Reactivity Trends in Radical Exchange Reactions

As an application, let us consider a typical class of radical exchange reactions, the hydrogen abstractions from alkanes. Equation 6.14 describes the identity process of hydrogen abstraction by an alkyl radical  

Identity reactions proceed without a thermodynamic driving force, and therefore project the role of promotion energy as the origin of the barrier (recall, the intrinsic barrier above). 

The barriers for a series of radicals have been computed (22), and were found to increase as the R H bond energy D increases the barrier is the largest for R = CF13 and the smallest for R = C(CH3)3. This trend has been interpreted by Pross et al. (23) using the VBSCD model. The promotion gap G that is the origin of the barrier involves the singlet-triplet excitation of the 

This correlation indicates that this is a reaction family with constants f= 0.3481 and B = 50kcal/mol. If this is indeed the case (can be checked by VB computations), this kind of correlation enables one to measure the resonance energy of the transition state of a chemical reaction. 

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