Radicals, disproportionation recombination

Butenyl (C4H7) mainly disappears by reacting with ethyl radicals. The leftover ethyl radicals either recombine (faster process) or undergo disproportionation (slower process) ... 

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],... 

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

The most important mode of termination of a radical polymerization is the bimolecular interaction of its reactive centers. Radicals rapidly recombine or disproportionate, and these reactions anihilate the growing ends. On the other hand, neither recombination nor disproportionation takes place in an ionic polymerization, and hence a collision between two ionically growing centers usually does not lead to termination. 

Polymer radicals may recombine upon bimolecular termination. When this cross-linking takes place between two polymer molecules the molecular yield increases. When there are more than one radical on the polymer chain, cross-linking between these two sites will form only a loop (Fig. 9.1), that is, no increase in the molecular weight ensues despite the fact that cross-linking has occurred. In competition to recombination, radicals may disproportionate, when at least... 

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 processes of macroradical decay have been studied in high vacuum and in oxygen. The main consequence of the mechanism of the process is that macroradicals are able to eliminate hydrogen atoms from the adjacent chains and thus transfer to them the radical state. The radical state is transmitted further on by a series of hydrogen elimination acts. Thus it migrates in the polymer until it collides with another radical state. This may result in the disappearance of both radicals by recombination or by disproportionation. 

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-... 

Encounters between silyl radicals in solution or in the gas phase usually result in recombination and disproportionation (45, 46). Disproportionation results in the production of silanes and highly reactive silenes. The disproportionation reaction is thermodynamically favorable because of the formation of a silicon-carbon double bond, which, although subsequently chemically reactive, is worth —39 kcal/mol (44). For pentamethyldisilanyl radicals, disproportionation is kinetically competitive with radical dimerization (46). In an earlier study, Boudjouk and co-workers (47) demonstrated conclusively by isotopic substitution and trapping that the silyl radicals generated by photolysis undergo disproportionation, as well as, presumably, dimerization (Scheme I). In deuterated methanol, the silanes produced were predominantly undeuterated, whereas methoxymethyldiphenylsilane was extensively deuterated in the a position. The results of these experiments strongly implicated the substituted silene produced by disproportionation. 

The kinetics of the self-reaction of amidogen radicals, NH2 + NH2, has been studied extensively using different techniques and bath-gases.228 239 The apparent bimolecular rate constant was found to be strongly pressure dependent. The amidogen radicals can recombine or disproportionate ... 

For example, in the case of ethyl radicals, the recombination and disproportionation reactions may produce n-butane, ethylene, and ethane ... 

While tert-hniyX radicals disproportionate predominantly, oyr-dimethyl benzyl radicals recombine. 

A reaction of hydrogen atoms with the CHj groups of a surrounding polymer chain and formation of secondary alkyl radicals and molecular hydrogen will compete with this reaction. Secondary aUcyl radicals either recombine or disproportionate but the probability of both reactions is not so different now. Not only disproportionation of... 

Not all ions and radicals will undergo geminate recombination in the spur, and the species that were not lost by recombination will escape from the spur expanded by diffusion in the bulk. After escape from the spur, the reactive species will undergo reactions, such as hydrogen abstraction, radical recombination, radical disproportionation, neutralization, etc. The active species react with not only solvent molecules but also the bulk solute. 

If one now jumps to some higher pressure at which steady-state reaction conditions can again prevail, similar semi-quantitative arguments can be used to explain the phenomenon known as the second explosion limit. At these somewhat higher pressures the large majority of the events by which chains are terminated will occnr in the gas phase. The higher pressure hinders the diffusion of radicals to the vessel surfaces and provides a number density of gas-phase radicals that is sufficient for radical disproportionation and recombination reactions to become mnch more significant than surface termination processes. 

Figure 20-7. End-group fraction in the polymerization of styrene at 60° C with AIBN as initiator for l%-3% conversion. Termination by mutual deactivation of two free radicals, t(pp), leads to one initiator free radical fragment (disproportionation) or two initiator free radical fragments (recombination) per chain with one monomer residue as end group in the case of disproportionation. With termination by initiator, /(/), each molecule has two initiator fragments, etc. tr(i), transfer to initiator tr(m), transfer to monomer tr(ah), transfer to the molecule. III, in Equation (20-32). (After Pryor and Coco.)...

Apolar, hydrophobic solutes with sufficient volatility, e.g., aromatics in dilute aqueous solutions, are vaporized into the bubble, where they undergo pyrolysis to acetylene,26 and oxidation to carbon mono- and dioxide.27/28 Tetrachloromethane and other chlorocarbons cleave homolytically. The chlorine atoms and organic radicals decompose, recombine, and disproportionate to give complex mixtures (Fig. 5).28,29,30 In most cases, a monomolecular pyrolytic mechanism is admitted, unaffected by frequency changes. Sc... 

Head-to-Head (H-H) and tail-to-tail (T-T) linkages may also be present in polymer structures. They have been identified in addition polymers prepared by radical or coordination polymerization (polypropylene). Introduction of H-H linkages into addition polymers by radical polymerization can proceed by two mechanisms (a) terminations of radical polymerization (recombination or disproportionation of the polymer radicals) (3). The recombination introduces one H-H linkage, disproportionation of a vinyl and a saturated polymer end. Recombination and disproportionations are not always equally important termination reactions what type of termination depends on the type of monomer and polymerization conditions, (b) H-H linkages are also introduced into polymers by reverse addition of the monomer in radical polymerization. This behavior is particularly noticeable in radical polymerizations of hal-... 

The phenoxy radicals are relatively long-lived and can undergo self-disproportionation, recombination vith alkylperoxy radicals, or isomeric rearrangement followed by recombination. The resulting compounds may have some stabilization activity. Propionate-type hindered phenols constitute a special class in this category during reaction, the phenol is transformed into phenolic cinnamates, which are known to be efficient chain-breaking antioxidants [Eq. (85)]. 

At a higher irradiation dose a number of free radical initiated reactions like chain scission, disproportionation, recombination of macroradicals take place (refer to Figure 8 - Scheme-3). With the increase of vinyl... 

Initiation occurs in aqueous phase as previously mentioned, and the resulting oligomers RM (n small) penetrate into the micelles and consume the monomer available inside until arrival of a second radical. In such a confined volume, the two radicals present recombine or disproportionate and chain growth is discontinued. 

Aliphatic aminyl radicals, as alkyl radicals, enter into disproportionation and recombination reactions. The reaction direction depends on the radical structure. For example, methylaminyl radicals predominantly recombine and diisopropylaminyl radicals disproportionate... 

Scheme 2 Termination of C-centered radicals by recombination and disproportionation.

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