Spectrometric and theoretical evidences for the occurrence of tautomeric structures of selected ketones (original) (raw)
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International Journal of Molecular Sciences, 2004
The mass spectra of ketones can provide valuable information with regards to keto-enol equilibria occurring in the gas phase. Mass spectra of selected ketones and thioketones have been analysed and specific fragmentation assignments have been done to characterised and weigh co-existing keto and enol tautomers. Thioketones are of particular interest due to their tendency to shift the tautomeric equilibrium towards the enethiol form. The predictive value of this methodology is not only supported by the influence of these compound's nature and size of the substituent on these equilibria but also by the good correlation found between the selected fragments abundances ratio and semi-empirical calculation (AM1) of the corresponding heats of tautomerization.
Tautomerism of lactones and related compounds. Mass spectrometric data and theoretical calculations
Arkivoc, 2003
The enolization degree of lactones and esters is favoured by the oxygen-sulfur exchange in the respective functional groups. The analysis of the corresponding mass spectra has allowed unambiguous assignment of some fragments to specific tautomers and establishment of an acceptable correlation between ion abundances ratios and AM1 theoretical calculations. This supports mass spectrometry as an adequate tool to evaluate the tautomerism of neutral species.
Tautomeric Equilibria Studies by Mass Spectrometry
2007
Tautomerism in organic chemistry has been extensively studied in condensed phase by spectrometric methods, mainly by IR and NMR techniques. Mass spectrometry studies start 40 years ago but just recently it has been recognized the importance of the mass spectral data for the study of tautomerism in the gas phase. Mass spectrometry can provide valuable information in regard to tautomeric
Tetrahedron, 1988
The enol content and equilibrium free energies for the keto-enol tautomerism have been determined for a series of 1,3-diketones, at 40 OC, in deuterochloroform and dimethylsulfoxide-d (DMSO), by lH n.m.r.. Compounds containing methyl, phenyl, t-butyl, E-thienyl, trifluorome t hyl and ethoxy groups have been examined. The equilibrium free energies in DMSO are characterized by a linear additivity effect, and substituent parameters, representing the variation produced by the substituent, have been calculated. Equilibrium enthalpy and entropy have been obtained in the range of temperature 20-60 OC. A correlation between enolic 15 coupling constants and equilibrium free energies in DMSO is discussed in terms of suig ytuent effects on the energy of the enol. r-The tautomeric equilibrium of simple enols2 as well as of /?-diketones has been the subject of continues interest in chemistry. In the latter system, the position of the keto-enol equilibrium is affected by the presence of substituent groups at the three carbon atoms of the diketone frame (Equation 1).
Spectrochimica Acta Part a Molecular and Biomolecular Spectroscopy, 2011
The study of tautomerics equilibria is really important because the reactivity of each compound with tautomeric capacity can be determined from the proportion of each tautomer. In the present work the tautomeric equilibria in some γ,δ-unsaturated β-hydroxynitriles and γ,δ-unsaturated β-ketonitriles were studied. The first family of compounds presents two possible theoretical tautomers, nitrile and ketenimine, while the second one presents four possible theoretical tautomers, keto-nitrile, enol (E and Z)-nitrile and keto-ketenimine. The equilibrium in gas phase was studied by gas chromatography-mass spectrometry (GC-MS). Tautomerization enthalpies were calculated by this methodology, and results were compared with those obtained by density functional theory (DFT) calculations, observing a good agreement between them. Nitrile tautomers were favored within the first family of compounds, while keto-nitrile tautomers were favored in the second family.
Separation of keto–enol tautomers in β-ketoesters: a gas chromatography–mass spectrometric study
Journal of Molecular Structure, 2001
The gas chromatographic behaviour for some b-ketoesters was studied. Additionally, the feasibility of the gas chromatographic separation of the corresponding tautomer forms was examined. In this work mass spectrometric detection allowed identi®cation of both keto and enol forms and an estimation of their relative amounts for methylacetoacetate, a-chloromethylacetoacetate, ethylacetoacetate and a-chloroethylacetoacetate. This ®nding demonstrates slow tautomerisation kinetics permitting the chemical identity maintenance of the tautomers. q
Mass spectrometry of β-ketoesters: evidences of their tautomerism
2007
Laboratorio de Estudio de Compuestos Organicos (LADECOR), Division Quimica Organica, Departamento de Quimica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, B1900 La Plata, Provincia de Buenos Aires, Argentina. INIFTA (UNLP-CONICET-CIC), Departamento de Quimica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Diagonal 113 y 64, Suc. 4, C. C. 16, B1900 La Plata, Argentina.
Journal of the American Chemical Society, 1973
An nmr method is applied to a study of prototropic shifts in a highly enolized system. The temperaturedependent nmr spectrum of 3-hydroxy-2,4-dimethylcyclobutenone (1) has been examined as a function of pH, and the nmr rates obtained were analyzed according to the general acid-base catalysis scheme. In the low pH limit investigated (pH NO), the nmr rates are insensitive to the concentration of 1. At higher pH, the rates become concentration dependent. Rates were found dependent on the water concentration. The pH-rate profile is explained on the basis of two competing reactions, both involving the enolate ion of 1 and proceeding by way of 2,4-dimethyl-1,3-cyclobutanedione. Activation parameters, E, of 13.6 and 13.2 kcal/mol and In A of 28.9 and 24.7 sec-', were calculated at pH 1.6 and 2.8, respectively, and a solvent deuterium isotope effect, kH20/kD20, of 5.0 1.0 and 3.0 f 1.0 was observed. nolization, and subsequently ketonization, comE prises the most familiar form of prototropic tautomerism. Numerous techniques that include halogen titrations, acidimetric methods, and spectrophotometric methods have been widely used to study equilibria in tautomeric systems. Recently, nuclear magnetic resonance spectroscopy including I7O studies and double resonance, relaxation method^,^ and other studies6 have also been applied to investigate this phenomenon. Often, however, unfavorable equilibrium constants limit the effectiveness of these techniques. A considerable number of cyclic systems, ranging from the aromatic tropolones' and phenols8 to 1,2and 1,3-diketones in small and medium size rings, favor one tautomer. Equilibrium constants and rates of protopropic shifts in these systems are not in general attainable by these methods. In fact, the kinetics of prototropic shifts have rarely been determined directly;6 instead rates of racemization, isotope exchange, and other indirect methods have commonly been employed. Recently, we reported a study of enol-keto tautom