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Nucleofugality of Carbonates

The leaving group abilities (nucleofugalities), defined by nucleofuge-specific parameters (Nf and sf), were determined for a series of aromatic and aliphatic carbonates from solvolysis rate constants of X, Y-substituted benzhydryl carbonates in a series of aqueous ethanol mixtures by applying the linear free energy relationship (LFER) equation: log k (25 °C) = sf (Ef + Nf). In this correlation equation k is the solvolysis first-order rate constant, sf (slope of the correlation line) and Nf (negative intercept on the abscissa) are the nucleofuge-specific parameters, whereas Ef represents the electrofugality parameter of variously substituted benzhydrylium electrofuges. Nf parameters can be employed for comparing heterolytic reactivities of diverse types of leaving groups in a wide range of reactivity. Furthermore, combining previously determined parameters, the duration of a particular solvolytic reaction can be estimated. The applicability of quantum-chemical model reaction (M06-2X method) has been examined for determination of Nf parameters of both aromatic and aliphatic carbonates (including HCO3−). A very good correlation between experimental and calculated reactivities makes possible to extend the nucleofugality scale significantly.

nucleofugality; carbonates; kinetics; solvolysis; DFT

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