New data on keto–enol equilibria of 3-alkylpentane-2,4-diones in aqueous solution

Abstract

For β-diketones MeCOCHRCOMe, there is a linear free energy relationship between the rate constant for ketone deprotonation and the tautomeric equilibrium constant which decreases as R increases. This relationship is valid only for R different from H and its slope is unity (1.00 ± 0.07). Our main conclusion is that the effect of the alkyl groups, with R different from H, on the keto–enol equilibrium position is mainly due to a preponderant structural effect on the keto-form and not on enol. However, when any alkylated compound is compered with pentane-2,4 dione (R = H) two distinct structural effects, one relating to the enol form and the other to the keto-form, are observed.