Third-order radical kinetics

The reactivity of NO towards atoms, free radicals, and other paramagnetic species has been much studied, and the chemiluminescent reactions with atomic N and O are important in assaying atomic N (p. 414). NO reacts rapidly with molecular O2 to give brown NO2, and this gas is the normal product of reactions which produce NO if these are carried out in air. The oxidation is unusual in following third-order reaction kinetics and, indeed, is the classic... 

The electrode mechanisms treated, along with the rate laws and the appropriate digital simulation parameters, are shown in Table 16. The symbols for mechanisms 5 and 6, RS-2 and RS-3, indicate that these reactions represent cases of radical (primary intermediate B) reacting with substrate (A). Mechanism 5 foDows second-order kinetics while third-order kinetics characterize mechanism 6. The theoretical data for the mechanisms are summarized in Tables 17—23. The calculations are for EX — f revI equal to 300 mV. Data are also available for EX — Eiev — 100 mV. In the following paragraph, the data are explained with reference to the eC mechanism, i.e. Table 17. 

The coupling of ethylmagnesium bromide and ethyl bromide to n-butane follows overall third-order kinetics, being first order in each component and the copper(l) catalyst. There is no evidence for alkyl radicals in the copper(I)-catalyzed coupling process, and we propose the following two-step mechanism ... 

It is worth noting that the dimer and trimer generated in reactions (8) and (9) can react with polymeric radicals as a chain transfer agent, and therefore their effect on the polymer molecular weight should not be neglected the quantitative estimation of the concentration of these byproducts depends on the fact that whether the rate of thermal initiation is a second- or third-order reaction of monomer concentration. More kinetic information for such transfer reactions can be found in a number of publications [14-19]. Nevertheless, detailed kinetic studies on such Diels-Alder byproducts remain scarce. Katzenmayer [20], Olaj et al. [21,22], and Kirchner and Riederle [23] have published some quantitative results on this matter. 

NH2 radicals are formed in the troposphere by reaction of OH radicals with NH2, and may be formed in the stratosphere by solar photolysis of NHj at u.v. wavelengths <220 nm. The kinetics of NH2 radical reactions with O2 and O3 have been studied by Patrick and Golden using a laser photolysis/laser resonance absorption arrangement. They obtained an upper limit of 1.5 x 10 cm molecule s for the third-order combination of NH2 and O2 which contrasts with the value of 3.5 x 10" cm molecule s reported by Hack et al. Andresen et al7 obtained rate data for the reaction of NH2 with NO, monitoring the products using various detection techniques, and Whyte and Phillips determined rate constants for the reactions NH2 + N, NH2+NO, and NH2+NO2. 

In 1934 Rice and Herzfeld assumed second-order termination, whereas some years later Taylor and Burton suggested a third-order recombination of methyl radicals in the acetaldehyde decomposition. Conclusions derived from other systems are also contradictory. The results of Laidler and Wojciechowski on ethane pyrolysis favour third-order combination. However, more recent investigations by Quinn , as well as by Trenwith , on the kinetics of ethane pyrolysis at around 500 °C and 100 torr pressure, show that the methyl recombination is practically second order. From the work of Lin and Back it follows that, at 550 °C, the recombination rate coefficient becomes pressure-dependent below approximately 200 torr. 

Although bromination, iodochlorination and iodobromination of unsaturated compounds in aprotic solvents are generally described as following a third-order rate law, chlorination was always found to obey, in the presence of radical inhibitors, second-order kinetics . However, it has been recently shown , through kinetic studies on chlorine addition to... 

This situation is typical of metal radicals, the relatively low M-H bond strengths prohibit direct attack on H2 for thermodynamic reasons. Kinetic studies75 showed that oxidative addition of H2 obeyed an overall third-order rate law d[PJ/dr /c "Cr (CO)3Cp ]2[H2] with All1 0 kcal/mol and AS = -47 cal/(molK). The proposed mechanism for the third-order reaction is shown in Equation 10.57. 

The kinetics of the reactions of various substituted thio phenols with liquid sulfur have been investigated, and intermediates of the general form RS H and HS H have been identified. The studies suggest that the reaction is usually free radical involving an initiation period in which a steady state concentration of sulfur radical species is established, The reaction appears to be second order in mercaptan and third order in sulfur. These findings have been interpreted using a complex sulfur radical species involving several Sg molecules. 

Hence, for the reaction of OH radicals with vinyl fluoride and other fluoroalkenes containing no Cl or Br atoms, the rate constant will, as observed for ethene, exhibit fall-off behavior from second- to third-order kinetics at low total pressures. 

The method of molecular absorption has been used for Br atoms in the author s laboratory to check the stoichiometry of the titration reaction, Br + CINO -> BrCl + NO. It has also been used to measure the kinetics of radiative and nonradiative third-order recombination of Br atoms, and to follow [Br] in the bimolecular disproportionation of two BrO radicals these were shown to decay by the reaction BrO -f BrO -> 2Br -f- O2. It appears that vibrational relaxation of Bra, which might affect the method, was complete under the conditions used. One advantage of the molecular absorption method is its relative instrumental simplicity. However, the method is clearly inapplicable to any system in which interfering absorption by transients, reactants or products can occur. 

Nitric oxide (NO) is an inorganic, colorless gas with good solubility in water. The half-life of NO in water is considerably longer, about 3 s, than would be expected for other free radicals. This is, in part, due to the reluctance of NO to dimerism and to the third-order kinetic of its reaction with oxygen. However, NO reacts rapidly... 

The second and third terms in equation (2.86) describe kinetics of annihilation of radicals corresponding to above processes developing in the first and second order of magnitude with respect to chemisorbed radicals. Note that the rate constant A" may be dependent on concentration of free radicals in volume. 

Tanaka et al. studied the decay reactions of PVB radical anions produced by electron pulses in MTHF [47]. At low concentration ( < 0.05 base-mol dm - 3) of polymers the decay reaction followed a simple second-order kinetics. The charge neutralization reaction is responsible for the decay curve as is the case of biphenyl radical anions. However, the rate constant of the polymer anions was only a half or one-third of that of the biphenyl anion, because of the small diffusion coefficient of the polymer ion in solution. At high concentration of the polymer, a spike was observed in the time-profile of the PVB anion this was attributed to the retarded geminate recombinations within micro-domains where the polymers were entangled with each other. 

The active centres of polymerization are produced by the addition of the primary radical to the monomer, i. e. to a n electron system. Only rarely is this simple process, and almost all branches of theoretical chemistry and chemical physics have contributed to its elucidation. The addition is a bimolecular reaction interpreted kinetically as a second-order reaction [125]. Unfortunately, most studies have been concerned with reaction in the gaseous phase. In the condensed phase, the probability that the excess energy of the reaction product will be removed by collision with a third molecule is very much higher thus the results obtained in the gaseous phase need not be valid generally. 

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