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The Impact of the Intrinsic Barrier on Relative Reactivities of Leaving Groups

A comprehensive nucleofugality scale, based on both LFER equation log k (25 °C) = sf (Ef + Nf)(1) and the set of reference benzhydryl electrofuges, has been established. The nucleofugality of a particular leaving group is determined with two parameters: Nf (the negative intercept on the abscissa of the log k vs. Ef correlation plot) and sf (the slope of the correlation plot). Whereas Nf values represent the approximate solvolytic reactivity of dianisylmethyl derivatives in a given solvent (in the logarithmic scale), the reaction constant sf indicates the variation in relative reactivities of leaving groups with electrofugality (Ef), i.e. the reactivity of an electrofuge in a substrate. Comparison of sf values of phenolate leaving groups with sf of carboxylates and carbonates with similar reactivities reveals the noticeably higher values for the former which is manifested with intersecting the corresponding log k vs. Ef correlation lines and inversion of relative reactivities of leaving groups in the region of kinetic measurements. The inversion of relative reactivities with a series of benzhydryl electrofuges clearly demonstrates that nucleofugalities of leaving groups are not determined only with stabilities of free anions but also with a differential impact of the intrinsic barrier, caused by different types of stabilizing effects that are operative in diverse leaving groups during heterolysis.

intrinsic barrier, reactivity, leaving groups, activation free energies

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