- Pressure Dependences of Rates of Nucleophilic Attack by Water in Aqueous Solution. Hydrolyses of Methyl p-Nitrobenzenesulfonate and Ethyl Trichloroacetate
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The pressure dependences of the rates of two formally uncatalyzed reactions of water in aqueous solution have been measured in the range 0-2 kbar at 25 deg C.A plot of ln k vs.P for the hydrolysis of ethyl trichloroacetate (water-catalyzed water addition, followed by ethanol elimination) is smoothly and gently curved with ΔV = -33 +/- 3 cm3 mol-1 near P = 0.The corresponding plot for the hydrolysis of methyl p-nitrobenzenesulfonate (direct methyl transfer from ArSO3-1 to H2O) shows an abrupt change in slope near 0.1 kbar, the slope between 0 and 0.07 kbar corresponds to ΔV = -24 +/- 5 cm3 mol-1, while that between 0.14 and 2.07 kbar corresponds to ΔV = -6.3 +/- 0.1 cm3 mol-1.Comparison to the data of Baliga and Whalley for CH3Br hydrolysis at higher temperatures suggests that the pressure dependences of rates of methyl transfer to water may commonly show such changes in slope and that increasing temperature shifts the slope change to higher pressure and decreases its sharpness.
- Kurz, Joseph L.,Lu, John Y.-W
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- Solvolytic Behavior of Aliphatic Carboxylates
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The leaving group abilities (nucleofugalities) of a series of aliphatic carboxylates have been obtained by determining the nucleofuge-specific parameters (Nf and sf) from solvolysis rate constants of X,Y-substituted benzhydryl carboxylates in a series of aqueous ethanol mixtures by applyication of the linear free energy relationship (LFER) equation: log k = sf (Ef + Nf). These values can be employed to compare reactivities of carboxylates with those of other leaving groups previously included in the nucleofugality scale, and also to estimate the solvolysis rates of various carboxylates. It is confirmed that the inductive effect is the most important variable governing the reactivities of halogenated carboxylates in solution. Moreover, both the Hammett correlation and the solvolytic activation parameters have revealed a strong influence of the inductive effect on the nucleofugality of alkyl-substituted carboxylates. The reaction constants (sf) indicate that carboxylate substrates with weaker leaving groups solvolyze via later, more carbocation-like, transition states, which is in accord with the Hammond postulate. In addition, due to the weaker demand for solvation of transition states that produce more strongly stabilized benzhydrylium ions, in which more efficient charge delocalization occurs, the reaction constants (sf) obtained with most of the leaving groups investigated here increase as the polarity of the solvent decreases.
- Matic, Mirela,Denegri, Bernard,Kronja, Olga
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p. 1477 - 1486
(2015/10/05)
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