Radical disproportionation reactions, solvent

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. 

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 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. 

The substituted hydroxylamine C NOPP from reaction 7) can take part in various dark reactions, even at ambient temperature. From a study of the low molecular weight model I in the liquid phase, two decomposition pathways are possible (reaction 8) (12). The products from the disproportionation reaction 8a were only observed in the absence of a radical trap such as O2. In a given solvent ks kaa-Uo (solvent air saturated and degassed respectively). Both k8a and ke were found to increase by an order of magnitude on going from a non-polar solvent (iso-octane) to a polar solvent (methanol or tert.-butyl hydro peroxide, BuOOH). 

The analogy between electron-transfer via addition/elimination (Eq. 2b,c) or abstraction/elimination (Eq. 2a, c) and classical solvolysis involving closed-shell molecules (nonradicals) is seen by comparing Scheme 1 with Scheme 3, in which XY, the precursor of the ions X and Y , is formally derived from the two radicals X and Y". Analogous to Scheme 1, on the way to the ionic products that result from the interaction between X and Y there are two possibilities if XY denotes a transition state, the reaction (Eq. 3a, a ) is a case of outer-sphere electron transfer. If, however, a covalent bond is formed between X and Y, the path (Eq. 3b, b ) is an example of inner- sphere electron transfer. Obviously, part b of the scheme describes the classical area of S l solvolysis reactions (assuming either X or Y to be equal to C) [9, 10]. If a second reaction partner for C (other than the solvent) is allowed for (the (partial) ions then represent transition states), then Eq. 3b also covers Sn2 reactions. If looked upon from the point of view of radical-radical reactivity, Eqs. 3a and b show well-known reactions radical disproportionation in Eq. 3a,a and combination in Eq. 3b. 

In processes I and II, the acetyl and methyl radicals react with solvent molecules giving respectively acetaldehyde and methane. In the third process chain degradation occurs followed eventually by disproportionation. Evidently the same photolysis carried out in the presence of a second monomer (acrylonitrile yields graft copolymers (I and II) and, possibly, some block copolymers (III), together with homopolymer initiated by the methyl and acetyl radicals. The formation of graft copolymers seems more likely on account of the greater stability of the radicals produced in reaction schemes I and II compared to scheme III. 

The donor ability (nucleophilicity) of R2M , including the stability of complexes being formed, increases as M becomes heavier. The acceptor properties of M in R2M (such as the ability to form the adducts with Lewis bases, for example, with pyridine and piperidine at — 30 °C) are determined by the low-lying unoccupied atomic d- and pz-orbitals160. Stable free radicals R3M (M = Ge, Sn) are obtained by a photochemical disproportionation reaction of R2M in a hydrocarbon solvent medium160 see equation 26, R = (Me3Si)2CH. 

Apart from the above described radical-forming reactions, the influence of solvent on the reactions of the radicals themselves has also been thoroughly investigated [159-166]. The most important elementary reactions of radicals are atom transfer, combination, addition, disproportionation, and electron transfer, as listed in Table 5-10 [15, 213]. 

The disproportionation reactions being controlled by the same factors as the coupling reaction would be represented by the same equation. Finally, relative to the attack on solvent by radicals that might escape into solution, they merely stated that it would depend on the design of the experiment (i.e., the nature of the free radicals and the nature of the solvent used). 

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