Ethyl radical, recombination rate

The rate constants for ethyl radical recombination in steps (5) and (6) do not depend on temperature,... 

Absolute radical concentrations could be determined by reference to the signal obtained from the stable free radical galvinoxyl. Hence it was possible to determine the absolute rate coefficient for the recombination of ethyl radicals in liquid ethane as 3 x 10 l.mole-1.sec-1 at 98 °K. The activation energy for this reaction was 780 cal.mole-1, which is essentially that for the diffusion controlled process. 

The recombination of methyl or ethyl radicals in the gas phase or in iso-octane solution have very similar rate coefficients [22]. In the gas phase, the number of collisions between, for instance, methyl radicals of concentration n and molecular diameter a, is cj2 where c is the... 

R, = a k4 [CHolCCoHglg., where R, is the rate of formation of ethane in reaction 3, and a = k5/(k5 + k4 [CH4]), the fraction of ethyl radicals formed in reaction 4 which decompose via reaction 5 if reaction 4R is negligible, a = 1. The concentration of methyl radicals will be given by [CH3] = ((R3 + k3 [C2Hg]55)/k3). Note that while reaction 3R, the reverse of reaction 3, must be taken into account in calculating the radical concentration and in fact becomes the dominant source of radicals, it does not enter directly into the steady-state equation for ethane because it does not lead to a net consumption of methyl radicals, since these ultimately can only recombine to reform ethane in the present system. 

Recombination of alkyl radicals, as that of atoms, occurs practically without an activation energy. In the gas phase at a sufficiently high pressure the recombination of methyl radicals is bimolecular with the rate constant close to (l/4)Zo (where Zo is the frequency factor of bimolecular collisions, and the factor 1/4 reflects the probability of collisions of particles with the antiparallel orientation of s ins). The theoretical estimation of the constant at a collision diameter of 3.5-10 m agrees with the experimental value = 2-10 ° l/(mol-s) (300 K). This k value agrees with the estimation by the theory of absolute reaction rates under the assumption that the free rotation of methyl groups is retained in the transition state. In the liquid the recombination of meth)d radicals is bimolecular with the rate constant of difiiision collisions (see Qiapter 5). For example, in water 2k = 3.2-10 l/(mol s) (298 K). Ethyl radicals react with each other by two methods recombine and disproportionate... 

To resolve this apparent incompatibility in the measured H + C2Hl J C2H5 unimolecular and bimolecular rate constants it has been proposed that either one of the following three dynamical processes or some combination is important(1) there is a dynamical steric factor of approximately 0.1 for H + C2HL,. C2H5 recombination (2) there is incomplete energy redistribution within the vibrationally excited ethyl radical and (3) the efficiency for collisional deactivation of C2H5 is approximately 10 times smaller than what is found for similar vibrationally excited molecules. 

The 2-butoxy, and 1-butoxy radicals formed in the reactions of 2-butyl and 1-butyl peroxy radicals with NO (path (d) and (j)), react with O2 to produce HO2 together with carbonyl compounds such as methyl ethyl ketone and butanal (path (g) and (1)), and complete the HO cycle. Under the typical NO concentrations in the polluted atmosphere, the formation rate of alkyl nitrates by the reactions of alkoxy radicals with NO2 is negligible, the recombination isomerization reaction by the alkylperoxy radicals with NO is the major pathway as the formation process of alkyl nitrate. 

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