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 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.