Peroxy radicals, initiation kinetics

Applying the steady-state approximation to the radicals R and OH- a rate law of the form above may be derived where [Cu J ofCu L j. The inverse dependence on [L l indicates that [Cu L] does not react with HgOj at a significant rate. A chain reaction has also been postulated in the oxidation of copper(n) dialkyldithiophos-phates with alkyl hydroperoxides. Initially, the redox process yields alkoxy and alkyl peroxy radicals. The kinetics of the initial reaction are second-order. For the dithiophosphate, the reaction is first-order with respect to each reagent, whereas for the corresponding dithiocarbamate complex there is a zero-order dependence on the metal species and a second-order dependence on the hydroperoxide concentration. 

Figure 6.2. Typical ignition delay of an alkane fuel as a function of the initial mixture s temperature. Three different kinetic models are shown (a) High temperature chemistry only that is, no peroxy radical chemistry, (b) Same as (a), but the Q OOH chain-branching channel of the peroxy radicals has been considered, (c) Same as (b), bnt the concerted elimination of RO2 to alkene + HO2 has been considered. (Figure courtesy of Timothy Barckholtz, ExxonMobil Research and Engineering.)...

The kinetics for the oxidation of leuco bases using oxygen has been studied.19 The oxidation involves complex formation between the proto-nated leuco base and the peroxy radical formed by air oxidation of the solvent. Addition of a radical initiator (AIBN) facilitates the reaction, while radical inhibitors retard the dye formation. In addition, oxidation reactions employing 2,3-dichloro-5,6-dicyanoquinone have shown large isotope effects in acetonitrile.20... 

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. 

The biperoxy radical produced by the ceric ion oxidation of 2,5-di-methylhexane-2,5-dihydroperoxide decays rapidly with first-order kinetics [k = ioio.e exp( -11,500 1000)/RT sec.1 = 180 sec."1 at 30°C. (30)]. After the first-order decay has run to completion, there is a residual radical concentration (—4% of the initial hydroperoxide concentration) which decays much more slowly by a second-order process. The residual second-order reaction cannot be eliminated or changed even by repeated recrystallization of the dihydroperoxide. This suggests that a small fraction of the biperoxy radicals react intermolecularly rather than by an intramolecular process and thus produce monoperoxy radicals. The bimolecular decay constant for this residual species of peroxy radical is similar to that found for the structurally similar radical from 1,1,3,3-tetra-methylbutyl hydroperoxide. Photolysis of the dihydroperoxide gave radicals with second-order decay kinetics which are presumed to be 2,5-hydroperoxyhexyl-5-peroxy radicals. 

According to the results shown in Figure 1, the kinetic chain length of the photooxidation of isooctane is very low. Even for the lowest rate of radical initiation applied, I 10 M/h, the kinetic chain length of the non-inhibited photooxidation did not exceed a value of 1. Radical termination, therefore, seems to dominate over a peroxy radical chain reaction according to equation (5) in Scheme I. 

Scheme IV. Kinetics for initiation of propagating peroxy radical chains by HOO".

The overall rate increases autocatalytically until a limiting rate is reached. At this point the concentration of hydroperoxide available for initiation reaches a constant value. In many cases the steady state concentration of hydroperoxide is low, autoxidation rates are high and kinetic chain lengths short. Most of the hydrocarbon is consumed by reactions with other than peroxy radicals . 

AIBN-initiated oxidation of cumene and Tetralin in the presence of deuterated amines were unsuccessful. They proposed an alternative mechanism involving reversible formation of a complex of antioxidant with peroxy radical as the kinetically controlling process. We observed an isotope effect, dAh = 1.8, consistent with the hydrogen-donation mechanism in the retarded oxidation of SBR polymer with deuterated amines (7,8). Our results were confirmed by observation of significant isotope effects in the initial stage of oxidation of purified cfc-l,4-polyiso-prene with both hindered phenols and amines (9). Table I shows the effect of temperature and antioxidant concentration on the rates of oxidation and the observed deuterium isotope effects. 

Kinetic studies have been performed on the individual steps occurring in the NO3 and OH initiated oxidation of VOCs. The studied reactions include essentially reactions of NO3 with alkenes, di-alkenes and dimethyl sulfide (DMS), reactions of NO3 with intermediate peroxy radicals (HO2, CH3O2, C2H5O2) and reactions of OH with methane and oxygenated VOCs (ethers, alcohols). The rate constants for these reactions have been measured, and mechanistic information has been determined. The experimental methods used were discharge-flow reactors coupled with mass spectrometry, electron paramagnetic resonance (EPR), laser-induced fluorescence (LIF) analysis and the laser photolysis associated with LIF analysis. The discharge-flow LIF and laser photolysis LIF experiments have been especially developed for these studies. 

One of the nice features of free-radical polymerization is that values of the preexponential coefficients and activation energies (or alternately half-life values at various temperatures) can be obtained in the literature (such as in Odian (1991)) or from their manufacturers (such as Wako Chemical Corp.) for a variety of initiators, and these numbers do not normally change no matter what the fluid environment the initiator molecules are in. Thus, if we want to decompose more than 99% of the starting initiator material in the reactor, we just have to wait for the reaction to proceed up to five times the initiator half-life. The other attractive feature of free-radical polymerization is that free-radical reactions are well known and radical concentrations can be directly measured. Thus, we know, for example, that if we want to preserve radicals in solution, we should not allow oxygen gas (O2) in our system, because reactive radicals will combine with oxygen gas to form a stable peroxy radical. That is why reaction fluids were bubbled with N2, CO2, Ar, or any inert gas, in order to displace O2 gas that comes from the air. Finally, Iree-radical polymerization is not sensitive to atmospheric or process water, compared to other polymerization kinetic mechanisms. 

The stoichiometric factors of inhibition and the rate constants of the ter-penephenols (TP) with isobornyl and isocamphyl substituents were determined by the reaction with peroxy radicals of ethylbenzene. The reactivity was found to decrease for o-alkoxy compared with o-alkyl substituent caused by the intramolecular hydrogen bond formation that is conformed by FTIR-spectroscopy. The inhibitory activity for mixtures of terpene-phenols with 2,6-di-ferf-butyl phenols in the initiated oxidation of ethylbenzene was also studied. In spite of the similar antiradical activities of terpenephenols with isobornyl and isocamphyl sunstituents, the reactivity of phenoxyl radicals formed from them are substantially different that is resulted from the kinetic data for mixtures of terpenephenols with steri-cally hindered phenols. 

Consistent with a radical chain mechanism, the rate of O2 insertion was found to be sensitive to light, and the addition of radical initiator AIBN was required in order to observe reproducible reaction rates. Based on analysis of the kinetics of O2 insertion into the Pd-C bond of 24, a mechanism involving mononuclear Pd(III) intermediates was proposed (Fig. 16). Palladium(III) intermediate 27, formed by the combination of dimethyl Pd(II) complex 24 with peroxy radical 26 [84], generates the observed Pd(II) peroxide 25 by homolytic Pd-C cleavage to reduce Pd(III) complex 27 and generate radical chain carrier Me. ... 

The initiation process constitutes the first reaction step in free radical polymerization, leading to the generation of (primary) radicals. The kinetics of the initiation process, ie its rate and effectiveness, are of fundamental importance in both theoretical studies and commercial applications. Commercial procedures mainly rely on the formation of primary radicals via thermal decomposition processes using azo- and peroxy-type compounds. Investigative kinetic studies are— to a large extent—carried out using photoinitiators, which decompose upon irradiation with UV or visible light. The main reason for this choice is the possibility to define exact start and end times of the initiation and subsequently the polymerization process. 

The seeondary reaetions of peroxy radieal with HO2 slow down the free radical driven photochemical oxidation reactions and reduce the formation of ozone. In addition, these reactions represent an important chemical sink for HOx radicals in the troposphere. Hence, the reactions of peroxy radical with HO2 are of comparable importance in the atmospheric fate of dimethylphenols. Previous studies on the reactions of dimethylphenol with OH radical have focused only on the initial H-atom abstraction step and its kinetics. Hence, this work focused mainly on the study of possible secondary reactions of the reaction between 2,3-dimethylphenol and OH radical. Theoretical calculations assess the feasibility of different reaction channels and provide thermochemical data for the reaction system. 

The potential energy surface, thermochemical and kinetic data for the reaction of 2,3-dimethylphenol with OH radical reveal several important aspects of alkylated aromatic compounds in the atmospheric chemistry. The reaction between 2,3-dimethylphenol and OH radical is initiated by H-atom abstraction from a methyl group and the resulting alkyl radical further reacts with O2 to form a peroxy radical, a key intermediate in the reaction mechanism. This peroxy radical has excess energy to undergo further reaction with the atmospheric species. 

Secondary cage recombination of peroxy radicals [698]. In a solid polymer, a pair of polymer peroxy radicals (POO 2) is trapped in the polymer matrix. When a radical pair, produced by photoinitiation, escapes the initial cage, the probability of its recombination remains high even after several propagation steps. This phenomenon, known as secondary cage recombination, has a pronounced effect on the kinetics of oxidation and on the distribution of kinetic chain lengths in the oxidation process. 

I) If the polymer peroxy radicals of the pair can diffuse or separately develop the oxidation chian, so that their descendants will not meet again, long kinetic chains are possible before they are terminated by pol3mier peroxy radicals from other pairs. In this case, the rate of oxygen consumption will be half the kinetic order in the initiator and sensitive to inhibitors. 

The kinetics shown in Figure 2.2 has been rationalized proposing that the formation of styrene oxide requires the generation of peroxy radicals derived from benzaldehyde. According to this, benzaldehyde should evolve at initial stages of the reaction and, when present in sufficient concentrations, can become oxidized to peroxyl radicals by the PINO radical present in NHPI-FeBTC. Experimental support to this proposal was obtained by performing a series of experiments in which increasing concentrations of benzaldehyde... 

The mechanism and kinetics of the atmospheric oxidation of alkynes, initiated by OH radicals, have been studied particularly to determine the role of alkyne oxidation in tropospheric ozone formation. A general mechanism for OH-initiated oxidation of alkynes has been developed with the aid of thermodynamic calculations. In general, the significance of atmospheric alkynes to the formation of tropospheric ozone was found to be smaller than for alkanes and alkenes, due to the absence of the hydroxy peroxy-forming product channel in the OH-initiated atmospheric oxidation of alkynes.227... 

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