Recombination ratios disproportionation

Table VII. Disproportionation-Recombination Ratios Involving Alkoxy Radicals...

A rather indirect mode of reaction accounts for 60% of the product trans-stilbene (reaction sequence 1, 2, 4, 5, and 6 in Table VII). The importance of this path may be traced to both the low disproportionation/recombination ratio for resonance stabilized radicals, and the high rate constant for 3-bond scission of the intermediate radical recombination product, 1,2,3,4-tetraphenylbutane. Analogous decomposition routes involving formation of an adduct, followed by irreversible destruction of the adduct, may be important reaction paths in many other coal related reaction systems. 

The question of cage recombination 22) merits special consideration in these systems. The most sensitive way to check for it was to test if meso/DL equilibrations occurred in the course of the thermolysis reaction of a pure diastereomer I8, 20). Additional evidence for the unimportance of cage dimerizations are the high disproportionation-recombination ratios found for most of the radicals involved 9- 23) and the high fluid-... 

To establish the effect of structure on the ratio of recombination and disproportionation, the knowledge of reactions of low-molecular analogs may be use. It may be deduced from the ratios of rate constants of disproportionation and combination determined for different types of alkyl radicals that primary alkyl radicals enter combination preferably while tertiary radicals dispoiporti[Pg.153]

The overall constant 2 k +kj) = 2,2-10 l/(mol s) in HjO at 298 K. The ratio = 0.13 in the gas phase and 0.18 in isooctane. This ratio increases with viscosity it is equal to 0.27 in ethylene glycol. This is explained by the fact that transition states for recombination and disproportionation are different, and disproportionation occurs through a more contact transition state. The difference V - Vj = 2.6 cm /mol (273 K), and the kjk ratio depends, therefore, on the internal pressure of the liquidpj... 

The kjkc ratio depends slightly on temperature kjkc = 0.087 exp(1.55/R 7). The ratio between recombination and disproportionation depends, naturally, on the structure of the alkyl radical, which can be seen from the presented data (decaline, 303 K)... 

Kinetic vs. material chain. Kinetically, a chain reaction exists throughout the "life" of the radical, that is, from the initiation of a radical up to its termination by recombination or by disproportionation. The lifetime of a radical determines the so-called kinetic chain length Lp defined as the number of monomers consumed per initiating radical. Lp, by definition, can be calculated from the ratio between the propagation rate Rp to the initiation rate R, or, using steady-state hypothesis (Equation (1)), from the ratio between propagation rate to the termination rate Rt (Equation (3)). 

It can be seen that primary and secondary R02 radicals disproportionate with the participation of the a-C—H bond. This explains why the substitution of D in the a-position for H retards the recombination of R02 [/tn//tD =1.9 for ethylbenzene, h/ d = 2.1 for styrene, and h/ d=1-37 for diphenylmethane [179]). Because of this, R02 radicals of unsaturated compounds with a double bond in the a-position to the peroxyl free valence disproportionate more rapidly than structurally analogous aliphatic peroxyl radicals (at 300 K, 2kt = 2x 107 and 3.8 x 106 L mol-1 s-1 for R02 radicals of cyclohexene and cyclohexane, respectively [180]). Among the products of secondary peroxyl radicals disproportionation, carbonyl compound and alcohol were found in a ratio of 1 1 at room temperature (in experiments with ethylbenzene [181], tetralin [103], and cyclohexane [182-184],... 

The study of the recombination (kc) and the disproportionation (/tdis) of ethyl radicals proved the dependence of the ratio kdis/kc on the solvent [230-232], Stefani [231] found that log( dis/ c) is a linear function of the solubility 8S, which is equal to the square root of the cohesive energy density of the solvents (Dce) ... 

An even more exaggerated behavior is presented by the branched radicals isopropyl, sec-butyl, terf-butyl for which the ratio of rate constants for disproportionation to recombination are, respectively, 0.64, 2.3, and... 

Both of these methods are subject to uncertainty that arises from the manner of termination, i.e., by recombination or by disproportionation. This latter question, if not resolved, can introduce an error of 2 into the estimates of / . Assuming that all of these problems have been satisfactorily resolved, the study of photo-stationary-state kinetic systems gives the ratio of rate constants kp ki>. Within these limitations the results of various laboratories, obtained by the use of different techniques, show reasonably good agreement, usually within 10 to 20 per cent for a given monomer-solvent system. ... 

The polymerization reaction is terminated by disproportionation and recombination, with the ratio depending mainly on the temperature. Chain termination is represented by... 

The constancy of product ratio relative to changes of light intensity and reactant pressure, indicate pripaary photochemical production of four radicals, CH3, CHO, H, and CH3CO, with, therefore, ten recombination reactions and a large number of disproportionation reactions. 

The disproportionation-combination ratio is temperature dependent. For sec-butyl radicals, this ratio is 2.3 at 375°K. (11) and 11 at 90°K. (10). Adopting the same values for pentyl radicals we calculate a ratio of 7.5 at 143°K. (melting point of n-pentane) at which recombination occurs. 

An increase of disproportionation at the expense of recombination when ing from primary to secondary and tertiary alkyl radicals may be expressed by the ratio 1 5 25. An almost equal ratio for disproportionation and recombination has been found for reactions of the methyl radical with ethyl, isopropyl and rer/.butyl radicals [13]. 

In the radiolysis of liquid cyclohexane the major products are H2, cyclohexane and the bicyclohexyl, with yields of 5.6, 3.3 and 1.8 (100 eV)" S respectively, which suggests an important contribution of cyclohexyl radicals. This has been substantiated by radical titration experiments, where a solute is added that reacts with the radicals R (e.g. I2) and the products (RI) can be measured Also, with radical scavengers present, the yields of cyclohexene and bicyclohexyl decrease with the ratio kjk for the cyclohexyl radical. When it is assumed that the dimer is formed only by the radical recombination reaction 37 [with a yield of 1.8 (100eV) ], it follows that c-C Hio is formed concomitantly with a yield of 1.1 x 1.8 = 2.0 (100 eV)" Since the total yield of c-C H q is 3.3 (100 eV)" it follows that a yield of c-C Hio equal to 1.3 (100 eV)" is found that is formed in another process than disproportionation of cyclohexyl radicals. It appears that this process is molecular H2 elimination of the Si state (that also fluoresces), with (/>h2 = 0.8 as obtained from photolysis at the lowest photon energies (equation 33). The... 

It seems that the rate constants of these processes have not been measured directly. The commonly measured value is the ratio A of the disproportionation rate constant to that of radical recombination. 

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