Radicals reaction with hydrocarbons

Opeida [46] compared the values of the rate constants of peroxyl radical reactions with hydrocarbons with the BDE of the oxidized hydrocarbon, electron affinity of peroxyl radical, EA(R02 ) ionization potential of hydrocarbon (/Rn), and steric hindrance of a-substituent R(Fr). They had drawn out the following empirical equation ... 

Rate Constants of Haloidalkylperoxyl Radicals Reactions with Hydrocarbons (348 K, Solvent is Oxidized Hydrocarbon)... 

The reaction enthalpy is not the single factor that influences the activation energy of this reaction. The parameters of the peroxyl radical reaction with hydrocarbon (ethylbenzene) and phenol (p-cresol) are compared in the following table [34]. 

Kinetic Parameters of Phenoxyl and Aminyl Radical Reactions with Hydrocarbons and Hydroperoxides in IPM Model [4,34,38]... 

Neeb, P. (2000), Structure-reactivity based estimation of the rate constants for hydroxyl radical reactions with hydrocarbons, J. Atmos. Chem., 35, 295-315. 

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... 

The isotope effect in peroxyl radical reactions with C—H/C—D bonds of attacked hydrocarbon shows the direct hydrogen atom abstraction as the limiting step of this reaction [15], For example, the cumylperoxyl radical reacts with the C—D bond of a-deuterated cumene (PhMe2CD) ninefold slower than with the C—H bond (cumene, 303 K [118]). The second isotope effect (ratio fcp(PhMe2CH)//tp(Ph(CD2)2CH) is close to unity, i.e., 1.06 per C—D bond [118],... 

Opeida proposed the following empirical equation for the rate constant of the peroxyl radical reaction with C—H bonds of alkylaromatic hydrocarbons [34] ... 

The traditional chain oxidation with chain propagation via the reaction RO/ + RH occurs at a sufficiently elevated temperature when chain propagation is more rapid than chain termination (see earlier discussion). The main molecular product of this reaction is hydroperoxide. When tertiary peroxyl radicals react more rapidly in the reaction R02 + R02 with formation of alkoxyl radicals than in the reaction R02 + RH, the mechanism of oxidation changes. Alkoxyl radicals are very reactive. They react with parent hydrocarbon and alcohols formed as primary products of hydrocarbon chain oxidation. As we see, alkoxyl radicals decompose with production of carbonyl compounds. The activation energy of their decomposition is higher than the reaction with hydrocarbons (see earlier discussion). As a result, heating of the system leads to conditions when the alkoxyl radical decomposition occurs more rapidly than the abstraction of the hydrogen atom from the hydrocarbon. The new chain mechanism of the hydrocarbon oxidation occurs under such conditions, with chain... 

The theory of chain co-oxidation of binary mixtures of organic compounds was described in Chapter 5. The experimental study of co-oxidation of alcohols (HRiOH) and hydrocarbon R H opens the way to measure the rate constants of one chosen peroxyl radical R OO with several alcohols HRiOH and on the reverse, the chosen alcohol HR1 OH with several peroxyl radicals RiOO. The parameters of co-oxidation of alcohols and hydrocarbons are collected in Table 7.6. The absolute values of peroxyl radical reactions with alcohols were calculated from these data using the values of kp from Table 2.8 (see Table 7.7). 

The polar interaction changes the geometry of the transition state of the reaction R02 + RH. Atoms C, H, O of the reaction center O H C of this reaction are in a straight line for the reaction of the peroxyl radical with a hydrocarbon. The reaction center O H C has an angular geometry in the reaction of the polar peroxyl radical with a polar molecule of the ketone. The interatomic distances rc H and i o n and angles peroxyl radical reactions with ketones calculated by the IPM method [79,80] are given in Table 8.15. 

When a molecule consists of a few similar fragments n, the rate constant of the reactant reaction with this molecule can be expressed as the product of the partial rate constants ky k = nx kj. This was proved many times for free radical reactions for groups of reactants where both reactants or one of them are nonpolar. For example, the rate constants of peroxyl radical reactions with nonbranched aliphatic hydrocarbons Me(CH2) Me can be presented in... 

A second process that has a central position in the analysis of the chemical properties of carbenes is their reaction with hydrocarbons. As is the case for alcohols, singlet and triplet carbenes react with hydrocarbons in distinctive ways. It has long been held that very electrophilic singlet carbenes can insert directly into carbon-hydrogen bonds (11) (Kirmse, 1971). On the other hand, triplet carbenes are believed to abstract hydrogen atoms to generate radicals that go on to combine and disproportionate in subsequent steps (12)... 

FIGURE 3 12 Possible initial steps for ozone, atomic oxygen, nitrogen dioxide, and hydroxyl-radical reaction with aromatic hydrocarbons. 

Thus hydrocarbons and CO are products of incomplete combustion, which occurs primarily when the engine (and the catalyst) are cold. NO, is produced primarily by free-radical reactions with atmospheric N2 in the high temperatures (2500°C) within the engine cylinders. The overall reaction can be written as... 

Kramp, F., and S. E. Paulson, On the Uncertainties in the Rate Coefficients for OH Reactions with Hydrocarbons, and the Rate Coefficients of the 1,3,5-Trimethylbenzene and m-Xylene Reactions with OH Radicals in the Gas Phase, J. Phys. Chem. A., 102, 2685-2690 (1998). 

The isotope effect in peroxyl radical reactions with C—H/C—D bonds of attacked hydrocarbon shows the direct hydrogen atom abstraction as the limiting step of this reaction... 

Heterocyclic substrates in SET processes have been widely studied, including the reactions of diliydroiiicotiiiamide,116 pyridine, and quinoline117 and also phenoxazine and phenotiiiazines.118 Phenothiazine has also been shown by ESR analysis to undergo an electron-transfer reaction witii its radical cation with an appreciable 15N/14N isotope effect.119 The reaction of phenazine di-A -oxidc radical cations with hydrocarbons shows evidence of non-radical processes.120... 

Before moving on to consider the fate of the carbonyl products, it is appropriate to discuss the atmospheric fate of CF30 radicals. The usual modes of alkoxy radical loss are not possible for CF30 radicals. Reaction with O2 and decomposition via F atom elimination are both thermodynamically impossible under atmospheric conditions. Instead, CF3O radicals react with NO and hydrocarbons. 

It is generally accepted that thermal cracking is a free radical chain reaction (a free radical is an atom or group of atoms with an unpaired electron). Free radicals react with hydrocarbons and produce new hydrocarbons and new free radicals ... 

The main sink for chlorine radicals is the reaction with hydrocarbons and especially with CH4 ... 

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