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)(3)CNH3+, 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.