Free radical transition states polar character

Why do free-radical reactions involving neutral reactants and intermediates respond to substituent changes that modify electron distribution One explanation has been based on the idea that there would be some polar character in the transition state because of the electronegativity differences of the reacting atoms ... 

The transition states of free radical reactions generally show evidence of polar character wherein electron transfer to or from the radical has occurred (20). Thus, the electron affinity or ionization potential of the radical involved should affect the reaction. The much higher electron affinity (16) of ROo than CH3 radicals no doubt alters the transition state so that the reactivities toward it show less selectivity. The results of Szwarc and Binks (22) center around the fact that only carbon radicals were used for the correlation, and thus the electron affinity does not vary sufficiently to show in the correlation any deviation from the expected reactivity-selectivity relationship. 

Free radicals and vinyl monomers are neutral, but variations in the reactivities of both species can be rationalized and predicted by considering that the transition states in their reactions may have some polar character. Appropriate substituents may facilitate or hinder a particular reaction because of their influence on the polarity of the reaction site. 

In many reactivity problems the transition state might be said to be more delocalized than the ground state. Dissociation reactions, whether polar or free radical, have this character. The ionization of allyl chloride involves a change... 

Inclusion of the ionic structures in the three-electron model made it possible to take into account the effects of polarization upon the mechanism of free-radical reactions and to arrive at the following conclusion addition of a free radical to a bond attacked by polar groups occurs in such a position for which the new bond that forms in the transition state possesses the most ionic character. If the reaction occurs under thermodynamic control, then the polarization effects stabilize the product thereby facilitating the development of reaction. Otherwise, products may form that are not thermodynamically the most stable. These qualitative conclusions have been supported [7] by ab initio calculations of the relative stability of reactants and products as well as the activation barriers for a series of the reactions ... 

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