Indices for predicting the quality of leaving groups
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The inherent quality of leaving groups in chemical reactions is related to their ionization potential and electron affinity using a quadratic model for the dependence of the energy on the number of electrons. A good leaving group for nucleophilic substitution/elimination reactions is one where the difference in energy between the system with the "optimum" number of electrons and the anion is small. Similarly, a good leaving group for electrophilic substitution/elimination reactions is one where the difference in energy between the system with the optimum number of electrons and the cation is small. This insight allows us to define indices for the quality of leaving groups in nucleophilic and electrophilic reactivity, which we term the nucleofugality and the electrofugality, respectively. These indices are useful not only for predicting the quality of leaving groups in organic reactions, but also for explaining the stability of carbocations, carbanions, and trends in pKa.
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