114915-58-1Relevant articles and documents
Keto-enol/enolate equilibria in the N-acetylamino-p-methylacetophenone system. Effect of a β-nitrogen substituent
Chiang,Griesbeck,Heckroth,Hellrung,Kresge,Meng,O'Donoghue,Richard,Wirz
, p. 8979 - 8984 (2001)
The cis-enol of N-acetylamino-p-methylacetophenone was generated flash photolytically and its rates of ketonization in aqueous HClO4 and NaOH solutions as well as in HCO2H, CH3CO2H, H2PO4-, (CH2OH)3CNH3+, and NH4+ buffers were measured. Rates of enolization of N-acetylamino-p-methylacetophenone to the cis-enol were also measured by hydrogen exchange of its methylene protons, and combination of the enolization and ketonization data gave the keto - enol equilibrium constant pKE = 5.33, the acidity constant of the enol ionizing as an oxygen acid pQaE = 9.12, and the acidity constant of the ketone ionizing as a carbon acid pQaK = 14.45. Comparison of these results with corresponding values for p-methylacetophenone itself shows that the N-acetylamino substituent raises all three of these equilibrium constants: KE by 3 orders of magnitude, QaE by 1 order of magnitude, and QaK by 4 orders of magnitude. This substituent also retards the rate of H+ catalyzed enol ketonization by 4 orders of magnitude. The origins of these substituent effects are discussed.
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 (2007/10/02)
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.
Mechanistic Change in Acid-Catalyzed Hydrolysis of Silyl Vinyl Ethers
Novice, Marilyn H.,Seikaly, Hani R.,Seiz, Annette D.,Tidwell, Thomas T.
, p. 5835 - 5838 (2007/10/02)
tert-Butyldimethylsilyl vinyl ethers t-BuMe2SiOCR=CH2 hydrolyze in 50percent CH3CN - H2O with general and specific acid catalysis and a linear dependence of log kH+ on ?p+(R), indicating that the reaction occurs with rate-determing proton transfer to carbon (the ADE2 mechanism).By contrast the rates of trimethylsilyl vinyl ethers Me3SiOCR=CH2 are not correlated by ?p+(R) but still depend on proton and general acid concentrations.Nucleophilic attack on silicon is implicated in the rate-determing step of the latter compounds, and the possibilities for the details of this process are considered.
