A method of determining individual keto-enol equilibrium and acid dissociation constants for systems in which unstable enols can exist in cis and trans isomeric forms is developed and is applied to phenylacetaldehyde in aqueous solution; the results give equilibrium cis: trans ratios of 35 : 65 in acidic and neutral solutions and 20 : 80 in a basic solution (where the enols are converted to enolate ions), but show considerably less cis: trans differentiation for enols formed under kinetic control.
This paper is dedicated to Professor Jacob Bigeleisen on the occasion of his 70th birthdayThe following kinetic isotope effects were determined for acid-catalyzed ketonization of isobutyrophenone enol and enolate ion through rate-determining hydron transfer from catalyst to substrate: enol, kH/kD = 3.30±0.07 (hydronium ion catalysis), /cH//cD = 4.0 + 2.8 (acetic acid catalysis); enolate ion, kH/kD= 1.00 + 0.21 (hydronium ion catalysis), kH/kD = 3A \ +0.20 (acetic acid catalysis), kH /kD = 7.48 + 0.23 (water catalysis). The magnitude of these isotope effects, when assessed in terms of the free energies of reaction for the processes in which they occur, are consistent with Melander-Westheimer-Bigeleisen theory. An equilibrium isotope effect of KH/KD = 5.88 + 0.32 was also determined for the ionization of isobutyrophenone enol as an oxygen acid.
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