Rate constants homolytic fragmentation

Three different mechanisms of perester homolytic decay are known [3,4] splitting of the weakest O—O bond with the formation of alkoxyl and acyloxyl radicals, concerted fragmentation with simultaneous splitting of O—O and C—C(O) bonds [3,4], and some ortho-substituted benzoyl peresters are decomposed by the mechanism of decomposition with anchimeric assistance [3,4]. The rate constants of perester decomposition and values of e = k l2kd are collected in the Handbook of Radical Initiators [4]. The yield of cage reaction products increases with increasing viscosity of the solvent. 

AN+- (Reitstoen and Parker, 1991). In other words, the triad of reactive fragments produced in (63) in the charge-transfer excitation of the EDA complex with A-nitropyridinium ion is susceptible to mutual (pairwise) annihilations leading to the Wheland intermediate W and the nucleophilic adduct N (Scheme 12), so that the observed second-order rate constant ku for the spectral decay of ArH+- in Table 3 actually represents a composite of k2 and k2. A similar competition between the homolytic and nucleophilic reactivity of aromatic cation radicals is observed in the reaction triad (55)... 

However, consider the corresponding situation for a closely related complex in which, instead of an iodide ion bound to pentaaquo chromium, there is an aliphatic fragment such as an alkyl group. As one example, we may consider the hydroxyiso-propyl chromium cation. Here the possibility for both homolytic and heterolytic substitution exists (Scheme II). With proper experiments, one can evaluate separately the rate constant values for cleavage by each of these pathways. 

Some homolytic fragmentation reactions are driven by formation of small, stable molecules. Alkyl acyloxyl radicals (RCOp decarboxylate rapidly (fe > 1 x 10 s ) to give alkyl radicals, and even aryl acyloxyl radicals (ArCOp decarboxylate to aryl radicals with rate constants in the 10 s range." Azo radicals produced in the homolysis of azo initiators eliminate nitrogen rapidly. Elimination of carbon monoxide from acyl radicals occurs but is slow enough (fe 10" -10 such that the acyl radical can be trapped in a bimolecular process,... 

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