Hydroperoxy radical, reaction with

The total 296 K rate constant was 1.6 x 10" cm s", with abstraction of O2 to yield the hydroperoxy radical (reaction (9)) noted as the major pathway. Exact branching ratios could not be defined since the product ions were also found to be very reactive towards H atoms, preventing any quantitative diagnostics. 

Alkyl radicals, R, react very rapidly with O2 to form alkylperoxy radicals. H reacts to form the hydroperoxy radical HO2. Alkoxy radicals, RO, react with O2 to form HO2 and R CHO, where R contains one less carbon. This formation of an aldehyde from an alkoxy radical ultimately leads to the process of hydrocarbon chain shortening or clipping upon subsequent reaction of the aldehyde. This aldehyde can undergo photodecomposition forming R, H, and CO or, after OH attack, forming CH(0)00, the peroxyacyi radical. 

Because of the importance of hydroperoxy radicals in autoxidation processes, their reactions with hydrocarbons arc well known. However, reactions with monomers have not been widely studied. Absolute rate constants for addition to common monomers are in the range 0.09-3 M"1 s"1 at 40 °C. These are substantially lower than kL for other oxygen-centered radicals (Table 3.7). 454... 

Figure 45-6. Interaction and synergism between antioxidant systems operating in the lipid phase (membranes) of the cell and the aqueous phase (cytosol). (R-,free radical PUFA-00-, peroxyl free radical of polyunsaturated fatty acid in membrane phospholipid PUFA-OOH, hydroperoxy polyunsaturated fatty acid in membrane phospholipid released as hydroperoxy free fatty acid into cytosol by the action of phospholipase Aj PUFA-OH, hydroxy polyunsaturated fatty acid TocOH, vitamin E (a-tocopherol) TocO, free radical of a-tocopherol Se, selenium GSH, reduced glutathione GS-SG, oxidized glutathione, which is returned to the reduced state after reaction with NADPH catalyzed by glutathione reductase PUFA-H, polyunsaturated fatty acid.)...

The photo-oxidation reactions are the last two in the scheme - these are listed as reactions G and T. In the former reaction, the initial radical abstraction is performed by pretty much any radical available in the polymer matrix. Reaction with oxygen to form the hydroperoxy radical followed by hydrogen abstraction to form a hydroperoxide has been suggested as a mechanism of gylcol oxidation [11, 25, 31] and is, of course, a very reasonable reaction path. Note that the... 

At these temperatures, singlet oxygen atoms could also react with hydrogen or methene to form OH. The OH reacts with 03 to produce hydroperoxy radicals H02. Both HO and H02 destroy ozone by an indirect reaction that sometimes involves O atoms ... 

Pemitric (or peroxynitric) acid is an example of a compound that could be present in significant quantities. This proposal is very speculative, because there is no evidence of this compound in the gas phase, although there is evidence of some such species in solutions.The reaction of hydroperoxy radical with nitrogen dioxide is usually written... 

Similarly, recent experiments" have been interpreted to mean that about 10% of the reaction of hydroperoxy radical with nitric oxide gives per-nitrous add, HOONO, instead of nitrogen dioxide and hydroxyl radical. Because this reaction is of major importance, even 10% of a second channel would be important, although it has been argued that such compounds would not be sufFidently stable to accumulate in the atmosphere." Whether such peroxynitrogen compounds are stable in the gas phase and whether they can be found in the atmosphere must await further experiments. 

The superoxide ion or its protonated form (hydroperoxy radical HO2 ) is produced in the system (Shugalei and Tselinskii 1993, 1994). Hydroquinone, which is known to interact effectively with superoxide ion (Afanas ev and Polozova 1978), exerts fairly strong inhibiting effect on the reaction. Addition of potassium ferricyanide to the system has, in contrast, an accelerating effect. The cause of the effect consists of transformation of the following type ... 

First, hydroperoxy radicals must react predominantly via Reaction 8 and not via Reaction 7. It is difficult to assess this competition because of the uncertain energetics of these reactions. Assuming that k7 — k3, Reaction 8 is faster than Reaction 7 when the hydrogen abstracted in 7 is primary, the rate constants are approximately equal when it is secondary, and 7 predominates when it is tertiary. Only under conditions where the yields of alkenes are considerable and the alkane has no tertiary C—H bonds will Reaction 8 be important. Even then, abstraction of allylic hydrogen from the alkene by HO2 will compete strongly with Reaction 8. 

This reaction competes favorably with other CH3O2 reactions, such as (R16) and (R20), and offers a fast pathway to the methoxy radical (CH3O). In a similar reaction, nitric oxide converts the hydroperoxy radical (HO2) to the more reactive hydroxyl radical,... 

Here Q denotes an alkyl radical with two unpaired electrons (in QOOH and QO) which may rearrange to form a stable alkene. The compound QO is a cyclic ether3 (which may break down to form an aldehyde4 and a smaller alkene). The sequence of reactions (R64) to (R67) is chain propagating, in that the initial alkyl radical has produced one HO2 or OH radical in addition to one or more stable components. However, it is also possible that a second oxygen molecule may add to QOOH to form a peroxy alkyl hydroperoxy radical,... 

Recent theories of knock (4, 33, 45, 207) have emphasized the role played by the hydroperoxy radical (H02-) under incipient knock conditions. H02 radicals would be formed by Reactions 9", 11, and 9-4. Reaction 9, with an activation energy of 40 kcal. (204), might be the reaction required to trigger ignition. The yields of both hydrogen peroxide (207) and hydrogen (107) have in fact been observed to increase rapidly near autoignition. 

Hendry, D.G. and D. Schuetzle. 1976. Reactions of hydroperoxy radicals. Comparison of reactivity with organic peroxy radicals. ]. Org. Chem. 41 3179-3182. 

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.,... 

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