Nucleofugality of Aliphatic Carboxylates in Mixtures of Aprotic Solvents and Water

摘要

The leaving group ability (nucleofugality) of fluoroacetate, chloroacetate, bromoacetate, dichloroace-tate, trifluoroacetate, trichloroacetate, heptafluorobutyrate, formate, isobutyrate, and pivalate have been derived from the solvolysis rate constants of the corresponding X,Y-substituted benzhydryl car-boxylates in 60 % and 80 % aqueous acetonitrile and 60 % aqueous acetone, applying the LFER equa-tion: log k = sf (Ef + Nf). The experimental barriers (∆G‡,exp) for solvolyses of 11 reference dianisylme-thyl carboxylates in these solvents correlate very well (r = 0.994 in all solvents) with ∆G‡,model of the model σ-assisted heterolytic displacement reaction of cis-2,3-dihydroxycyclopropyl trans-carboxylates cal-culated earlier. Linear correlation observed between the log k for the reference dianisylmethyl car-boxylates and the sf values enables estimation of the reaction constant Using the ∆G‡,exp vs. ∆G‡,model correlation, and taking the estimated the nucleofugality parameters for other 34 ali-phatic carboxylates have been determined in 60 % and 80 % aqueous acetonitrile and 60 % aqueous acetone. The most important variable that determines the reactivity of aliphatic carboxylates in apro-tic solvent/water mixtures is the inductive effect of the group(s) attached onto the carboxylate moiety.