- Acid-catalyzed enolization of acetophenone: catalysis by bisulfate ion in sulfuric acid solutions
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Rates of acid-catalyzed enolization of acetophenone in dilute aqueous solution, measured under conditions where the solvated proton is the only acidic species present, give a hydrogen ion catalytic coefficient, kEH+= (1.21+/-0.01) x
- Keeffe, J. R.,Kresge, A. J.,Toullec, J.
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- Direct Observation of Acetophenone Enol Formed by Photohydration of Phenylacetylene
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The photohydration of phenylacetylene yields acetophenone enol as a transient primary product which was detected by flash photolysis.The identification of the transient intermediate rests on a quantitative comparison of its decay kinetics with that of authentic acetophenone enol generated by Norrish type-II photoelimination from γ-hydroxybutyrophenone in aqueous HCl and aqueous AcOH buffer solutions.
- Chiang, Yvonne,Kresge, A. Jerry,Capponi, Marco,Wirz, Jakob
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- Flash-Photolytic Generation of Acetophenone Enol. The Keto-Enol Equilibrium Constant and pKa of Acetophenone in Aqueous Solution
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Acetophenone enol has been generated by Norrish type II photoelimination of γ-hydroxybutyrophenone, and its rate of ketonization has been measured in dilute aqueous HCl solutions.These data, in combination with the specific rate of acid-catalyzed enolization of acetophenone, give a value of the keto-enol equilibrium constant which is in good agreement with another result based upon enolization and ketonization rate constants measured in NaOH solutions.The average of these determinations gives pKE = 7.90 +/- 0.02.This, when combined with the known acid-dissociation constant of acetophenone enol, leads to pKaK = 18.24 +/- 0.03 for the acidity constant of acetophenone ionizing as a carbon acid.The present results allow an estimate of the specific rate of proton transfer from H3O+ to the β-carbon atom of acetophenone enolate ion, k = (4.2 +/- 1.2) x 1010 M-1 s-1, which is so great as to suggest that this reaction occurs through proton transfer down hydrogen-bonded solvent bridges between acid and substrate.
- Chiang, Yvonne,Kresge, A. Jerry,Wirz, Jakob
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- Enolization of aldehydes and ketones: Structural effects on concerted acid-base catalysis
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The third-order term (k(AB)) for the concerted acid-base catalyzed enolization of a selection of simple aldehydes and ketones has been measured in a series of substituted acetic acids at 25°C at constant ionic strength 2.0 (NaNO3). While there is no direct correlation of the magnitude of the third-order term with either the rate constants for acid (k(A)) or base (k(B)) catalysis, a simple log-log relationship exists between the product of the consecutive rate constants (k(A)·k(B)) and the concerted (third order) rate constants (k(AB)). This implies that the concerted pathway is important only when both the general acid and the general base terms are significant; this will be useful in designing other systems which might show such concerted catalysis. In the case of aldehydes, a slope of 0.97 was found for this plot, which compares to the result for 4-substituted cyclohexanones (0.51) and other ketones (0.59), as measured in acetic acid buffers. The resultant Bronsted β(AB) value of 0.20 found for propanal (2) is consistent with the overall observation that concerted catalysis is largely independent of the buffering species, and that process is overall base catalyzed. The solvent isotope effect on the concerted acid-base catalyzed enolization rate term, k(AB)(H2O)/k(AB)(D2O) = 1.33, indicates that the transition state for proton transfer to the carbonyl is more advanced than in the case of ketones. In general we have found that carbonyl compounds with large measured (or estimated) enol contents show significant third-order terms.
- Hegarty, Anthony F.,Dowling, Joseph P.,Eustace, Stephen J.,McGarraghy, Michelle
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p. 2290 - 2296
(2007/10/03)
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- Characterization of the indan-1-one keto-enol system
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3-hydroxyindene was generated by Norrish type II photoelimination of 2-methoxyindan-1-one, and its subsequent rate of ketonization was measured in dilute aqueuos perchloric acid, sodium hydroxide, acetic acid buffer and ammonia buffer solutions.These data, when combined with acid-catalysed rates of enolization of indan-1-one determined bby both bromine and iodine scavenging, give pKE = 7.48 for the keto-enol equilibrium constant, pKaE = 9.48 for the acidity constant of the enol ionising as an oxygen acid, And pKaK = 16.96 for the acidity constant of the ketone ionizing as a carbon acid.These results are compared with corresponding values for the acetophenone keto-enol system, using the redetermined acetophenone enol acidity constant, pKaE = 10.40, obtained here from rates of ketonization of the enol in sodium hydroxide and ammonia buffer solutions.All the present equilibrium constants refer to wholly aqueous solution at 25 deg C and ionic strength = 0.10 mol dm-3.
- Jefferson, Elizabeth A.,Keeffe, James R.,Kresge, A. Jerry
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p. 2041 - 2046
(2007/10/03)
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- The ketonization of acetophenone enol in concentrated aqueous sulfuric and perchloric acid solutions. Implication on X acidity function correlations of the enolization reaction and determination of the keto-enol equilibrium constant as a function of acidity
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Rates of ketonization of the enol of acetophenone, generated by flash photolytic photohydration of phenylacetylene, were measured in aqueous sulfuric and perchloric acid solutions over the concentration range 1-50 wt.percent acid; rates of enolization of acetophenone, monitored by bromine scavenging, were also measured in aqueous perchloric acid solutions over the same concentration range.The results suggest that the curvature observed in a previous X acidity function correlation of the rate of enolization in sulfuric acid solutions was an artifact produced by insufficiently efficient scavenging, and that introduction of the activity of water in the correlating expression, used previously to eliminate the curvature and believed to reflect covalent involvement of water in the enolization reaction, is unnecessary.The present results also show that the keto-enol equilibrium constant for acetophenone decreases with increasing acidity in these concentrated sulfuric and perchloric acid solutions.
- Chiang, Y.,Kresge, J.,O'Ferrall, R. A. More,Murray, B. A.,Schepp, N. P.,Wirz, J.
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p. 1653 - 1656
(2007/10/02)
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- Enol Content of α-Pyridyl- and Pyridinio-acetophenones
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A comparison of the effects of phenyl, pyridyl, and pyridinio substituents upon keto-enol tautomerisation and enol ionisation equilibria of acetophenone shows that polar effects upon bond hybridisation and resonance interaction with 'neutral' double bonds are important influences upon enol stability.
- Carey, A. R. Edwin,Al-Quatami, Shaikha,O'Ferrall, Rory A. More,Murray, Brian A.
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p. 1097 - 1098
(2007/10/02)
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- Scavenging of 1,4-Biradicals by Bis(acetylacetonato)copper(II). The Importance of Paramagnetic Effects
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Cu(acac)2 is shown to interact with the triplet biradical (CH3)2C.CH2CH2C.(OH)C6H5 promoting its intersystem crossing to the product forming singlet species.The scavenging of the biradicals does not lead to any new products, but the product ratios (cyclobutanol and acetophenone) are modified, showing that paramagnetic scavengers can exert some control on the conformation of the nascent singlet biradical.The rate of scavenging in methanol at 27 deg C is 2.0E9 M-1 s-1, as determined by laser flash photolysis.
- Scaiano, J. C.,Lee, Carolyn W. B.,Chow, Yuan L.,Marciniak, Bronislaw
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p. 2452 - 2455
(2007/10/02)
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- Spin-trapping with 2-methyl-2-nitrosopropane: photochemistry of carbonyl-containing compounds. Methyl radical formation from dimethyl sulfoxide
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The photochemical reactions of several carbonyl-containing compounds investigated by spin-trapping with 2-methyl-2-nitrosopropane revealed different modes of scission depending on the structure of the initial compound.Thus, in photo-Fries rearrangements, the acyl radical was detected. 1,3-diphenyl-2-propanone decarbonylated to yield the benzyl radical.Finally, valerophenone yielded the radicals expected by γ-hydrogen abstraction.In a dark reaction, dimethyl sulfoxide reacts with NaOH to generate methyl radicals.The latter result suggests the need for caution in the use of dimethyl sulfoxide with 2-methyl-2-nitrosopropane for the detection of hydroxyl radicals.
- Rosenthal, Ionel,Mossoba, Magdi M.,Riesz, Peter
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p. 1486 - 1492
(2007/10/02)
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- Kinetics and Thermodynamics of Keto-Enol Tautomerism of Simple Carbonyl Compounds: An Approach Based on a Kinetic Study of Halogenation at Low Halogen Concentrations
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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.
- Dubois, Jaques-Emile,El-Alaoui, Mohiedine,Toullec, Jean
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p. 5393 - 5401
(2007/10/02)
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