Ionization and Tautomerization of 2-Nitrocyclohexanone in Aqueous Solution

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• 作者：Guido Angelini ; Paolo De Maria ; Antonella Fontana ; Marco Pierini ; Gabriella Siani
• 刊名：Journal of Organic Chemistry
• 出版年：2007
• 出版时间：May 25, 2007
• 年：2007
• 卷：72
• 期：11
• 页码：4039 - 4047
• 全文大小：255K
• 年卷期：v.72,no.11(May 25, 2007)
• ISSN：1520-6904

文摘

The keto-enol tautomerism of 2-nitrocyclohexanone (2-NCH) was studied in aqueous solution underdifferent experimental conditions. Ketonization rate constants were measured spectrophotometrically at25 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 forthe enol form was obtained from the rate-pH profile of the reaction. A value of pK_a^KH = 5.97 for theketo form was directly obtained from the UV-vis spectra of 2-NCH recorded at different pHs. Theequilibrium constant for the keto-enol tautomerism, pK_T = -log([enol]/[ketone]) = 1.19, was obtainedby combining the two pK_a values (pK_T = pK_a^KH - pK_a^EH). A comparison of these results with thecorresponding 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 -nitrosubstituent on the keto-enol acidities and the tautomerization constant of alicyclic ketones. Rates andequilibria were discussed in the light of the Brnsted equation, the principle of nonperfect synchronization,and the Marcus theory. It turns out that, on passing from nitroalkanes to nitroketones, the resonancecontribution to pK_a and deprotonation rate decreases, being overwhelmed by steric and inductive effects.