THE ACIDITY AND TAUTOMERISM OF BETA-DIKETONES IN AQUEOUS-SOLUTION

The acidity and keto-enol tautomerism of a series of symmetrical beta-diketones (RCOCH(2)COR (1): R = methyl (a), phenyl (b), 3-pyridinyl (c), 4-pyridinyl (d), 3-(N-methyl)pyridinio (e), and 4-(N-methyl)pyridinio (f)) and two series of unsymmetrical beta-diketones (RCOCH(2)COCH(3) (7a-7f) and RCOCH(2)COC(6)H(5) (8a-8f)) have been investigated in aqueous solution at 25 degrees C and ionic strength 0.1. Values of pK(a)(eq) were measured spectrophotometrically, and the acidities of the enols (pK(a)(K)) were obtained from the analysis of the pH dependence of the buffer catalysis for the general acid-catalyzed protonation of the enolate conjugate bases. These data in turn allowed the calculation of the acidities of the keto tautomers (pK(a)(K)) and the equilibrium constants for enolization (K-E = [Enol]/[Keto]). In general, K-E is greater for the symmetrical ketones (1) than for the corresponding R-substituted unsymmetrical ketones (7 and 8). K-E is much more sensitive to the nature of the R substituent in these three series of beta-diketones than in the corresponding series of beta-keto esters and amides. Correlations between pK(a)(K) for 8 and 7 combined with the known acidities of the corresponding beta-keto esters and amides provide the first accurate estimates of the acidities of dimethyl malonate (pK(a) = 13.0)and malonamide (pK(a) = 12.5) in aqueous solution.