Radical disproportionation reactions, solvent effects

Depending on the reaction solvent and conditions, the disproportionation of hydroperoxy radical (Eq. (14.5)) may contribute to the reaction (estimated rate constant 10 -10 s ) [23]. While no autocatalytic effects were observed in... 

In many polymerization systems the polymer molecular weight is observed to he lower than predicted on the basis of the experimentally observed extents of temtination by coupling and disproportionation. This effect is due to the premature temtination of a growing polymer by the transfer of a hydrogen or other atom or species to it from some compound present in the system—the monomer, initiator, or solvent, as the case may be. These radical displacement reactions are termed chain-transfer reactions and may be depicted as... 

Russell (10) suggested that the bimolecular self-reaction of S-RO2 involves the concerted decomposition of a cyclic tetroxide formed by combination of the radicals. This mechanism was deduced from a consideration of the results of a kinetic and product study of the autoxidation of ethylbenzene. Thus Russell found that almost one molecule of acetophenone is produced per two kinetic chains and that CeHsCHCCHa)O2 interact to form non-radical products nearly twice as fast as CsHsCDCcHs) O2. The former result is only compatible with (29) if all the alkoxy radicals disproportionate in the solvent cage (30) while the deuterium isotope effect requires a H-atom transfer reaction to be rate controlling, which is unlikely for the radical pathway. 

The values of k ]k for cyclohexyl radicals in the gas phase and in liquid cyclohexane (23°C) have been found to be 0-5 and respectively, (Becke aZ., 1954 Cramer, 1967). Comparison of these values with those determined in the present work show that there is little effect of temperature, phase or solvent on kf k for the cyclohexyl radical. This behaviour is in accord with the conclusions drawn from the photolysis studies but is in marked contrast to those from the hydrogen atom-olefin experiments. The absence of large effects of temperature and solvent suggest that the disproportionation and combination reactions of the cyclohexyl radical proceed through activated complexes in which the interactions between the radicals are very similar, if not identical. 

The participation of radicals in acid/base reactions with the solvent occurs for several species, and it often has profound effects on the radical reactivity. For example, deprotonation of H02 generates 02, which is stable with respect to disproportionation. Table 9.3 is a sampling of radicals having known p/fa values. 

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