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Nucleofugality of Aliphatic Carboxylates in Mixtures of Aprotic Solvents and Water

The leaving group ability (nucleofugality) of fluoroacetate, chloroacetate, bromoacetate, dichloroacetate, trifluoroacetate, trichloroacetate, heptafluorobutyrate, formate, isobutyrate, and pivalate have been derived from the solvolysis rate constants of the corresponding X, Y-substituted benzhydryl carboxylates in 60% and 80% aqueous acetonitrile and 60% aqueous acetone, applying the LFER equation: log k = sf (Ef + Nf). The experimental barriers (∆G‡, exp) for solvolyses of 11 reference dianisylmethyl 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 calculated earlier. Linear correlation observed between the log k for the reference dianisylmethyl carboxylates and the sf values enables estimation of the reaction constant (sf estim). Using the ∆G‡, exp vs. ∆G‡, model correlation, and taking the estimated sf estim, the nucleofugality parameters for other 34 aliphatic 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 aprotic solvent/water mixtures is the inductive effect of the group(s) attached onto the carboxylate moiety.

nucleofugality; reactivity; aliphatic carboxylates; benzhydryl; hydrolysis; solvolysis; model reaction; correlation

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