Solvent Effects on Free-Radical Transition State Reactions

4 Solvent Effects on Free-Radical Transition State Reactions 

According to Kosower, a third category of reactions involving free-radical activated complexes may be defined. These complexes are formed either through the creation of unpaired electrons during radical pair formation or atom-transfer reactions [15, 468] cf. 

Section 5.3). These two different types of free-radical reactions have been investigated for solvent effects (for reviews, see references [159-166]). Reactions in which the medium affects the reactivity of free radicals have received greater attention than radical-forming reactions. The solvent often affects the course of free-radical reactions by participating as a reactant. This is evidenced by the incorporation of the solvent into the products of the reaction. This Section, however, will be limited to those solvent effects which are truly medium effects. 

Free radicals may be generated by oxidation, reduction, or by homolytic cleavage of one or more covalent bonds, such as C—C bonds e.g. dimers of triarylmethyl radicals), N—N bonds e.g. tetrasubstituted hydrazines), O—O bonds e.g. hydroperoxides, dialkyl and diacyl peroxides, peroxycarboxylic esters), C—N bonds e.g. dialkyl azo compounds), and N—O bonds (as in the thermolysis of nitrogen pentoxide O2N— O—NO2). Two typical examples, which have been investigated in different solvents, are given in Eqs. (5-56) and (5-57) cf. also reaction (5-39a) in Section 5.3.2. 

The dissociation rate of the dimer of the triphenylmethyl radical in 28 solvents was studied by Ziegler el al. [167]. The decomposition rate of azobisisobutyronitrile in 36 solvents was measured by different authors [183-185, 562], Despite the great variety of solvents, the rate constants vary only by a factor of 2... 4. This behaviour is typical for reactions involving isopolar transition states and often indicates, but does not prove, a radical-forming reaction. The lack of any marked solvent effects in most free-radical forming reactions will become more apparent after an examination of some further reactions presented in Table 5-8. 

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