Keto-enol/enolate equilibria in the N-acetylamino-p-methylacetophenone system. Effect of a beta-nitrogen substituent
| Autor: | ‖ A. C. O'Donoghue, H. Heckroth, Yvonne Chiang, B. Hellrung, A. G. Griesbeck, J. Wirz, and J. P. Richard, Q. Meng, A. J. Kresge |
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| Rok vydání: | 2001 |
| Předmět: |
chemistry.chemical_classification
Ketone Photolysis Substituent Acetophenones Stereoisomerism General Chemistry Keto–enol tautomerism Photochemistry Biochemistry Medicinal chemistry Enol Catalysis Acid dissociation constant Orders of magnitude (entropy) chemistry.chemical_compound Kinetics Colloid and Surface Chemistry chemistry Methylene Equilibrium constant |
| Zdroj: | Journal of the American Chemical Society. 123(37) |
| ISSN: | 0002-7863 |
| Popis: | The cis-enol of N-acetylamino-p-methylacetophenone was generated flash photolytically and its rates of ketonization in aqueous HClO(4) and NaOH solutions as well as in HCO(2)H, CH(3)CO(2)H, H(2)PO(4)(-), (CH(2)OH)(3)CNH(3)(+), and NH(4)(+) buffers were measured. Rates of enolization of N-acetylamino-p-methylacetophenone to the cis-enol were also measured by hydrogen exchange of its methylene protons, and combination of the enolization and ketonization data gave the keto-enol equilibrium constant pK(E) = 5.33, the acidity constant of the enol ionizing as an oxygen acid pQ(a)(E)= 9.12, and the acidity constant of the ketone ionizing as a carbon acid pQ(a)(K)= 14.45. Comparison of these results with corresponding values for p-methylacetophenone itself shows that the N-acetylamino substituent raises all three of these equilibrium constants: K(E) by 3 orders of magnitude, Q(a)(E) by 1 order of magnitude, and Q(a)(K)by 4 orders of magnitude. This substituent also retards the rate of H+ catalyzed enol ketonization by 4 orders of magnitude. The origins of these substituent effects are discussed. |
| Databáze: | OpenAIRE |
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