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