Methylperoxy reactions

Tertiary peroxyl radicals also produce chemiluminescence although with lower efficiencies. For example, the intensity from cumene autooxidation, where the peroxyl radical is tertiary, is a factor of 10 less than that from ethylbenzene (132). The chemiluminescent mechanism for cumene may be the same as for secondary hydrocarbons because methylperoxy radical combination is involved in the termination step. The primary methylperoxyl radical terminates according to the chemiluminescent reaction just shown for (36), ie, R = H. 

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). 

Pate, C. T., B. J. Finlayson, and J. N. Pitts, Jr. A long path infrared spectroscopic study of the reaction of methylperoxy free radicals with nitric oxide. J. Amer. Chem. Soc. % 6554-6558, 1973. 

Methoxy (CH3O ) and methylperoxy (CH3O2 ) radicals have been subjected to substantial study. Zaslonko et al. have reviewed several reactions involving the... 

We now use the following reasonable simplifying assumptions to consider why oxidation rates in both the gas phase and dilute solution are proportional to [ferf-BuH]3/2—i.e., all the interactions represented by kx are nonterminating (Reaction 5), all termination is by Reaction 10, no methylperoxy radicals propagate (because they terminate so fast), and all initiation is by tert-BuO radicals from terf-Bu202. Then Relation 8 is replaced by... 

The increased temperature results in an increased rate of destruction of the branching intermediate (methyl hydroperoxide) with a consequent further increase of the rate, but also a decreased rate of formation of fresh hydroperoxide since Equilibrium 5 is displaced to the left, and the alternative reactions of methylperoxy increase in rate faster than that leading to formation of hydroperoxide. Consequently the quasi-stationary concentration of methyl hydroperoxide falls, and the rate of reaction declines since the new product of methyl oxidation—formaldehyde— cannot bring about branching at these temperatures. The temperature of the reaction mixture falls (because the rate has fallen), and when it has fallen sufficiently, provided sufficient of the reactants remain, the whole process may be repeated, and several further flames may be observed. 

The last argument which has been raised favoring this reaction is that the 28 kcal. of bond energy liberated in forming the methylperoxy radical produce a hot species, which in the next step then can use these 28 kcal. to abstract internally the hydrogen atom and isomerize to the hydroperoxy alkyl radical, shown in the diagram above as Step 10. However, such a step is very slow compared with the back reaction of dissociation, Step —1, or the more probable thermalization of the hot methylperoxy radical by collisions with other molecules. 

The formation of CH3O2 and subsequent reactions of this component are most important below 1000 K. At higher temperatures, the equilibrium for reaction (R15) is shifted toward the reactants, and methylperoxy radical is no longer thermally stable.1... 

The methyl radical may recombine with 02 (R15), followed by reaction of methylperoxy with formaldehyde,... 

High-Temperature Oxidation The high-temperature oxidation of methane has been studied extensively, and the mechanism is better established than that of the low temperature oxidation. At high temperatures the methylperoxy radical (CH302) is no longer stable, and the low-temperature oxidation pathway initiated by reaction (R15) is not active. Furthermore the chain-branching reaction... 

Reaction 92 with H02 results in formation of a hydroperoxide. Since the nature of the haloalkyl group is not likely to affect greatly the reactivity for this process, rate constants similar to that of 5 x lCT12cm3 molecule-1 s-1 for methylperoxy are expected. In view of the low concentrations of H02 radicals in the troposphere, this reaction is expected to be of only minor significance, except under conditions where the NO and N02 concentrations are also low. 

Rate coefficients for reactions of methylperoxy radicals with other radicals (From ref. 59.)... 

Simulations demonstrate, however, that variations in kinetic parameters of reactions under consideration lead to substantial consequences. Figure 15 shows how relatively small variations in the rate constant for reaction (30) influence the SID in methane-ethane mixtures. In such a reaction system (which models real compositions of natural gas) competition of different channels of ethyl-oxygen reaction overlaps (and very probably interferes) with methyl-oxygen chemistry. The latter is even somewhat qualitatively different there are no variations in mono-molecular reactions of methylperoxy radicals at temperatures below 900 K (only dissociation to methyl and 02) and all their bi-molecular reactions lead to branching as a nearest consequence. As to the ethyl-oxygen chemistry, it is much more rich and much less definite at the same time. So in this particular case, small variations in kinetic parameters lead to very substantial consequences. 

The oxidation of methyl radical with molecular oxygen is a complicated process that involves different competing reactions. The first step of CH3 oxidation is the formation, without energy barrier, of the methylperoxy radical that is a common intermediate for the subsequent reactions. In the pathway that give CH2O + OH as final products, the existence of a CH300 >CH200H isomerization process is postulated to occur. Because of the well known difficulties in the theoretical characterization of radical reactions and the importance of the role that these reactions plays in the combustion processes, we have undertaken the study of the entire reaction profile with both HF-CI and DF methods. The localization of the transition state for the above mentioned isomerization at ab-initio HF level is possible only when the correlation is considered [49]. 

The methylperoxy radical can react with NO, N02, HOz radicals, and itself (also other organic peroxy radicals R02 ). The reaction with NO, analogous to reaction (6) (and 19),... 

Table 6-12. Comparison of Atmospheric Reaction Rates for Methylperoxy Radicals ...

K yields a value five times larger than that previously assumed for atmospheric modelling calculations. Further quantitative studies of reactions of atmospheric importance include measurements of the temperature dependences of process (24) and of the recombination rate of methylperoxy radicals with N02, and... 

Under some experimental conditions, including low hydrocarbon concentration and high rate of radical formation, the interaction of methyl-peroxy and cumylperoxy radicals rather than self-reaction of cumyl-peroxy radicals accounts for most of the terminating interactions. The competing reaction for methylperoxy is abstraction from cumene, viz. 

The term a is the fraction of terminations per self-reaction of two cumylperoxy radicals. When there are no methylperoxy radicals, a equals 0.1 at 60° C. At high rates of initiation and low [RH], the value of a increases and in principle could reach 2.1 if every cumoxy is converted to methylperoxy and terminates with another cumylperoxy radical. Thus kt can range only from fe38 to 2.1 k3S. 

At night the thermal decomposition of PAN in an atmosphere with a relatively high mixing ratio of NO (e.g.,10 ppb) can initiate reaction 5.96 that converts NO to NO2 and produces a methylperoxy radical, which itself may convert another NO to NO2 and produce a molecule of formaldehyde, reaction 5.45. This sequence could lead to a reservoir of photo-chemically active species when the Sun rises. 

The methylperoxy radical (and probably other peroxy radicals) reacts very rapidly with NO (4.41) relative to hydrogen abstraction (4.40), so only in very clean environments will the latter reaction occur to a significant extent (McFarland et al., 1979 Hanst and Gay, 1983). In polluted atmospheres, nitrogen oxides interfere with several other of the above reactions, leading to the formation of nitrate and peroxy-nitrate esters (see Section 4.A.4). 

In addition, bimolecular decay of peroxy radicals is commonly observed. The intermediate in this type of reaction is generally accepted to be a tetroxide (23), that is, a species with four successive oxygen atoms (Russell, 1957). These intermediates break down by several routes to afford oxidized products and O2 or H2O2 (Figure 4.10). It has even been suggested that such reactions could occur in the unpolluted troposphere (in the absence of NO), although it appears likely that in most environments the reaction with HOO- to form a hydroperoxide would prevail. This appears to be true at least for methylperoxy radicals (Cox and Tyndall, 1980). 

The kinetics of the reaction CH3O2 + NO3 was studied by modulated photolysis spectroscopy [7] and later in a discharge flow reactor combined with molecular beam mass spectrometry [8]. In the latter experiment, the CD3O2 radicals were used instead of CH3O2, and their first order decay monitored in the presence of excess NO3 radicals. It was however observed that the first order decay of the methylperoxy radicals did not extrapolate to a common intercept and that the second order plot showed a large positive intercept. This is caused by the regeneration of CH3O2 radicals via reaction... 

Percent product distribution acetone 24.5 5.1, 2-methyl-2-propanol 18.8 4.0, 2-methyl-2-hydroperoxypropane 36.7 7.5, 2-methyl-propanal 14.0 3.9, 2-methyl-propanol 4.4 1.3, tertiary butylperoxide < 1.7. The peroxy radicals involved are primary 2-methyl-1-propylperoxy, primary methylperoxy and tertiary 2-methyl-2-propylperoxy. The relatively large yield of tertiary butanol is due to the interaction between CH3OO and tertiary butylperoxy radicals. Computer simulations based on the known rate coefficients for the self-reactions of these radicals [2] gave = 3 x 10" cm molecule s for the cross combination reaction. To simulate the observed ratio of primary alcohol and aldehyde requires a rate coefficient p 3 x 10" cm molecule s for the interaction between 2-methyl-1-propylperoxy and tertiary 2-methyl-2-propyl-peroxy radicals. The oxidation mechanism is quantitatively well understood. 

Methylperoxy self-reaction Products and branching ratio between 223 and 333 K,... 

Methylperoxy self-reaction products and branching ratio between 223 and 333 K, in G. Restelli, G. Angeletti (eds). Fifth European Symp. on Physico-Chemical Behaviour of Atmospheric Pollutants, Kluwer Academic Publ., Dordrecht 1990, pp. 341-346. 

UV-absorption spectrum of the methylperoxy radical and the kinetics of its disproportionation reaction at 300 K,... 

Lightfoot PD, Veyret B, Lesclaux R. Flash photolysis study of the methylperoxy + hydroper-oxy reaction between 248 and 573 K. J Phys Chem. 1990, 94,708-14. 

The c jmyloxy radicals produced by non-terminating interactions either abstract a H-atom from cumene or undergo B-scission at ambient temperatures to give acetophenone and methyl radicals, the latter being converted to methylperoxy by reaction with oxygen. 

Methylperoxy radicals either propagate autoxidation by reacting with cumene or terminate the reaction by reacting with cumyl-peroxy radicals. 

The magnitude of this rate constant depends on ki3, K12, f, the fraction of cimyloxy radicals which undergo 3-scission, and the fraction of methylperoxy radicals which are consumed in the termination reaction (19). 

Figiore 2). This increase in rate was attributed (31) to reaction of cumyloxy and methylperoxy radicals with the hydroperoxide thus preventing CH3O2 from undergoing chain termination reactions. 

Acetyl peroxide, methyl acetate, dimethyl peroxide and ethane are not produced in these reactions indicating that there is no cage collapse of the radicals. Instead the methyl radicals diffuse out of the cage and react with oxygen to give methylperoxy. This means that autoxidation of acetaldehyde is terminated either by reaction of methylperoxy with acetylperoxy or by self-reaction of methylperoxy (23) and (2i+). 

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