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All total 3 Articles be foundElectronic effects on enol acidity and keto-enol equilibrium constants for ring-substituted 2-tetralones
Yao,Pollack
, p. 634 - 638 (2007/10/03)
Equilibrium constants for the ionization of a variety of phenyl-substituted 2-tetralones (pK(a)(K)), for the ionization of their enols (pK(a)(E)), and for keto-enol tautomerization (PK(E)) were determined. Hammett plots of pK(a)(K) and pK(a)(E) vs. σ are linear with slopes (-ρ) of -1.66 ± 0.06 and -0.90 ± 0.03, respectively, except for deviations of the points corresponding to 6-nitro-2-tetralone (1b) and its enol. We have previously attributed the negative deviation of 1b from the correlation for the acidities of the ketones obtained with the more limited set of data to the lack of a free electron pair on C-1 of the free tetralone (Nevy et al.). The negative deviation of the point for 1b from the correlation for the acidities of the enols suggests that charge transfer from the hydroxyl group of the enol to the nitro group is less important than it is for phenols. This study represents the first systematic study of electronic effects on equilibria among ketone, enol, and enolate in aqueous solution.
Transition state imbalance in the deprotonation of substituted 2- tetralones by hydroxide ion
Nevy, John B.,Hawkinson, David C.,Blotny, Grzegorz,Yao, Xudong,Pollack, Ralph M.
, p. 12722 - 12726 (2007/10/03)
Rate and equilibrium constants for the deprotonation of a series of phenyl-substituted 2-tetralones in aqueous sodium hydroxide have been determined. A Bronsted plot of log k for deprotonation vs pK(a) of the appropriate 2-tetralone is linear with a slope (-α) of -0.60 ± 0.01, except for the point corresponding to 6-nitro-2-tetralone (1b). The negative deviation of 1b from the correlation indicates that the transition state for deprotonation of 2-tetralone is imbalanced, with delocalization of charge into the phenyl ring lagging behind proton transfer. A semiquantitative assessment of the charge distribution in both the fully formed anion and the transition state for deprotonation was calculated from these results and 13C NMR spectra of the 2-tetralone anion in methanol/water mixtures. Although approximately twice as much negative charge is localized on the oxygen than on the enolate carbon in the anion, slightly more charge is on the enolate carbon in the transition state.
