Peroxy radicals ketone

Analogous bonding occurs in several important classes of free radicals, including nitroxides, peroxy radicals, ketone radical anions, and superoxide, all of which also have a spin-bearing center adjacent to an atom bearing an unshared pair of electrons. 

Oxidation begins with the breakdown of hydroperoxides and the formation of free radicals. These reactive peroxy radicals initiate a chain reaction that propagates the breakdown of hydroperoxides into aldehydes (qv), ketones (qv), alcohols, and hydrocarbons (qv). These breakdown products make an oxidized product organoleptically unacceptable. Antioxidants work by donating a hydrogen atom to the reactive peroxide radical, ending the chain reaction (17). 

Acids are usually the end products of ketone oxidations (41,42,44) but vicinal diketones and hydroperoxyketones are apparent intermediates (45). Acids are readily produced from vicinal diketones, perhaps through anhydrides (via, eg, a Bayer-ViUiger reaction) (46,47). The hydroperoxyketones reportedly decompose to diketones as well as to aldehydes and acids (45). Similar products are expected from radical— radical reactions of the corresponding peroxy radical precursors. 

The decomposition of the peroxyketals (53) follows a stepwise, rather than a concerted mechanism. Initial homolysis of one of the 0-0 bonds gives an aikoxy radical and an a-peroxyalkoxy radical (Scheme 3.36).306"08"210 This latter species decomposes by P-scission with loss of either a peroxy radical to form a ketone as byproduct or an alkyl radical to form a peroxyester intermediate. The peroxyester formed may also decompose to radicals under the reaction conditions. Thus, four radicals may be derived from the one initiator molecule. 

The formation of this ketone is believed to proceed via internal abstraction of H in the initial peroxy radical (128 cf. p. 328), followed by migration of Me. It may be that the vigorous conditions employed now make a 1,2-alkyl shift feasible, or that the shift of Me may involve fragmentation followed by re-addition, rather than direct migration. 

First the interaction of selected tetramethylpiperidine (TMP) derivatives with radicals arising from Norrish-type I cleavage of diisopropyl ketone under oxygen was studied. These species are most probably the isopropyl peroxy and isobutyryl peroxy radicals immediately formed after a-splitting of diisopropyl ketone and subsequent addition of O2 to the initially generated radicals. Product analysis and kinetic studies showed that the investigated TMP derivatives exercise a marked controlling influence over the nature of the products formed in the photooxidative process. The results obtained point to an interaction between TMP derivatives and especially the isobutyryl peroxy radical. 

In the presence of nitroxide I, diisopropyl ketone photooxidation takes a course differing considerably from that without this additive (Fig. 5). In this case high yields of isobutyric acid and acetone were obtained, presumably as products arising from the postulated peroxy radicals c and d. On the other hand, the formation of isopropanol is almost completely suppressed. 

Kinetic analysis of the results of ketone oxidation in the presence of amine II reveals that the velocity constant of the oxidation of amines by acyl per-oxy radicals must be greater (by a factor of 2 - 3) than that of the interaction of these radicals with the nitroxide-i. In this reaction, acyl peroxy radicals are captured and destroyed by amines. 

Also in the case of a polymer therefore, provided the acyl peroxy radicals are formed by ketone photolysis in the presence of oxygen, the oxidation of amines by these radicals would make a significantly greater contribution to stabilization than the nit-roxide. The latter is in any case present in only very small amount as secondary producti - -. 

As mentioned in the introduction, there are conflicting views as to the contributions made to polymer degradation by various initiating species. Among these species, in addition to ketones, hydroperoxides are some of the more important chromophores. As it is known, the photolysis of hydroperoxides yields alkoxy and hydroxy radicals. In polymers, in the presence of oxygen, these radicals lead to the secondary formation of peroxy radicals. The latter in turn are converted by hydrogen abstraction into new hydroperoxides (Scheme I) ... 

Separate experiments in which tert.-butoxy radicals were produced thermally in benzene from di-tert.-butyl peroxyoxalate failed to reveal any direct reaction of these radicals with amine II. Even at higher temperatures (A/ 150°C, dichlorobenzene, +00+ decomposition), the +0 radicals attacked neither amine II nor nitroxide I. The earlier described experiments of ketone photooxidation showed additionally that amine II displays no specially marked reactivity towards peroxy radicals. 

A similar statement could probably be made concerning ketones. These compounds are commonly used as solvents, and they are known to form fi radicals when photolyzed. Many chlorinated hydrocarbons, which are also widely used as solvents, can be attacked by hydroxyl radicals and thus contribute to peroxy radical formation. 

In summary the concentration of ozone in the polluted atmosphere is controlled by the intensity of sunlight and the ratio of nitrogen dioxide to nitric oxide. Hydrocarbons and other pollutants—such as aldehydes, ketones, chlorinated hydrocarbons, and carbon monoxide—react to form peroxy radicals. These, in turn, react with nitric oxide, causing the ratio [NOjjilNO] to increase. As a consequence of Equation 2-5, the ozone concentration also increases. 

The reactions of aldehydes at 313 K [69] or 323 K [70] in CoAlPO-5 in the presence of oxygen results in formation of an oxidant capable of converting olefins to epoxides and ketones to lactones (Fig. 23). This reaction is a zeolite-catalyzed variant of metal [71-73] and non-metal-catalyzed oxidations [73,74], which utilize a sacrificial aldehyde. Jarboe and Beak [75] have suggested that these reactions proceed via the intermediacy of an acyl radical that is converted either to an acyl peroxy radical or peroxy acid which acts as the oxygen-transfer agent. Although the detailed intrazeolite mechanism has not been elucidated a similar type IIaRH reaction is likely to be operative in the interior of the redox catalysts. The catalytically active sites have been demonstrated to be framework-substituted Co° or Mn ions [70]. In addition, a sufficient pore size to allow access to these centers by the aldehyde is required for oxidation [70]. 

The effect of the medium on the rates and routes of liquid-phase oxidation reactions was investigated. The rate constants for chain propagation and termination upon dilution of methyl ethyl ketone with a nonpolar solvent—benzene— were shown to be consistent with the Kirkwood equation relating the constants for bimolecular reactions with the dielectric constant of the medium. The effect of solvents capable of forming hydrogen bonds with peroxy radicals appears to be more complicated. The rate constants for chain propagation and termination in aqueous methyl ethyl ketone solutions appear to be lower because of the lower reactivity of solvated R02. .. HOH radicals than of free RO radicals. The routes of oxidation reactions are a function of the competition between two R02 reaction routes. In the presence of water the reaction selectivity markedly increases, and acetic acid becomes the only oxidation product. 

The effect of solvents on the oxidation route as regards the composition of reaction products was investigated for methyl ethyl ketone at 145° to 160°C. at a pressure of 50 atm. (9, 10). The composition of products under these conditions is a function of the competition between two routes of the peroxy radical reactions ... 

Zaikov (9) showed that isomerization and decomposition of a methyl ethyl ketone peroxy radical may occur as shown below. 

When methyl ethyl ketone is oxidized in aqueous solutions, the over-all reaction rate drops because of solvation of the peroxy radicals, and w1 decreases more than w2. The reaction rates for formation of methyl ethyl ketone oxidation products in aqueous solutions are shown, as an example, in Tables VII and VIII. 

Methylglyoxal and other a-carbonyl aldehydes MGLY Peroxy radicals formed from ketone, KET KETP... 

Other evidence that peroxy radical rearrangements are essentially high temperature phenomena is the formation of methyl ethyl ketone in cool flames where the carbon atom of the carbonyl group was originally tertiary in the alkane (17). Since cool flame temperatures are around 500°C., heterogeneous processes are unlikely, and all products must presumably arise homogeneously. 

However, the ratios of the unsaturated materials to the saturated materials and of the ketones to the alcohols (Table I) indicate that the yields of unsaturated materials are higher than those of saturated products, especially cyclohexenol. [The excess alcohol might come from 2ROO - 2RO- + 02 however, its importance in the gas phase is unknown.] It was suggested from the above that some cyclohexenol and possibly cyclo-hexenone may be formed from cyclohexenyl hydroperoxide which is produced from chain reactions initiated by the cyclohexyl peroxy radical and cyclohexenyl peroxy radical as shown below. 

Lee S-H, Mendenhall GD (1988) Relative yields of excited ketones from self-reactions of alkoxyl and alkylperoxyl radical pairs. J Am Chem Soc 110 4318-4323 Leitzke A, Reisz E, Flyunt R, von Sonntag C (2001) The reaction of ozone with cinnamic acids - formation and decay of 2-hydroperoxy-2-hydroxy-acetic acid. J Chem Soc Perkin Trans 2 793-797 Lodhi ZH, Walker RW (1991) Oxidation of allyl radicals kinetic parameters for the reactions of allyl radicals with H02 and 02 between 400 and 480 °C. J Chem Soc Faraday Trans 87 2361-2365 Martini M, Termini J (1997) Peroxy radical oxidation of thymidine. Chem Res Toxicol 10 234-241... 

Of special attention is the single-stage method of olefm oxide production by their conjugated oxidation with another, more easily oxidizing compound (aldehyde, ketone, etc.) [134,139], For the epoxidation of an olefm, active oxygen of peroxy radicals is used in such a system ... 

The second route of getting free radical center near is mediated by low-molecular free radicals or compounds which are either present in the polymer or are gradually formed there by fragmentation reactions. While tertiary peroxy radicals propagate only by abstraction of hydrogen atom from surrounding C —H bonds, secondary peroxy radicals may easily cleave to hydroxy radicals and ketones as follows ... 

There exists another pathway of self-reaction of two secondary or primary alkyl peroxy radicals which is even more favorable from the viewpoint of exothermicity than it is Russell s mechanism. It is the reaction in which two molecules of ketone and hydrogen peroxide (or hydrogen and oxygen) are formed as follows ... 

Holroyd and Noyes69 have some difficulty in explaining how the chain length of the reaction was increased by ketene at constant absorbed intensity. If other reactions of hydroxyl or hydroperoxy radicals were introduced that were chain ending then, by competition with reactions (155) and (157), dependence on ketone concentration might be increased. They did not suggest the probability that the decomposition of methylene peroxy radicals would be second order and not instantaneous, i.e.,... 

To describe detailed kinetics for systems containing peroxy radicals and phenols in mixed water-methyl ethyl ketone media, six kinds of... 

The ultimate products of the oxidation of any hydrocarbon are carbon dioxide and water vapour, but there are many relatively stable partially oxidised organic species such as aldehydes, ketones and carbon monoxide that are produced as intermediate products during this process, with ozone produced as a by-product of the oxidation process. Figure 10 shows a schematie representation of the free radical catalysed oxidation of methane, whieh is analogous to that of a hydrocarbon. As previously discussed, the oxidation is initiated by reaction of the hydrocarbon with OH and follows a mechanism in with the alkoxy and peroxy radicals are chain propagators and OH is effectively catalytic, viz... 

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