engleski

Solvolytic Reactivity of Organophosphates and Organophosphinates

Solvolytic reactivities of some aliphatic and aromatic phosphate and phosphinate leaving groups (i.e. their nucleofugalities, Nf) in various aqueous binary mixtures have been determined according to Eq. log k (25 °C) = sf (Ef + Nf). For that purpose, logarithms of solvolysis rate constants (log k) for the series of benzhydryl phosphates and phosphinates have been correlated with corresponding electrofugalities (Ef) of the reference benzhydryl electrofuges. The obtained Nf values enable comparison of nucleofugality of the phosphorus leaving groups with nucleofugality of a large number of other leaving groups, constituted from various functionalities. At the moment the nucleofugality scale covers a reactivity range of 15 orders of magnitude. Employing nucleofuge-specific parameters (Nf and sf) of the organophosphate and organophosphinate leaving groups and previously established Ef values of corresponding electrofuges, rate constants for solvolysis of various organophosphates and organophosphinates in a given solvent (i.e. their half-lives and kinetic stability) can be predicted. Organic phosphates demonstrate noticeably higher solvolytic reactivity than organic phosphinates (more than 3.5 orders of magnitude) which, according to NBO analysis, might be rationalized with an additional orbital stabilizing effect (namely the negative hyperconjugation) that is operative in both TS and organic phosphate free anions.

Phosphates, phosphinates, solvolysis, reactivity, nucleofugality, negative hyperconjugation

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