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Different Solvolytic Behavior of Aryl/Alkyl Carbonates, Carboxylates and Phenolates

Mayr's LFER equation log k (25 °C) = sf (Ef + Nf) has been used to determine heterolytic reactivities of a large number of leaving groups, i.e. nucleofugalities (Nf), in various solvents. The procedure includes the construction of the log k vs. Ef correlation plot, in which k represents first-order rate constants for solvolysis of a series of substituted benzhydryl substrates with a leaving group of interest, whereas the Ef parameter represents electrofugalities of corresponding benzhydryl electrofuges from the Ef reference scale. Eq. log k (25 °C) = sf (Ef + Nf) defines Nf as the negative intercept on the abscissa (Ef axis). Although the goal of this LFER approach is to determine Nf parameters, whose application is described elsewhere, the second nucleofuge-parameter, namely the sf parameter (the slope of the log k vs. Ef correlation plot), might also provide useful information concerning the nature of solvolytic transition states. Similarly as the Hammett–Brown ρ+ parameter, the sf parameter represents the solvolysis reaction constant that implies the degree of charge separation in TS. Different pattern of variation of the sf parameters with reactivities of leaving groups in the series of aryl/alkyl carbonates, carboxylates and phenolates, shown by the sf vs. log k (dianisylmethyl-LG) correlation, reveals the different degree of charge separation in the transition states that are constituted from a common electrofuge and leaving groups with similar reactivity but different functionalities. Correlation of log k for solvolysis of dianisylmethyl aryl/alkyl carbonates, carboxylates and phenolates with log k for solvolysis of corresponding benzhydryl derivatives might mimic the reactivity-stability correlation whereas the difference between electrofugality of the common electrofuges in these two series is 6 orders of magnitude. However, this correlation also reveals the separation of data points according to the type of leaving groups and predicts the breakdown of linear relationship between the reactivity of leaving groups of different functionalities and the stability of corresponding free anions. The both correlations indicate a different impact of the intrinsic barrier on the nucleofugality of different types of leaving groups which might be attributed to the lag in developing electronic stabilizing effects along the heterolysis reaction coordinate.

carbonates, carboxylates, phenolates, intrinsic barrier, electronic effects, solvolysis, transition states

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