The acidity and tautomerism of β-diketones in aqueous solution

Abstract

The acidity and keto–enol tautomerism of a series of symmetrical β-diketones (RCOCH2COR (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 β-diketones (RCOCH2COCH3(7a–7f) and RCOCH2COC6H5(8a–8f)) have been investigated in aqueous solution at 25 °C and ionic strength 0.1. Values of [Formula: see text] were measured spectrophotometrically, and the acidities of the enols [Formula: see text] were obtained from the analysis of the pH dependence of the buffer catalysis for the general acid-catalyzed protonation of the enolateconjugate bases. These data in turn allowed the calculation of the acidities of the keto tautomers [Formula: see text] and the equilibrium constants for enolization (KE = [Enol]/[Keto]). In general, KEis greater for the symmetrical ketones(1) than for the corresponding R-substituted unsymmetrical ketones (7 and 8). KEis much more sensitive to the nature of the R substituent in these three series of β-diketones than in the corresponding series of β-keto esters and amides. Correlations between [Formula: see text] for 8 and 7 combined with the known acidities of the corresponding β-keto esters and amides provide the first accurate estimates of the acidities of dimethyl malonate (pKa = 13.0)and malonamide (pKa = 12.5) in aqueous solution. Keywords: acidity, tautomerism, β-diketones.