Kinetics and Thermodynamics of Keto-Enol Tautomerism of Simple Carbonyl Compounds: An Approach Based on a Kinetic Study of Halogenation at Low Halogen Concentrations

Article abstract of DOI:10.1021/ja00408a020

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 k_II = K_HS^SHk₂ (where K_HS^SH 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 K_HS^SH from data on k_II.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.