engleski

Nucleofugality of Some Aryl and Alkyl Carbonates

The leaving group abilities (nucleofugalities), defined by nucleofuge-specific parameters (Nf and sf), were determined for some aryl and alkyl carbonates from solvolysis rate constants of X, Y-substituted benzhydryl carbonates in 80% aqueous ethanol by applying the previously established linear free energy relationship equation log k (25 °C) = sf (Ef + Nf), which had also been employed in determining nucleofugality of other types of leaving groups. Nf values can be utilized for comparing heterolytic reactivities of diverse types of leaving groups in a wide range of reactivity. Furthermore, combining previously determined parameters in equation log k (25 °C) = sf (Ef + Nf) for various electrofuges and nucleofuges, the duration of a particular solvolytic reaction can be estimated. Values of sf parameter (which has a related meaning as the Hammett–Brown ρ+ parameter) correlate well with corresponding solvolytic log k of benzhydryl carbonates, indicating that more reactive carbonate leaving groups (smaller values of sf) yield earlier TS which is in agreement with Hammond postulate. The applicability of DFT model reaction (M06-2X method) has been examined for determination of Nf parameters of carbonate leaving groups (including HCO3−). A very good correlation between experimental and calculated reactivities enables further extending the nucleofugality scale.

carbonates; nucleofugality; reactivity; kinetics; dft

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