Vinyl Alcohol: Generation and Decay Kinetics in Aqueous Solution and Determination of the Tautomerization Equilibrium Constant and Acid Dissociation Constants of the Aldehyde and Enol Forms

Article abstract of DOI:10.1021/ja00247a028

Vinyl alcohol was generated in aqueous solution by Norrish type II photoelimination of 5-hydroxy-2-pentanone and by the acid-catalyzed hydrolysis of a ketene acetal (1-(vinyloxy)-1-methoxyethene) and an ortho ester (2-(vinyloxy)-2-phenyl-1,3-dioxolane), and rates of isomerization of this enol to acetaldehyde catalyzed by hydronium and hydroxide ions and formic and acetic acid buffers were measured.Rates of enolization of acetaldehyde, determined by iodine scavenging and catalyzed by hydronium and hydroxide ions, were also measured.These data lead to two concordant, independent determinations of the tautomerization equilibrium constant for acetaldehyde in aqueous solution whose average is K_E=(5.89+/-0.81)x10^-7, pK_E=6.23+/-0.06.They also give K_a^E=(3.13+/-0.17)x10^-11, pK_a^E=10.50+/-0.02, for the dissociation constant of vinyl alcohol ionizing as an oxygen acid, and K_a^K=(1.85+/-0.27)x10^-17, pK_a^K=16.73+/-0.06, for the dissociation constant of acetaldehyde ionizing as a carbon acid; all equilibrium constants are concentration quotients and refer to aqueous solution at ionic strength 0.10 M.Kinetic solvent isotope effects on both enolization and ketonization suggest that reaction occurs by a stepwise rather than a concerted mechanism.