Ionization and Tautomerization of 2-Nitrocyclohexanone in Aqueous Solution

Abstract

The keto-enol tautomerism of 2-nitrocyclohexanone (2-NCH) was studied in aqueous solution under different experimental conditions. Ketonization rate constants were measured spectrophotometrically at 25 degrees C at an ionic strength of 0.4 mol dm-3 (NaCl) in diluted hydrochloric acid, in diluted sodium hydroxide, and in several buffers by using NaHSO3 as the scavenger of the keto form. A value of pK(a)(EH) = 4.78 for the enol form was obtained from the rate-pH profile of the reaction. A value of pK(a)(KH) = 5.97 for the keto form was directly obtained from the UV-vis spectra of 2-NCH recorded at different pHs. The equilibrium constant for the keto-enol tautomerism, pK(T) = -log([enol]/[ketone]) = 1.19, was obtained by combining the two pKa values (pK(T) = pK(a)(KH) - pK(a)(EH)). A comparison of these results with the corresponding values (Keefe, J. R.; Kresge, A. J. In The Chemistry of Enols; Rappoport, Z., Ed.; Wiley & Sons: New York, 1990; pp 399-480) for cyclohexanone shows the dramatic effects of an alpha-nitro substituent on the keto-enol acidities and the tautomerization constant of alicyclic ketones. Rates and equilibria were discussed in the light of the Brønsted equation, the principle of nonperfect synchronization, and the Marcus theory. It turns out that, on passing from nitroalkanes to nitroketones, the resonance contribution to pKa and deprotonation rate decreases, being overwhelmed by steric and inductive effects.

Determination of thermodynamic parameters of tautomerization in gas phase by mass spectrometry and DFT calculations: Keto-enol versus nitrile–ketenimine equilibria

“…While keto-enol tautomeric equilibria of carbonylic compounds have been studied extensively [1,2], there are few reports on the occurrence of nitrile-ketenimine tautomerism [3][4][5].…”

A rational approach to predict and modulate stereolability of chiral α substituted ketones

“…29 Brønsted equations have been widely used to quantitatively correlate the rate constants of proton transfer and the acid strength of carbon acids involved in the process, using the same catalysing base. [30][31][32] An excellent linear Brønsted correlation extending over 16 log U in equilibrium and 5 log U in rate has recently been reported 33 We will demonstrate that these correlations can be used to obtain effective prediction of kinetic quantities.…”

Theory and Practice of UHPLC and UHPLC–MS

“…On the other hand, configurational isomerizations (i.e., processes involving rupture and reformation of chemical bonds) are commonly promoted by species that act as catalysts and that therefore do not modify their concentration during the interconversion (a bimolecular mechanism). Tautomeric equilibria (Ballini et al, 2000;Fontana et al, 2002;Angelini et al, 2007;Siani et al, 2008), enantiomerizations (Gasparrini et al, 2003;Cirilli et al, 2007Cirilli et al, , 2009aTrapp et al, 2002), and diastereomerizations driven by reversible proton abstraction from Calculation of free energy activation barriers and their enthalpic and entropic contributions.…”