Disproportionation, of radicals

The transition state for disproportionation requires overlap of the p C—H bond undergoing scission and the p-orbital containing the unpaired electron.18 This requirement rationalizes the specificity observed in disproportionation of radicals 29 (Section 1.4,2) and provides an explanation for the persistency of the triisopropylmcthyl radical (33) and related species (Section 1.4.3.2).166 In the case of 33, the P-bydrogens are constrained to lie in the nodal plane of the p-orbital due to stcric buttressing between the methyls of the adjacent isopropyls. 

The disproportionation of radicals can occur only in solution. In polar solvents, it follows that... 

The retarder may be a free radical which is too unreactive to initiate a Polymer chain (e.g. triphenyl methyl or diphenyl-picrylhydrazyl). The mechanism of retardation is simply the combination or disproportionation of radicals. 

This inference is checked stereochemically. If hydroperoxide produces alcohol at the expense of disproportionation of radicals not leaving the cage, the enantiomeric hydroperoxide should give alcohol that retains its optical activity. And this is what actually takes place (Davies and Feld 1958). 

It may not always be clear from the conditions for electrochemical generation which species is the effective EGB. In some cases a possible complication is fast disproportionation of radical-anion to dianion (Scheme 12). This can mean that for electrogeneration at, say, the first reduction potential E Jl) it is possible for either the radical-anion or the dianion to act as base, depending on the relative rates of protonation by acid HA (k and kp, the value of the disproportionation constant (Kj), and the rate at which equilibrium between radical-anion and dianion is attained. In principle, of course, it is also possible that electrogeneration at E p2) could lead to a situation where radical-anion was the effective base as a consequence of rapid reproportionation causing it to be present in high concentration, thus offsetting its probably much lower kinetic basicity. These points are discussed in more detail on p. 157. 

It would appear from the foregoing that the reactions of recombination and disproportionation of radicals fall into a reasonable relation with each other and with abstraction reactions if in the approach of the radical pair, either head-to-head as in recombination or head-to-tail as in disproportionation, there occurs an appreciable contribution of ionic structures to the attractive potential of the radical pair. These then also fall into a consistent relation with the insertion reactions of carbenes and the snap-out reactions of stable molecules from which latter, the present experimental evidence requires a large charge separation in the transition state. 

The process of addition of monomer units to the growing chain can be interrupted in different ways. One is chain termination by combination or disproportionation of radicals. Explicitly, two growing-chain radicals can combine to form a carbon-carbon bond, or disproportionation can occur with a hydrogen atom being transferred from one chain to the other ... 

Disproportionation of radicals by electron transfer to yield the negative and positive ions, and the subsequent reactions of the cation to either add OH- or lose H+, has been discussed in the previous section. Several aromatic and heterocyclic ions have been suggested to be intermediates in such processes, and the lifetime of these intermediates appears to be long enough to allow an effect of pH on their subsequent reaction (e.g. Haysom et al., 1972). 

For all reactions (2) with at least one component uncharged, AG = 0 if, as is normally done, we exclude dipole-dipole interactions. This means that AGf can be neglected for many organic electron-transfer reactions. Otherwise, in media that are often used for organic redox reactions the electrostatic effects are formally fairly large and certainly not negligible. The effect of cations upon the disproportionation of radical anions (41) in ethereal media is an example of... 

Chain ending occurs through association or disproportionation of radicals. 

The disproportionation of radical ions is a special case which has been studied rather extensively. Electron transfer between two anion radicals... 

Salt 7 was smoothly reduced by tetrakis(dimethylamino)ethene in acetonitrile to the intense green radical 10. Crystals of 10 as well as its hexane solution decompose at ambient temperature. Thereby diphosphirane 9 was generated in ca. 30% yield in addition to diphosphane Ar(Me)PP(H)Ar (43%), diphosphene 6 (ca. 10%) and phosphane ArP(Me)(H) (ca. 17%). The formation of diphosphirane 9 and Ar(Me)PP(H)Ar was explained by the disproportionation of radical 10 (Scheme 3) <2001AGE723>. 

CIDNP (chemically induced dynamic nuclear polarization) Non-Boltzmann nuclear spin state distribution produced in thermal or photochemical reactions, usually from colligation and diffusion, or disproportionation of radical pairs, and detected by nuclear magnetic resonance spectroscopy by enhanced absorption or emission signals. 

Thermal oxidation occurs when heat and oxygen are exposed to the seal. The term auto-oxidation is used for temperatures between ambient and 200°C. Shelton proposed the kinetic scheme for rubber shown in Figure 8.19. Autotermination by coupling or disproportionation of radicals will occur [18]. Hydrogen peroxide is assumed to be a source of radicals that will initiate oxidation in polymers [18]. 

In the Soviet study110, the following elementary stages were taken into account in the kinetic scheme of vinyl acetate polymerization chain transfer to the monomer, solvent, and polymer, and chain termination caused by the disproportionation of radicals. It was assumed that long-chain branches could be formed by chain transfer both to the acetate group hydrogen atoms and to the main chain hydrogen. 

Figure 4 indicates that the yields of olefins produced by radiolysis are independent of the absorbed dose. These unsaturated products are pentene, light hydrocarbons, and eventually decenes produced by Reaction 15. Cage disproportionation of alkyl radicals produced by C—C bond scission and molecular fragmentation account for the formation of light olefins. Pentene would be formed either by elimination of molecular hydrogen (Reaction 5) or by disproportionation of radicals when the temperature of the sample is raised (Reactions 9 and 16). The latter process is thus a postirradiation production of pentene. 

Such a reaction occurs, as well, in other conditions of mechanical stress applied to the polymers, in the presence of aromatic diamines, regardless of their physical state or phase. These reactions are termed mechano-chemical polycondensation [85], In the absence of radicalic acceptors, interruption occurs through disproportionation of radicals, the final product showing a lower molecular weight ... 

Benson SW. (1983) Molecular-models for recombination and disproportionation of radicals. Can. J. Chem. 61 881-887. 

Recombinations and disproportionations of radicals give rise to other radicals and other products. Some of these reactions are illustrated by the following equations ... 

The free-radical mechanism involves several reaction types (1) Initiation - the introduction of free radicals into the reaction system. In pyrolysis, the reaction is initiated thermally with a probable homolytic cleavage of a carbon-carbon bond. (2) Propagation - a series of reactions that converts reactants to products while leaving the radical concentrations unchanged (radical decomposition and isomerization, hydrogen transfer and radical addition). (3) Termination - the combination or disproportionation of radicals to give stable products (Rebick, 1983). 

During disproportionation of radicals bearing an a-methyl substituent (e.g., those derived from MMA), there is a strong preference for transfer of a hydrogen from the a-methyl group rather than the methylene group. 

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