accelerates the oxidation of cumene in emulsion [3], The higher the surfactant concentration the faster the cumene autoxidation in emulsion [17]. The rates of cumene emulsion oxidation after an induction period are given below (T = 353 K, [RH] [H20] = 2 3 (v/v), p02 = 98 kPa [17]). 

The formation of free radicals and alcohol (in addition to the products of hydroperoxide heterolysis) implies that the catalytic decomposition of hydroperoxide occurs both hetero-lytically and homolytically. The mechanism of homolytic hydroperoxide decomposition was proposed by Van Tilborg and Smael [48]. 

Emulsion oxidation of alkylaromatic compounds appeared to be more efficient for the production of hydroperoxides. The first paper devoted to emulsion oxidation of cumene appeared in 1950 [1]. The kinetics of emulsion oxidation of cumene was intensely studied by Kucher et al. [2-16]. Autoxidation of cumene in the bulk and emulsion occurs with an induction period and autoacceleration. The simple addition of water inhibits the reaction... 

The hydroperoxides formed in the autoxidation of unsaturated fatty acids are unstable and readily decompose. The main products of hydroperoxide decomposition are saturated and unsaturated aldehydes. The mechanism suggested for the formation of aldehydes involves cleavage of the isomeric hydroperoxide (I) to the alkoxyl radical (II), which undergoes carbon-to-carbon fission to form the aldehyde (III) (Frankel et al. 1961). 

Milk is characterized as having a pleasing, slightly sweet taste with no unpleasant after-taste (Bassette et al., 1986). However, its bland taste makes it susceptible to a variety of flavor defects. Autoxidation of unsaturated fatty acids gives rise to unstable hydroperoxides, which decompose to a wide range of carbonyl products, many of which can contribute to off-flavors in dairy products. The principal decomposition products of hydroperoxides are saturated and unsaturated aldehydes (Frankel et al., 1961), with lesser amounts of unsaturated ketones (Stark and Forss, 1962), saturated and unsaturated hydrocarabons (Forss et al., 1961), semialdehydes (Frankel et al., 1961) and saturated and unsaturated alcohols (Hoffman, 1962 Stark and Forss, 1966). 

Ans. (a) False. E° values are good indicators of catalytic efficiency only for the initiation steps, (b) False. The organic substrate (e.g., p-xylene, toluene, etc.) donate electrons to Co3+. (c) and (d) False. In the absence of a radical chain there is no evidence to show that metal ions help in the production of hydroperoxide. Similarly, there is no evidence to show that metal-dioxygen complexes are involved in the initiation or propagation steps, (e) False. Cyclohexanol is also formed by 8.19. The ratio under steady state is 1 2. 

At low temperatures fairly long chains leading to the production of hydroperoxides can be set up ... 

Of the main group hydrides, only those of carbon undergo insertion of oxygen to give products that are stable enough to be isolated. The production of hydroperoxides by autoxidation of carbon-hydrogen bonds ... 

Analysis of the Decomposition Products of Hydroperoxides. Some authors have monitored formation of some of the decomposition products of the lipid hydroperoxides. Direct spectrophotometric measurements of the formation of oxo-octadecadienoic acids at 280 nm are possible , as are measurements of secondary oxidation products like a-diketones and unsaturated ketones at 268 nm. The formation of various aldehyde products of lipid peroxide decomposition can be monitored by reacting them with 2,4-dinitrophenylhydrazine and, after HPLC separation, measuring at 360-380 mn the DNPH derivatives formed , althongh the sensitivity of this particular technique makes it very susceptible to interference. 

Vapor phase oxidation processes prevail over liquid phase processes, although the latter are sometimes used inlarge-scale chemical production when the products (i) can be easily recovered from the reaction medium, as interephthalic acid production, for example (ii) are thermally unstable (i.e., in the production of hydroperoxides and carboxylic acids, except for P-unsaturated compounds) and (iii) are very reactive at high temperature (i.e., epoxides, aldehydes and ketoses, with the exception of ethene oxide and formaldehyde). Liquid-phase oxidation is also preferred in fine chemicals production, although most processes are still non-catalytic. 

Production of hydroperoxides (i) isobutene to t-butyl hydroperoxide and ethylbenzene to ethylbenzene hydroperoxide, both subsequently used as the oxidizing agent... 

NHPI has been introduced as an effective system for C—H activation by hydrogen abstraction on several different substrates [30i,j,s,31a-e]. In 2001, Daicel commercialized the process used to synthesize dihydroxyadamantane, and has carried out pilot trials not only for the oxidation of cyclohexane but also for the oxidation of p-xylene to terephthalic acid. In the latter case, the advantage lies in being able to avoid using special anticorrosive metals currently required in the production of terephthalic acid because of the use of bromine. NHPI can also be used as a catalyst for the in situ production of hydroperoxides, reactants for epoxidation and for the oxidation and ammoxidation of cyclohexanone to caprolactone and caprolactam, respectively. 

Safety caution the production of hydroperoxides should always be performed with caution. In our case with 100% molecular oxygen, we worked above the explosion limit, and monitored the oxygen uptake by a burette, making sure that the conversion stayed below 40%. Laboratory glassware was used behind safety screens, and the scale was limited to 10 ml solutions. Hydroperoxides were stored at 4°C as dilute solutions, with the corresponding alkylbenzenes as solvent. 

The products of autoxidation and photo-oxidation of ethers are the same [187,287,288]. Aldehydes, alcohols, acids, and esters are the main products of hydroperoxide decomposition [186,187,202,283—285,289,290]. For example, ethanol, acetaldehyde, acetic acid, ethyl acetate, and ethyl formate were found in the products of diethyl ether oxidation [186,188, 202,203]. Their formation may be explained by the scheme... 

Hydroperoxide determination by iodometric titrations are quite common in the literature, the essential differences in the methods lies in the end point determination [Mielewski et al., 1989]. The propagation mechanism in the standard UV degradation process involves the production of hydroperoxides by the abstraction of a hydrogen atom from the polymer by the peroxy radical. 

A variety of compounds such as hydrocarbons, alcohols, furans, aldehydes, ketones, and acid compounds are formed as secondary oxidation products and are responsible for the undesirable flavors and odors associated with rancid fat. The off-flavor properties of these compounds depend on the structure, concentration, threshold values, and the tested system. Aliphatic aldehydes are the most important volatile breakdown products because they are major contributors to unpleasant odors and flavors in food products. The peroxidation pathway from linoleic acid to various volatiles is determined in several researchs, - by using various techniques (Gas chromatography mass spectrometry, GC-MS, and electron spin resonance spectroscopy, ESR), identified the volatile aldehydes that are produced during the oxidation of sunflower oil. In both cases, hexanal was the major aldehyde product of hydroperoxide decomposition, whereas pentanal, 2-heptenal, 2-octenal, 2-nonenal, 2,4-nonadienal, and 2,4-decadienal were also identified. 

Secondary Oxidation Products. This refers particularly to those degradation products of hydroperoxides which are significant because of their influence on flavour even at very low concentration. Three papers relate to volatile oxidation products from linoleate at temperatures between 70 and 250°C and attention is drawn to the... 

We have seen earlier that in the presence of an antioxidant the rate of hydroperoxide formation is related to the ratio of lipid concentration to that of the antioxidant concentration, which is dependent on the rate of inhibition reaction (4). The effectiveness of an antioxidant is generally based on the balance between the inhibition rate (k ) of reaction (4) and the transfer reactions ( ), (9) and (13). Therefore, the effect of antioxidants on hydroperoxide decomposition reactions (10) and (11) is an important property that needs to be evaluated. However, most studies of antioxidant actions measure initial events of lipid oxidation based on oxygen absorption, hydroperoxide formation, and peroxide values (Chapters 5 and 7). Very few studies have measured the effect of antioxidants on decomposition products of hydroperoxides, such as aldehydes and carbonyl compounds. Yet these volatile decomposition aldehydes are most relevant to the development of rancidity and to the ultimate quality and stability of food lipids. 

Diallylsulfide can also be oxidized selectively to the sulfone without oxidation of the unsaturated side chain [144]. Coordination of the sulfur is apparently strong enough to exclude the olefinic ligand from the reactive center since in the absence of the sulfur, olefins are readily epoxidized by r-BuOOH in the presence of molybdenum complexes. The product of hydroperoxide oxidation of sulfides depends on the reaction conditions [143]. Sulfoxides are obtained at temperatures below 50 °C using excess sulfide whereas sulfones are the predominant product at temperatures above 55 °C using excess hydroperoxide. 

Sheldon has considered the competing process of homolytic decomposition of hydroperoxides during the epoxidation of olefins with tert-h xty hydroperoxide in the presence of molybdenum complexes. It was found that homolytic decomposition of the hydroperoxide is initiated by electron transfer reactions of Mo(V) and Mo(VI) complexes with the hydroperoxide giving rise to free radical species. Reaction rates and products of hydroperoxide decomposition were dependent on the solvent and on the presence or absence of an olefin. The rates and selectivities of epoxidation were highest in polychlorinated hydrocarbons and very poor in coordinating solvents such as alcohols or ethers [387]. 

Stoehler et al. [44] studied the thermal properties of polyethylene-montmorillonite nanocomposites and found evidence for accelerated formation and decomposition of hydroperoxides during the thermooxidative degradation of the nanocomposites in the range of 170°C to 200°C, as compared to unfilled polyethylene, as weU as the formation of intermolecular chemical cross-links in the nanocomposites above 200°C due to recombination reactions involving the radical products of hydroperoxide decomposition. 

The data also indicate that there were quantitative differences in the volatile constituents of the three varieties. ANOVA indicated that of these differences, 13 were statistically significant. The aldehydes were by far the most prominent of the volatiles, amounting to 50% of the total volatile composition, which is in good agreement with data published previously for olive oil (1). Hexanal, trans-2-htxtnd and 3-methyl butanal were the most prominent aldehydes, the former two volatiles being major products of hydroperoxide lyase activity. In most instances Picual produced the greatest amount of volatiles, particularly of the aldehydes. The identification of these volatile constituents in the callus cultures verifies the use of callus cultures as a model system for the study of the LOX pathway in olive. 

A specific type of rancidity, known as reversion in the United States, occurs during the oxidation of soybean oil. Reversion is caused by some furan derivatives, such as 2-pentylfuran or (Z)- and ( )-2-(pent-2-en-l-yl)furan and other decomposition products of hydroperoxides. Pentylfuran arises (Figure 3.48) as a by-product of the decomposition of 9-hydroperoxyoctadeca-10,12-dienoic acid derived from linoleic acid via the terminal radical. The content of linoleic acid in soybean oil is about 51% of the total fatty acids. Similarly, both isomers (cis and trans) of 2-(pent-2-en-l-yl) furan (3-139) arise by decomposition of 9-hydroperoxyoctadeca-10,12,15-trienoic acid formed by autoxidation of linolenic acid. 

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