Radical reactions disproportionation

The second method is to generate the quinone methide via radical coupling reactions, analogously to the way they are produced via radical coupling during lignification (Fig. 12.4c) orto exploit radical disproportionation reactions (Fig. 12.4d). 

In order to document the radical disproportionation reaction, we have used FT-IR spectroscopy to characterize the irradiation products. Upon irradiation of 1 in pentane, the formation of the characteristic peak near 2100 cm-1 due to Si-H stretching vibrations was readily apparent. The IR spectrum obtained in perdeuterated pentane was identical, suggesting that radical processes other than abstraction from the solvent are involved. Furthermore the ESR spectrum obtained in this solvent is identical to that already described. This raises the question whether the initially formed silyl radicals really abstract hydrogen from carbon with the formation of carbon-based radicals as suggested (13), particularly in light of the endothermicity of such a process. 

For proteins and peptides having sulfydryl or mercaptal side chains, free radical disproportionation reactions such as the ones reported by... 

To examine the potential importance of molecular disproportionation, a means for estimating ki3 must be found. Estimates of ki3 will be obtained from estimates of both the rate constant for the reverse reaction (radical disproportionation, reaction -13), and the equilibrium constant, Ki3 i.e., ki3 = Ki3 k x3. 

Baldwin and Walker [103] suggest that the decrease in the yield of conjugate alkenes with increase in pressure is probably due to the concomitant increase in [C H2 +iOO] and hence stress the importance of alkylperoxy radical disproportionation reactions. 

The available evidence suggests, therefore, that alkylperoxy radical disproportionation reactions are important for low molecular weight alkanes, but reliable values of the Arrhenius parameters for reactions (42) and (14a ) are urgently needed to confirm this. However, since is greater than fei4a by ca. 10 this will not be the case for alkanes which can form secondary or tertiary alkylperoxy radicals capable of undergoing extensive isomerization involving 1 5 or 1 6 H-transfer from further secondary or tertiary carbon atoms. 

In addition, we have seen that a composite operator constructs new reactions through the execution of three methods, Kj, get-sites> nd K. In the context of radical disproportionation reactions ... 

TABLE 9.2 Rate Constants for Radical Disproportionation Reactions... 

The unsaturation level in the polymer is at most 0.1 double bonds per chain and unsaturated oligo-mers are not detected at all. B-Scission and radical disproportionation reactions which occur in Fawcett-type (high-temperature, high pressure) ethylene polymerizations are therefore of minor importance in silver perchlorate solution at 0-40°C. 

Consider now the alkyl radical disproportionation reactions. In these reactions the attacking radical abstracts a hydrogen atom from the radical attacked to form an alkane molecule and an olefin molecule. If the disproportionation involves two different radicals, two reaction paths are possible ... 

Table 10. Rate constants (in cm mol s ) and activation energies of alkyl radical disproportionation reactions [232]...

It is generally true that almost all the hydroxybiradicals formed by internal hydrogen abstraction partially revert to starting ketone by an internal radical disproportionation reaction in which the carbon-centered radical site abstracts the hydrogen from the hydroxyl group. This process often is the major reaction of the intermediate biradical, so that the overall quantum yield of product formation is low even when hydrogen abstraction by the excited state is 100% efficient Should the efficiency of hydrogen abstraction be low because of electronic or conformational problems, then biradical disproportionation lowers quantum efficiency even further. 

Tlie formation of initiator radicals is not the only process that determines the concentration of free radicals in a polymerization system. Polymer propagation itself does not change the radical concentration it merely changes one radical to another. Termination steps also occur, however, and these remove radicals from the system. We shall discuss combination and disproportionation reactions as modes of termination. 

Another method for producing petoxycatboxyhc acids is by autoxidation of aldehydes (168). The reaction is a free-radical chain process, initiated by organic peroxides, uv irradiation, o2one, and various metal salts. It is terrninated by free-radical inhibitors (181,183). In certain cases, the petoxycatboxyhc acid forms an adduct with the aldehyde from which the petoxycatboxyhc acid can be hberated by heating or by acid hydrolysis. If the petoxycatboxyhc acid remains in contact with excess aldehyde, a redox disproportionation reaction occurs that forms a catboxyhc acid ... 

The main reason that the decreases as the polymerization temperature increases is the increase in the initiation and termination reactions, which leads to a decrease in the kinetic chain length (Fig. 17). At low temperature, the main termination mechanism is polystyryl radical coupling, but as the temperature increases, radical disproportionation becomes increasingly important. Termination by coupling results in higher PS than any of the other termination modes. 

The chain is broken by a combination or disproportionation reaction between the two radicals. 

Each radical undergoes reaction with another dimerization, disproportionation, or the like. The cross-coupling process between R1 and R2 also occurs. With these, the reaction scheme takes the following form ... 

FIGURE 1. Proposed mechanism for the disproportionation reaction of the arenesulfonyl radicals. 

The number of reported reactions in which the radical derived from the decomposition of AIBN plays a role in the termination process has increased considerably. Often these reactions are not radical chain reactions, since the initiator is used in stoichiometric amounts. A few examples of rearomatization of cyclohexadienyl radicals by disproportionation have been reported herein. Below are some other examples, where the phenyl selenide 61 reacts with (TMSfsSiH (3 equiv), AIBN (1.2 equiv) in refluxing benzene for 24 h to give the coupling product of radicals 63 and 64 in good yields (Scheme 9).i24,i25 these cases,... 

Coupling and disproportionation reactions of R radicals are a minor process, usually not observed. Hence, these radicals, if formed, are not mobile in the matrix. 

Chain reactions do not continue indefinitely, but in the nature of the reactivity of the free radical or ionic centre they are likely to react readily in ways that will destroy the reactivity. For example, in radical polymerisations two growing molecules may combine to extinguish both radical centres with formation of a chemical bond. Alternatively they may react in a disproportionation reaction to generate end groups in two molecules, one of which is unsaturated. Lastly, active centres may find other molecules to react with, such as solvent or impurity, and in this way the active centre is destroyed and the polymer molecule ceases to grow. 

On condensation at low temperatures, on dissolution in inert solvents or on raising its partial pressure substantially above 1 mbar (100 Pa) S2O polymerizes with partial disproportionation. Since sulfur radicals have been detected in such condensates by ESR spectroscopy [10] it has been proposed that a radical-chain reaction takes place according to Scheme 5. 

The dinitrobenzenes display the characteristics of inhibitors for the more reactive vinyl acetate chain radicals. Two radicals are terminated during the induction period by each molecule of dinitrobenzene, indicating disappearance of inhibitor radicals by a disproportionation reaction. 

A different behavior is observed if TMA is replaced by TEA. Even though the final catalysts are equally active for ethylene polymerization, radicals have never been observed. This is in line with expectations based on indirect evidence [126-128] proposing a disproportionation reaction for the activation with TEA according to ... 

The third fact that seemed to argue in favor of the occurrence of radicals 10 was the observation that reactions of a-tocopherol under typical radical conditions, that is, at the presence of radical initiators in inert solvents or under irradiation, provided also large amounts of two-electron oxidation products such as o-QM 3 and its spiro dimerization product 9 (Fig. 6.8).16,25,26 This was taken as support of a disproportionation reaction involving a-tocopheroxyl radical 2 and its hypothetical tautomeric chromanol methide radical 10, affording one molecule of o-QM 3 (oxidation) and regenerating one molecule of 1 (reduction). The term disproportionation was used here to describe a one-electron redox process with concomitant transfer of a proton, that is, basically a H-atom transfer from hypothetical 10 to radical 2. 

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