Radicals hydroperoxy

Peroxy and hydroperoxy radicals play important roles ia the knock process. A number of good reviews have discussed the details of the chemical mechanisms (16). Ignition delay (tau) has also been used for description of the chemical tendency to knock (17). The chemical factors affecting knock are... 

This proposal, however, has been criticized on the basis of transition state theory (74). Hydroperoxy radicals produced in reaction 23 or 24 readily participate in chain-terminating reactions (eq. 17) and are only weak hydrogen abstractors. When they succeed in abstracting hydrogen, they generate hydrogen peroxide ... 

Little is known about the existence of alkyl hydrotetraoxides, R—OOOOH. There is some kinetic evidence supporting methyl hydrotetraoxide [23594-84-5] as a very labile intermediate in the reaction of methylperoxy radical, , and hydroperoxy radical, OOH (63). 

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. 

It has been proposed that oxygen adds to the excited keto group [- (112)]. The rearrangement of the resulting hydroxyhydroperoxy diradical (112) could then proceed by intramolecular hydrogen abstraction involving a six-membered cyclic transition state, followed by fission of the former C —CO bond to form the unsaturated peracid (113) as the precursor of the final product. Such a reaction sequence demands a hydrogen atom in the J -position sterically accessible to the intermediate hydroperoxy radical. 

With some systems, the hydroperoxide is reduced to hydroperoxy radical by the metal ion in its higher oxidation state (Scheme 3.39). Thus, it is possible to set up a catalytic cycle for hydroperoxide decomposition. 

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... 

Not all oxidants formed biolc cally have the potential to promote lipid peroxidation. The free radicals superoxide and nitric oxide [or endothelium-derived relaxing aor (EDRF)] are known to be formed in ww but are not able to initiate the peroxidation of lipids (Moncada et tU., 1991). The protonated form of the superoxide radical, the hydroperoxy radical, is capable of initiating lipid peroxidation but its low pili of 4.5 effectively precludes a major contribution under most physiological conditions, although this has been suggested (Aikens and Dix, 1991). Interestingly, the reaction product between nitric oxide and superoxide forms the powerful oxidant peroxynitrite (Equation 2.6) at a rate that is essentially difiiision controlled (Beckman eta/., 1990 Huie and Padmaja, 1993). 

This oxidative process has been successful with ketones,244 esters,245 and lactones.246 Hydrogen peroxide can also be used as the oxidant, in which case the alcohol is formed directly.247 The mechanisms for the oxidation of enolates by oxygen is a radical chain autoxidation in which the propagation step involves electron transfer from the carbanion to a hydroperoxy radical.248... 

Polymer hydroperoxy radical, formation of oxidized groups, 197 Polymer media, preparation of elaborate metal species, 238... 

A closer examination by ex situ analysis using NMR or gas chromatography illustrates that intrazeolite reaction mixtures can get complex. For example photooxygenation of 1-pentene leads to three major carbonyl products plus a mixture of saturated aldehydes (valeraldehyde, propionaldehyde, butyraldehyde, acetaldehyde)38 (Fig. 33). Ethyl vinyl ketone and 2-pentenal arise from addition of the hydroperoxy radical to the two different ends of the allylic radical (Fig. 33). The ketone, /i-3-penten-2-one, is formed by intrazeolite isomerization of 1-pentene followed by CT mediated photooxygenation of the 2-pentene isomer. Dioxetane cleavage, epoxide rearrangement, or presumably even Floch cleavage130,131 of the allylic hydroperoxides can lead to the mixture of saturated aldehydes. 

Fig. 36 A Wagnerova Type II hydroperoxy radical chain initiated autooxidation.

Saylor, R. D. An estimate of the potential significance of heterogeneous loss to aerosols as an additional sink for hydroperoxy radicals in the troposphere, Atmos. Environ., 31, 3653-3658,1997. 

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... 

Air is required for conversion of the keto-enol to the endoperoxide. The most likely reaction is autoxida-tion. The O2 makes bonds to C2 and C6, neither of which has an H atom attached for abstraction. But abstraction of H from 07 gives a radical, A, that is delocalized over 07 and C2. Addition of O2 to C2 gives a hydroperoxy radical, which abstracts H from 07 of the starting material to give a hydroperoxide and A again. The hydroperoxide thus obtained can then add to the C6 ketone in a polar fashion to give the observed hemiketal. 

The significant step is represented by reaction (8.84). One should recall that there can be appreciable amounts of H02 in the early parts of a flame. The appearance of the N02 is supported further by the fact that reaction (8.84) is two orders of magnitude faster than reaction (8.85). The importance of the hydroperoxy radical attack on NO appeared to be verified by the addition of NO to the cold-fuel mixtures in some experiments. In these tests, the NO disappeared before the visible region was reached in oxygen-rich and stoichiometric flames, that is, flames that would produce H02. The N02 persists because, as mentioned previously, its reduction to N2 and 02 is very slow. The role of H02 would not normally be observed in shock tube experiments owing to the high temperatures at which they usually operate. 

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 ... 

Reaction 2-6 is sufficiently fast to be important in the atmosphere. For a carbon monoxide concentration of 5 ppm, the average lifetime of a hydroxyl radical is about 0.01 s (see Reaction 2-6 other reactions may decrease the lifetime even further). Reaction 2-7 is a three-body recombination and is known to be fast at atmospheric pressures. The rate constant for Reaction 2-8 is not well established, although several experimental studies support its occurrence. On the basis of the most recently reported value for the rate constant of Reaction 2-8, which is an indirect determination, the average lifetime of a hydroperoxy radical is about 2 s for a nitric oxide concentration of 0.05 ppm. Reaction 2-8 is the pivotal reaction for this cycle, and it deserves more direct experimental study. 

The methyl radical rapidly (in 10 s) combines with oxygen to form the methylperoxy radical, CH3O3. A recent study has confirmed that nitric oxide is oxidized by methylperoxy, although the rate constant is still unknown. The meffioxy radical, CH3O, should then react predominantly with oxygen to form formaldehyde, CHjO, and hydroperoxy radical. The net result of this sequence is the oxidation of one molecule of nitric oxide to nitrogen dioxide and the conversion of an alkyl radical into a hydro-... 

There were two important innovations in the development of these oxidative cycles the use of carbon monoxide which had previously been considered a relatively inert molecule in the atmosphere to regenerate the hydroperoxy radical via Reactions 2-6 and 2-7 and the use of peroxy radicals HO, and RO, to oxidize nitric oxide to nitrogen dioxide. 

The participation of hydroxyl and hydroperoxy radicals in the oxidation of nitric oxide raises the possibility that these radicals might also attack hydrocarbons. In the case of hydroxyl these reactions are known to be fairly rapid. On the basis of e rate constants that have been measured and estimates of those which have not the rates of attack of hydroxyl and hydroperoxy radicals appear to be large enough to explain the excess consumption of propylene shown in Figure 2-5. 

Although the above reactions generate a few fi radicals, most of the oxidation of nitric oxide to nitrogen dioxide is carried out by the alkyl-peroxy, RO, and hydroperoxy radicals that are formed in later reactions involving reactive hydrocarbons, aldehydes, or even carbon monoxide. One such example is shown in Figure 2-7. There is still considerable uncertainty as to the mechanism of these secondary reactions. The modeling studies should be consulted for details. ... 

The importance of the secondary reactions can be expressed as a radical chain length, which is the total rate of all reactions involving a particular radical divided by the primary rate of formation of that radical. For a particular set of conditions chosen in the modeling studies, chain lengths of about 4 for the hydroperoxy radical and about 8 for the alkyl-peroxy and hydroxyl radicals were calculated early in the irradiation period. After further irradiation, the chains became shorter. 

Direct determinations of rate constants are needed for almost all the reactions of hydroperoxy radical and ROj. 

Reactions of the hydroxyl radical dominate the removal of hydrocarbons. However, several other reactants make significant contributions, including hydroperoxy radical, ozone, and oxygen atoms. (This conclusion depends on the hydrocarbon being considered it is claimed that some terpenes in air are attacked mainly by ozone. )... 

The hydroperoxy radical has the highest concentration of all the free radicals in smog. The concentrations of both hydroperoxy and hydroxyl radicals are rather insensitive to primary pollutant concentration. 

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