Enol Equilibrium Research Articles

New data on keto–enol equilibria of 3-alkylpentane-2,4-diones in aqueous solution

For β-diketones MeCOCHRCOMe, there is a linear free energy relationship between the rate constant for ketone deprotonation and the tautomeric equilibrium constant which decreases as R increases. This relationship is valid only for R different from H and its slope is unity (1.00 ± 0.07). Our main conclusion is that the effect of the alkyl groups, with R different from H, on the keto–enol equilibrium position is mainly due to a preponderant structural effect on the keto-form and not on enol. However, when any alkylated compound is compered with pentane-2,4 dione (R = H) two distinct structural effects, one relating to the enol form and the other to the keto-form, are observed.

Photoinduced ketonization of some cyclic β-ketoesters in solution

The influence of u.v. irradiation on the keto:enol ratio of 2-ethoxy- carbonylcyclopentanone, 2-ethoxycarbonylcyclohexanone and 2-ethoxy- carbonylcycloheptanone in hexane solution was examined. It was shown that the keto—enol equilibrium is displaced to the keto form. In all cases a good linear relation between the time of irradiation and the concentration of the enol form was established. The rate constants of the photoinduced ketonization have been calculated.

Preparation and properties of β-oxo-thionesters

O-Ethyl S-phenacyl dithiocarbonate (I) and its S-acetonyl analogue (III) are converted respectively into O-ethyl-(benzoyl)thioacetate (II) and acetothioacetate (IV) by sulphide contraction. Keto–enol equilibria and alkylation of the anions of these β-oxo-thionesters have been investigated briefly.

Hindered rotation about the amido NC bond and the tautomeric equilibrium in N,N‐dimethylacetoacetamide

AbstractThe molecule N,N‐dimethylacetoacetamide, which is subject to a keto–enol equilibrium process in solution, also exhibits hindered rotation about the amido NC bond. The hindered rotation rates have been studied by lineshape fit methods of the nuclear magnetic resonance spectra. In spite of some overlap of the keto and enol N‐methyl proton signals, the simultaneous measurement of the two distinct energy barriers in the two forms is possible as well as a determination of the keto–enol equilibrium. The differences in free energy of activation between keto and enol forms for the rotation barrier can be related to the conjugation energy of the NC π system with the enolic hydrogen bonded ring. Appeal to the model compound acetylacetone reveals a consistent set of energies for the keto and enol forms in the ground and transition states for internal rotation. The opportunity has been taken to reexamine and compare the keto–enol system ethylacetoacetate. Long range, solvent, concentration and temperature sensitive scalar couplings 4J(HH) between the enolic –OH and the adjacent methyl group in acetoacetic ester have not hitherto been reported.

Cinetique complexe de l'halogenation de cetones en milieu acide—I : Iodation des enols

The kinetics of the iodination of acetone, diethylketone and di-isopropylketone in aqueous media ([H 2SO 4] = 0·1 to 1·0 N; [I 2] a o = 10 −7 to 10 −5M) have been studied by couloamperometry under irreversible conditions. At these concentrations the rates of formation of the enol and of its iodination are similar. The general equation, which assumes the steady state approximation for the enol, is applicable, and is used to separate the rate constants of enolisation (k 1) and the apparent enol iodination rate constant (k II I 2 = K Ek 2 I 2 ). For acetone, the value given by Schwarzenbach for the enol equilibrium constant (K E = 2·5 x 10 −6) leads to an elementary rate constant for the addition of iodine to the enol (k 2 I 2 = 6·5 x 10 6 M −1s −1). This value is not, however, consistent with k I 2 = 1·5 x 10 8 M −1s −1, the rate constant for the iodination of the corresponding ether 2-ethoxypropene.

Ab initio Molecular orbital study of the effects of α-substitution on keto–enol tautomerism

The effects of α-substitution on the keto–enol equilibrium in acetaldehyde are investigated by ab initio molecular orbital theory; for all substituents considered, the enol structure is stabilized preferentially to the keto.

An n.m.r. study of keto–enol tautomerism in β-diketones

Thermodynamic functions for the enolisation of a series of β-diketones have been obtained from measurements of equilibrium constants and their temperature dependence. The values of –ΔH range from 1·3 to 4·6 kcal. mole–1 and of –ΔS from 3·4 to 9·5 cal. mole–1 deg.–1, and the results are discussed in terms of steric and electronic effects. The effect of solvent on the keto–enol equilibrium has been studied for pentane-2,4-dione and the results in polar solvents are explicable in terms of preferential solvation of the keto tautomer.

A STUDY OF TAUTOMERISM IN CYCLIC β-DIKETONES BY PROTON MAGNETIC RESONANCE

The keto–enol equilibrium of cyclohexane-1,3-dione in chloroform is best interpreted from proton resonance measurements as[Formula: see text]K1 and K2 may be separately determined from chemical shift measurements of the enol-OH proton and intensity measurements of peaks assigned to keto and enol forms. K1 and K2 are satisfactorily independent of concentrations except in very dilute solutions where intensity measurements become unreliable. The overall equilibrium constant K = K1 × K22 can be obtained for the same molecule in acetonitrile solutions where the enol monomer form is in very low concentration. 5,5′-Dimethylcyclohexane-1,3-dione in chloroform has less enol form than the unsubstituted molecule. The enthalpy change associated with 'K' for cyclohexane-1,3-dione in chloroform is 2.05 ± 0.5 kcal mole−1.

SPECTROSCOPIC STUDIES OF KETO–ENOL EQUILIBRIA: PART 2. ANOMALOUS ULTRAVIOLET ABSORPTION SPECTRA OF SATURATED 1,2-DICYANO ESTERS

1,2-Dicyano esters show an absorption band around 245 mμ in alcoholic solutions which is not found in non-polar and other polar solvents. This anomalous behavior has been found to be due to the unusual solvent dependence of the equilibrium between the keto and enol forms. The intensity of the absorption band in alcoholic solutions decreases with increase in concentration of the 1,2-dicyano ester, indicating association of the enol form. The keto–enol equilibrium is also found to be sensitive to the substituent R of the alcohol ROH.

Keto-cis-trans-enol-equilibrium of 3-alkylacetylacetones

Abstract The effect of solvents on the position of keto- cis - trans -enolic equilibrium of a series of 3-alkylacetylacetones has been studied. The bromometric and IR evidence for 1% solutions gives a sound qualitative picture but owing to insufficient dilution does not elucidate pertinent quantitative regularities. The latter was achieved by studying UV spectra with dilution down to 10 −4 m/l. As a measure of concentration of tautomeric forms integral densities of absorption bands assigned to cis - and trans -enolic forms were used. The data obtained led to the conclusion that cis -enolization of 3-alkylacetylacetones follows Meyer's rule whereas trans -enolization is independent of the nature of the solvent. The overall enolization is expressed as K T = EL + E 1 . The specificity of β-dicarbonyl compounds as a particular chemical type in terms of the Bronsted-Izmailov theory is discussed.

NUCLEAR MAGNETIC RESONANCE MEASUREMENTS OF PROTON TRANSFERS IN ACETYLACETONE IN DIETHYLAMINE AND TRIETHYLAMINE

Solutions of acetylacetone in triethylamine are completely enolic at all dilutions studied (0.1 to 0.85 mole fraction). The nature of the proton resonance signals in the NMR spectrum indicates a proton exchange between the OH and ==CH— groups of the enol tautomer. Addition of diethylamine to acetylacetone results in the formation of a white crystalline 1:1 molecular complex which melts at 41.5 °C. The NMR spectrum of the complex reveals a simultaneous proton exchange between the amine NH group and the OH and ==CH— groups of the enol tautomer. The mechanism of this exchange is discussed. In effect the addition of amines to acetylacetone not only alters the keto–enol equilibrium very markedly, but also increases the keto–enol conversion rate. With diethylamine the conversion rate becomes sufficiently high to "average out" the individual NMR spectra of the two tautomers.

Some Keto--Enol Equilibria in the Gas Phase

Some Keto--Enol Equilibria in the Gas Phase

THE KETO—ENOL EQUILIBRIUM OF ETHYL ALPHA-PHENYL ACETOACETATE

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTTHE KETO—ENOL EQUILIBRIUM OF ETHYL ALPHA-PHENYL ACETOACETATEHoward W. Post and Gladys A. MichalekCite this: J. Am. Chem. Soc. 1930, 52, 11, 4358–4362Publication Date (Print):November 1, 1930Publication History Published online1 May 2002Published inissue 1 November 1930https://doi.org/10.1021/ja01374a024RIGHTS & PERMISSIONSArticle Views145Altmetric-Citations9LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (332 KB) Get e-Alerts Get e-Alerts