Why Do Reactions Choose Ion-Radical Mechanism

In a range of anions PhZ (Z = O, S, Se, Te), for example, the thiophenolate ion (PhS ) effectively traps aryl radicals (Ar ), whereas the anion of phenyl selenide (PhSe ) is 20 times less active, and the phenolate anion (PhO ) is absolutely inactive. The reaction of aryl radicals with phenyl-telluride ions (PhTe ) proceeds in an abnormal fashion—both asymmetrical and symmetrical tellurides are produced (Rossi and Pierini 1980). 

As MO calculations show (Vilar et al. 1982), the energy levels for pairs (Ar + TePh) and (ArTe + Ph ) are equal. This makes a dual direction of decomposition possible for the intermediate anion-radical as follows  

To disrupt the carbon-chloride bond at position 5 of the substrate anion-radical, population of this bond with an unpaired electron should be increased. However, if a spin density at carboncarrying chlorine is too great, the initial chlorine-containing anion-radicals dimerize instead of cleaving the chloride ion. Thus, in the isomeric 6-chloro-27/,3//-benzo[b]thiophenedione-2,3 anion-radical, unpaired electron density at C-6 is five times greater than at C-5, and the dimerization proceeds much more rapidly than the cleavage of carbon-chlorine bond (Alberti et al. 1981). 

A solvent role can play a decisive role when a reaction chooses the ion-radical pathway. Thus, the solvent effect on thermodynamic contribution to activation free energy causes an increase in the 

The yields of the first two products are insensitive to changes in solvent polarities. The yield of the third (the last) product increases with changing the solvent from CD2CI2 to CDjCN and, especially, addition of LiC104 to CD3CN reaction medium. Obviously, the third product is formed on a parallel reaction having ion-radical mechanism (for details, see Painter and Blackstock 1995). The dependence of reaction kinetics on ionic strength can supposedly be an indicator of ion-radical participation in rate control. 

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