Generation of Simple Enols in Aqueous Solution from Alkali Metal Enolates. Some Chemistry of Isobutyraldehyde Enol

Article abstract of DOI:10.1021/ja00280a032

The enol isomer of isobutyraldehyde was generated in aqueous solution by reaction of its lithium and potassium enolates with water and of the trimethylsilyl enol ether with fluoride ion, and rates of ketonization of the enol were measured in HCl, DCl (in D2O), and NaOH solutions and in CNCH2CO2H, HCO2H, CH3CO2H, CH2ClPO3H^-, and H2PO4^- buffers.Rates of enolization of isobutyraldehyde were also determined, by iodine scavenging, in HClO4 and NaOH solutions.The reaction rates in HCl and NaOH give two independent estimates of the keto-enol equilibrium constant for isobutyraldehyde in aqueous solution at 25 deg C, which are in good agreement with each other and whose average is K_E = (1.37 +/- 0.09)*1E-4, pK_E = 3.86 +/- 0.03.The ketonization rates in NaOH solution also provide an estimate of the acidity constant of isobutyraldehyde enol ionazing as an oxygen acid, K_a^E = (2.37 +/- 0.14)*1E-12 M, pK_a^E = 11.63 +/- 0.03, which, when combined with K_E, gives the acidity constant of the keto form of isobutyraldehyde ionizing as a carbon acid, K_a^K = (3.23 +/- 0.29)*1E-16 M, pK_a^K = 15.49 +/- 0.04.The ketonization reaction in buffer solutions shows both general-acid and general-base catalysis, consistent with two parallel reaction paths involving rate-determing β-carbon protonation of both enol and enolate ion.Analysis of the data in terms of this scheme shows enolate to be 1E8 times more reactive than enol.Arguments are advanced to the effect that all of the present data are consistent with stepwise reaction mechanisms and do not require a concerted reaction path.