title: Keto-enol/enolate equilibria in the N-acetylamino-p-methylacetophenone system. Effect of a beta-nitrogen substituent.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada.
authors.forename: Y
authors.initials: Y
authors.lastname: Chiang
authors.forename: A G
authors.initials: AG
authors.lastname: Griesbeck
authors.forename: H
authors.initials: H
authors.lastname: Heckroth
authors.forename: B
authors.initials: B
authors.lastname: Hellrung
authors.forename: A J
authors.initials: AJ
authors.lastname: Kresge
authors.forename: Q
authors.initials: Q
authors.lastname: Meng
authors.forename: A C
authors.initials: AC
authors.lastname: O'Donoghue
authors.forename: J P
authors.initials: JP
authors.lastname: Richard
authors.forename: J
authors.initials: J
authors.lastname: Wirz
mesh_terms: D000098:Acetophenones / Q000737:chemistry*; D007700:Kinetics; D010782:Photolysis; D013237:Stereoisomerism
chemical_list: D000098:Acetophenones