Keto−Enol/Enolate Equilibria in the N-Acetylamino-p-methylacetophenone System. Effect of a β-Nitrogen Substituent

The cis-enol of N-acetylamino-p-methylacetophenone was generated flash photolytically and its rates of ketonization in aqueous HClO4 and NaOH solutions as well as in HCO2H, CH3CO2H, H2PO4 -, (CH2OH)3CNH3 +, and NH4 + 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 p = 9.12, and the acidity constant of the ketone ionizing as a carbon acid p = 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, by 1 order of magnitude, and 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.