Ketones from hydroperoxide decomposition

Depending on the temperature, we cannot exclude that ketones can also originate from hydroperoxides decomposition, via a closed process, which does not create new radicals and then does not spread oxidation. Furthermore, because the process takes place without further chain scissions, it does not induce any variation of the molecular mass and only minimal changes of the mechanical properties of UHMWPE. 

The aldehydes and ketones are least abundant of all the compounds found which may be considered as derived from the fat. The carbonyl compounds are probably produced by an indirect route, which is most likely similar to that involved in autoxidation of a fat. The alkyl free radical can absorb oxygen, form a hydroperoxide, and then follow the many decomposition paths which are familiar in the oxidation chemistry of fats. The more abundant aldehydes found are unsaturated, which further agrees with the hypothesis that they are derived from the decomposition of hydro-... 

Two different cases may occur. If this radical is formed in a succession of styrene units (1), it reacts in the same way as in PS. If it is formed on a styrene unit linked to an acrylonitrile unit (2), three reaction pathways may be envisaged. The alkoxy radical resulting from the decomposition of the hydroperoxide formed on this polystyryl radical may react by 3-scission. Scissions (a) and (b) yield chain ketones, acetophenone end-groups and phenyl and alkyl radicals as previously observed in the case of PS photooxidation mechanism. Scission (c) leads to the formation of an aromatic ketone and an alkyl radical. This alkyl radical may be the precursor of acrylonitrile units (identified by IR spectroscopy at 2220 cm-1), or may react directly with oxygen and after several reactions generates acid groups, or finally this radical may isomerize to a more... 

The thermal or photochemical homolysis of the hydroperoxide leads to the formation of an alkoxy radical. The alkoxy radical is the precursor of unsaturated alcohols, acids and ketones. The decrease in intensity of the band at 807 cm-1 indicated saturation of the double bond, which could result from a radical addition to the double bond (for example, by reaction with the hydroxyl radicals resulting from the decomposition of hydroperoxides). Saturation reactions result in the formation of saturated alcohols, acids and ketones. 

The oxo-oxidation products from cyclohexane with tBuOOH arc cyclohcxyl peroxide and its decomposition products, cyclohexanol and cyclohexanone. The relative ratio of the hydroperoxide decomposition products depends on its decomposition mechanism (see scheme). After a homolytic 0-0 bond cleavage in the peroxide, the formed alkoxy radical can undergo disproportionation, yielding equal amounts of ol/onc. A high ketone yield results from the peroxide dehydration with a Lewis acid, such as l e(OII) formed by H2O2 decomposition on free Fe. 

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. 

The oxidation of alkanes by r-butyl hydroperoxide (TBHP) has been catalysed by titanium alkoxides, producing the corresponding alcohols and ketones. A radical mechanism is proposed in which r-butoxyl radical formed from TBHP and titanium alkoxide initiates the reaction. The evolution of oxygen (from the decomposition of peroxide) and the abstraction of hydrogen from alkane to form alkyl radical occur competitively. A method for the determination of both the primary and secondary KIEs at a reactive centre based on starting-material reactivities allows the determination of the separate KIEs in reactions for which neither product analysis nor absolute rate measurements are applicable. It has been applied to the FeCls-catalysed oxidation of ethylbenzene with TBHP, which exhibits both a primary KIE and a substantial secondary KIE the findings are in accordance with previous mechanistic studies of this reaction. The oxidation of two l-arylazo-2-hydroxynaphthalene-6-sulfonate dyes by peroxy-acids and TBHP catalysed by iron(III) 5,10,15,20-tetra(2,6-dichloro-2-sulfonatophenyl)porphyrin [Fe(ni)P] is a two-step process. In single turnover reactions, dye and Fe(in)P compete for the initially formed OFe(IV)P+ in a fast reaction and OFe(IV)P is produced the peroxy acid dye stoichiometry is 1 1. This is followed by a slow phase with 2 1 peroxy acid dye stoichiometry [equivalent to a... 

Thirdly, a problem with all allylic peroxides, but particularly severe here, the product a,3-unsaturated aldehyde (or ketone) polymerizes, incorporating other interesting products as well as some undecomposed hydroperoxide. The polymerization of acrolein by itself is gloriously complicated, and the nonvolatile residue from thermal decomposition of allyl hydroperoxide in toluene has so far defied analysis. Since the polymer constitutes the major, if not the only, chain termination product, some knowledge of its structure is necessary in order to obtain a free radical count and determine the efficacy of reaction 15. 

Post-irradiation oxidation of UHMWPE at room temperature results in the formation of ketones as main products of the oxidation cycle, together with hydroperoxides and variable amounts of acids, alcohols, esters, and lactones [21]. It has been shown [21] that hydroperoxides in UHMWPE are stable at room temperature and start to decompose at temperature higher than 70°C, therefore ketones and the other oxidized species observed during post-irradiation oxidation cannot result only from the decomposition of hydroperoxides. Considering these results, it can be supposed that ketones are also formed during the first step of oxidation of UHMWPE as a consequence of a direct reaction between macroalkyl radicals and oxygen. A modified Bolland s cycle is proposed in Scheme 9 [21]. 

Butenes were subjected to photosensitized reaction with molecular oxygen in methanol. 1-Butene proved unreactive. A single hydroperoxide, l-butene-3-hydroperoxide, was produced from 2-butene and isolated by preparative gas chromatography, Thermal and catalyzed decomposition of pure hydroperoxide in benzene and other solvents did not result in formation of any acetaldehyde or propionaldehyde. The absence of these aldehydes suggests that they arise by an addition mechanism in the autoxidation of butenes where they are important products. l-Butene-3-hydroperoxide in the absence of catalyst is converted predominantly to methyl vinyl ketone and a smaller quantity of methyl vinyl carbinol —volatile products usually not detected in important quantities in the autoxidation of butene. 

The cleanest product composition may be effected by decomposition of the pure hydroperoxide or solutions in the injection block of the gas chromatograph. In carbon tetrachloride solution only methyl vinyl ketone and methyl vinyl carbinol were produced, the ratio of ketone to alcohol being 2.9. No definite traces of products from isomerized hydroperoxide were observed. 

Such a rearrangement was detected only in the presence of sulfuric acid, and furthermore at 100°C. it was supplanted by a homolytic breakdown. The products found in the purely thermal decomposition—methyl vinyl ketone and methyl vinyl carbinol—are in fact consistent with the behavior of alkyl hydroperoxides and are analogous to the products produced from the cyclic allylic hydroperoxide from cyclohexene (2). 

Polymeric a-Oxygen-Suhstituted Peroxides. Polymeric peroxides (3) are formed from the following reactions ketone and aldehydes with hydrogen peroxide, ozonization of unsaturated compounds, and dehydration of ur-hydroxyalkyl hydroperoxides consequently, a variety of polymeric peroxides of this type exist. Polymeric peroxides are generally viscous liquids or amorphous solids, are difficult to characterize, and are prone to explosive decomposition. 

It should be noted that both linoleic and a-linolenic acids form hydroperoxides that absorb UV radiation at 233 nm (i.e., the same wavelength as that of CDs). Furthermore, CDs are formed upon decomposition of hydroperoxides from a-linolenic acid, absorbing at 233 nm, whereas secondary oxidation products, particularly ethylenic diketones and a-unsatu-rated ketones, show a maximum absorbance at -268 nm. Carotenoid-containing oils may interfere in the assay by giving higher than expected absorbance values at 233 nm, due to the presence of double bonds in the conjugated structures of carotenoids. 

In the presence of molecular oxygen, the oxidation of initially pure paraffins and cycloparaffins takes place at temperatures from 120 to 160 °C. In parallel with other products, there are formed acids, aldehydes, and ketones which induce the decomposition of hydroperoxides. This is documented by kinetic curves of hydroperoxide accumulation which show a maximum [78]. 

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