117067-79-5Relevant articles and documents
Kinetics and Thermodynamics of Keto-Enol Tautomerism of Simple Carbonyl Compounds: An Approach Based on a Kinetic Study of Halogenation at Low Halogen Concentrations
Dubois, Jaques-Emile,El-Alaoui, Mohiedine,Toullec, Jean
, p. 5393 - 5401 (1981)
Kinetic data for bromination and/or iodination of cycloalkanones and aryl-substituted acetophenones in water, using very low halogen concentrations (10-7 - 10 -5 M) (i.e., when the rate limiting step is not enolization but partly halogen addition to enol) provide the hydronium-catalyzed enolization rate coefficients (k1) and the apparent second-order rate coefficients kII = KHSSHk2 (where KHSSH is the keto-enol equilibrium constant and k2 the enol halogenation rate constant).As previously suggested, the rate of enol halogenation is usually encounter controlled.This makes it possible to estimate k2 and calculate KHSSH from data on kII.The enol ketonization rate coefficients are deduce and compared with those for methyl and ethyl enol ether hydrolysis.It is shown that the ratio enol ketonization/enol ether hydrolysis of parent compounds varies in the range 15-150 and depends on enol structure.For the different rate and equilibrium coefficients which intervene in the two-step mechanism of keto-enol interconversion of acetophenones, a variety of linear free-energy relationships are established, using the Young-Jencks modified Yukawa-Tsuno equation.A Broensted relation is observed by plotting the rate constants for enol formation from the conjugated acids of acetophenones vs. the CH acidity constants of these ions.The Broensted exponent, α = 0.4, is in agreement with a transition state model in which the proton is less than half-transferred.Data on entalpy and enthropy of activation for enolization of cycloalkanones, compared with those for keto-enol equilibrium, are also in favor with an early transition state.
