Simple Enols. 2. Kinetics and Mechanism of the Ketonization of Vinyl Alcohol

Article abstract of DOI:10.1021/ja00390a030

The kinetics of the conversion of vinyl alcohol into acetaldehyde in aqueous solution at 15 deg C were found to depend on pH according to the equation k₀ = k_H2O + (k_H+*10_-pH) + (k_OH-*k_W)/10_-pH where K_H2O = 1.38*10_-2 s^-1, K_H+ = 20.2 M^-1 s^-1, and K_OH- = 1.50*10⁷ M^-1 s^-1.Under the same conditions, K_H+ for the hydrolysis of ethyl vinyl ether is 1.19 M^-1 s^-1.The solvent deuterium isotope effrect, K_H+/k_D+, for the ketonization of vinyl alcohol is 4.75 and for the hydrolysis of ethyl vinyl ether is 2.98; the entropies of activation for the two reactions are -13.3 (esd = 1.7) cal deg^-1 mol^-1 at 25 deg C.General-base catalysis (β = 0.77) was observed for the ketonization of vinyl alcohol with eight carboxylate ions, and general-acid catalysis (α = 0.45) was observed with five carboxylic acids stronger than and including formic acid.The variation of k_H+ with solvent composition in H2O-Me2SO mixtures was much less for the ketonization of vinyl alcohol than for the hydrolysis of ethyl vinyl ether, and as result of this, k_H+ for the former reaction is 325 times greater than for the latter in 92.5 mol percent Me2SO; the isotope effect k_H+/k_D+, for the two reactions in 85.9 mol percent Me2SO are 5.33 and 2.05.It is concluded that these experimental results are best explained by a mechanism for the acid-catalyzed ketonization of vinyl alcohol in which protonation of the double bond by the acid catalyst is concerted with removal of the enolic proton by water acting as a general base and by a mechanism for the base- and water-catalyzed reactions that involves a rate-limiting C-protonation of the enolate anion.The pK_enol for acetaldehyde at 25 deg C was estimated to be 6.66 or 6.44 from the value of k_H+ for the ketonization of vinyl alcohol determined in this investigation and previously reported values of k_H+ for the enolization of acetaldehydeu