Keto-enol Equilibrium Constants Research Articles

Size controlled synthesis of gold nanostructures using ketones and their catalytic activity towards reduction of p-nitrophenol

This work reports a rapid, facile, surfactant free and size controlled microwave assisted synthesis of colloidal gold nanostructures in aqueous medium. The growth of gold nanostructures was taking place using auric chloride with different ketones. The various ketones such as cyclohexanone, 2,4-pentanedione, 2,5-hexanedione, 1,4-cyclohexanedione, 1,3-cyclopentanedione, cyclopentanone, 2,3-butanedione and 2-butanone were used for the synthesis of gold nanostructures. Ketone plays a crucial role in synthesis and it act as a reducing, stabilizing as well as structure directing agent. The nanoparticle size and their rates of formation were found to be dependent on the enol content (keto-enol equilibrium constant) and water solubility of the ketones. The size of the gold nanoparticles was tunable in the range of 20–80nm and 200–500nm for the nanoplates with the specific selection of ketone. The phase structure, elemental composition, morphology and surface plasmon resonance (SPR) properties of the synthesized gold nanostructures were studied using XRD, EDS, FEG-SEM and UV–Visible spectroscopy respectively. Furthermore, the catalytic activity and recyclability of gold nanoparticles for the reduction of p-nitrophenol to p-aminophenol in the presence of sodium borohydride was evaluated using UV–Visible spectroscopy at room temperature. Interestingly, the catalytic activity was found to be depended on the particle size of gold nanoparticles.

Keto-Enol Tautomeric Equilibrium of Acetylacetone Solution Confined in Extended Nanospaces.

We aim to clarify the effects of size confinement, solvent, and deuterium substitution on keto-enol tautomerization of acetylacetone (AcAc) in solutions confined in 10-100 nm spaces (i.e., extended nanospaces) using (1)H NMR spectroscopy. The keto-enol equilibrium constants of AcAc (K(EQ) = [keto]/[enol]) in various solvents confined in extended nanospaces of 200-3000 nm were examined using the area ratios of -CH3 peaks in keto to enol forms. The results showed that the keto form of AcAc in hydrogen-bonded solvents such as water and ethanol increased drastically with decreasing space sizes below about 500 nm, but the size confinement did not induce equilibrium shifts in aprotic solvents such as DMSO. The magnitudes of K(EQ) enhancement were well correlated with solvent proton donicity. It followed from the determination of thermodynamic parameters that the stabilization of intermolecular interactions between protons in water and carbonyl oxygen (C═O) in the keto form of AcAc were promoted by size-confinement, and that the keto form could be energetically and structurally favored in extended nanospaces vis-à-vis the bulk space. Furthermore, the measurements of deuterium dependence of the K(EQ) values verified that the nanoconfinement-induced shifts of keto-enol tautomerization of AcAc are attributable to high proton mobility via a proton hopping mechanism of the confined water.

Effect of Ring Size on the Tautomerization and Ionization Reaction of Cyclic 2-Nitroalkanones: An Experimental and Theoretical Study

The keto-enol tautomerism of some cyclic 2-nitroalkanones was studied in cyclohexane. Keto-enol equilibrium constants, K(T), at 25 °C were obtained from (1)H NMR spectra. The relative enol content for the investigated ketones as a function of ring size decreases in the order 6 > 7 > 11 > 12 > 15. This trend apparently is different from that observed in water. Density functional theory (DFT) calculations have been performed to rationalize the effects of ring size and of the solvent on tautomerism. The acidity constants, K(a)(KH), for the different keto tautomers were measured spectrophotometrically at 25 °C in buffered aqueous solutions. No simple correlations between K(a)(KH) and ring size was observed, and this is in agreement with a DFT analysis performed on the same compounds.

Synthesis and properties of aromatic 1,3-diketones and bis-(1,3-diketones) obtained from acetophenone and phtalic acids esters

Dibenzoylmethane and six aromatic 1,3-diketones containing a dibenzoylmethane moiety were synthesized from acetophenone and the appropriate ester in crossed-Claisen condensations. The synthesized diketones include derivatives containing carboxyl and ester groups; bis- (1,3-diketones) were also prepared. The absorption of UV radiation of the obtained compounds was investigated in various solvents, and their molar absorption coefficients were calculated. The ratio of tautomers and keto-enol equilibrium constants were calculated using 1 H NMR techniques. Aromatic bis- (1,3-diketones) demonstrated strong hyperchromic effects. The keto-enol equilibrium of the investigated compounds is strongly shifted to the enol form, especially in non-polar solvents.

ChemInform Abstract: Keto-Enol Equilibria in the Pyruvic Acid System: Determination of the Keto-Enol Equilibrium Constants of Pyruvic Acid and Pyruvate Anion and the Acidity Constant of Pyruvate Enol in Aqueous Solution.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Iodomethane oxidative addition and CO migratory insertion in monocarbonylphosphine complexes of the type [Rh((C 6H 5)COCHCO((CH 2) nCH 3))(CO)(PPh 3)]: Steric and electronic effects

The chemical kinetics, studied by UV/Vis, IR and NMR, of the oxidative addition of iodomethane to [Rh((C 6H 5)COCHCOR)(CO)(PPh 3)], with R = (CH 2) n CH 3, n = 1–3, consists of three consecutive reaction steps that involves isomers of two distinctly different classes of Rh III-alkyl and two distinctly different classes of Rh III-acyl species. Kinetic studies on the first oxidative addition step of [Rh((C 6H 5)COCHCOR)(CO)(PPh 3)] + CH 3I to form [Rh((C 6H 5)COCHCOR)(CH 3)(CO)(PPh 3)(I)] revealed a second order oxidative addition rate constant approximately 500–600 times faster than that observed for the Monsanto catalyst [Rh(CO) 2I 2] −. The reaction rate of the first oxidative addition step in chloroform was not influenced by the increasing alkyl chain length of the R group on the β-diketonato ligand: k 1 = 0.0333 ([Rh((C 6H 5)COCHCO(CH 2CH 3))(CO)(PPh 3)]), 0.0437 ([Rh((C 6H 5)COCHCO(CH 2CH 2CH 3))(CO)(PPh 3)]) and 0.0354 dm 3 mol −1 s −1 ([Rh((C 6H 5)COCHCO(CH 2CH 2CH 2CH 3))(CO)(PPh 3)]). The p K a ′ and keto–enol equilibrium constant, K c, of the β-diketones (C 6H 5)COCH 2COR, along with apparent group electronegativities, χ R of the R group of the β-diketones (C 6H 5)COCH 2COR, give a measurement of the electron donating character of the coordinating β-diketonato ligand: (R, p K a ′, K c, χ R) = (CH 3, 8.70, 12.1, 2.34), (CH 2CH 3, 9.33, 8.2, 2.31), (CH 2CH 2CH 3, 9.23, 11.5, 2.41) and (CH 2CH 2CH 2CH 3, 9.33, 11.6, 2.22).

Halogenation of ketones with N-halosuccinimides under solvent-free reaction conditions

Several aryl substituted ketones, cyclic ketones, 1,3-diketones and a β-ketoamide were halogenated with N-halosuccinimides under solvent-free reaction conditions (SFRC) at various temperatures (20–80 °C), whereas less enolized ketones required the presence of an acid catalyst ( p-toluenesulfonic acid, PTSA). Bromination of substituted acetophenones obeys first order kinetics v= k Br[ketone] and the following correlation with the keto–enol equilibrium constant: log k Br=0.3p K E+ C 1, less enolized substrates being more reactive; the moderate positive charge developed in the rate determining step was confirmed by the Hammett correlation ( ρ=−0.5). On the other hand, in cyclic ketones an opposite relation was observed: log k Br=−0.6p K E+ C 2, indicating higher reactivity of substrates with higher enolization constant ( K E). The important role of the nature of the solvent (MeCN, MeOH) in preorganization of the ketone–NBS–PTSA mixture prior to SFRC bromination was found.

The 2-Oxocyclobutanecarboxylic Acid Keto−Enol System in Aqueous Solution: A Remarkable Acid-Strengthening Effect of the Cyclobutane Ring

Flash photolysis of 2-diazocyclopentane-1,3-dione in aqueous solution produced 2-oxocyclobutylideneketene, which underwent hydration to the enol of 2-oxocyclobutanecarboxylic acid; the enol then isomerized to the keto form of this acid. Rates of the ketene and enol reactions were measured in acid, base, and buffer solutions across the acidity range [H+] = 10(-1)-10(-13) M, and analysis of these data, together with rates of enolization of the keto form of 2-oxocyclobutanecarboxylic acid determined by bromine scavenging, gave keto-enol equilibrium constants as well as acidity constants of the keto and enol forms. The keto-enol equilibrum constants proved to be 2 orders of magnitude less than those reported previously for the next higher homolog, 2-oxocyclopentanecarboxylic acid, reflecting the difficulty of inserting a carbon-carbon double bond into a small, strained carbocyclic ring. The acidity constant of the enol group of 2-oxocyclobutanecarboxylate ion, on the other hand, is greater, by 4 orders of magnitude, than that of the corresponding enol in the cyclopentyl system. This remarkable increase in acidity with diminishing ring size is consistent with the enhanced s character of the orbitals used to make the exocyclic bonds of the smaller cyclobutane ring.

Inclusion Complexes of β-Cyclodextrin with Keto/Enol Tautomers of 2-Acetyl-1-tetralone. A Comparative Study

The keto–enol interconversion of 2-acetyl-1-tetralone (ATLO) and of 2-acetyl-cyclohexanone (ACHE) occurs at measurable rates in aqueous acid or neutral medium. This finding allowed us to determine the keto–enol equilibrium constants, KE, by following two distinct methods. Both methodologies afford results in complete agreement. The first one is a test of the Beer-Lambert law under two different experimental conditions that contain the substrate only in the enol form or in a mixture of both tautomers in equilibrium. The second method analyses the UV-absorption spectrum of each substrate under keto–enol equilibrium in aqueous β-cyclodextrin (β-CD) solutions of variable concentration: the presence of β-CD increases the percentage of the enol due to the formation of 1:1 inclusion complexes between this tautomer and β-CD. Rates of keto–enol tautomerization, in neutral and acid medium, and of nitrosation in acid medium under non equilibrium conditions have also been measured. Throughout the study, the presentation of the results is done by comparing the different behaviour observed between ATLO and ACHE. While the enol of ACHE included into the β-CD cavity shows to be unreactive either in tautomerization or in nitrosation, in the case of ATLO it is observed tautomerization through the complexed enol. In addition, with ACHE only the enol tautomer forms inclusion complexes with β-CD, whereas with ATLO the keto tautomer entries also to the β-CD cavity, however the stability constant with the enol is near 3-fold that of the keto isomer. These main differences can be rationalized on the basis of the molecular structure of these diketones.

Application of Organized Microstructures to Study Keto-Enol Equilibrium of β-Dicarbonyl Compounds

Keto-enol tautomerism has been the subject of continuous interest in chemistry. Enols and enolate ions are essential intermediates in many important chemical reactions, and a number of biological transformations also involve enol formation. Enolization, moreover, has long been of interest as an example of tautomeric rearrangement. One of the things that one would like to know about enols is the magnitude of keto (KH) / enol (EH) equilibrium constants (KH EH, KE). This tautomeric equilibrium has been studied for many years and a large number of methods have been applied for investigating the keto-enol conversion. We review some of the work that we have been doing in the field of keto-enol / enolate equilibria characterization and on enol reactivity in constrained media, such as aqueous solutions of surfactants forming micelles or cyclodextrins. A recently developed method of studying keto-enol equilibrium that combines the UV-vis spectroscopy and aqueous solutions of organized microstructures will be presented. The emphasis of the exposition is the comparative analysis of the several methods used in continuing research to evaluate keto-enol equilibrium constants, by paying detailed attention to the difficulties, and advantages / disadvantages of this new method in comparison with previous others. Therefore, after a brief description of common properties of 1,3- dicarbonyl compounds and of micro-organized media, we report studies of keto-enol / enolate equilibria and the methods and techniques used.

Keto–enol/enolate equilibria in the 2-acetylcyclopentanone system. An unusual reaction mechanism in enol nitrosation

The keto–enol equilibrium of 2-acetylcyclopentanone (ACPE) was studied in water by analysing the effect that aqueous micellar solutions produce in the UV absorption spectrum of this compound. Aqueous solutions of anionic, cationic, or nonionic surfactants forming micelles have been used. The quantitative treatment of absorbance changes measured at the maximum absorption wavelength as a function of surfactant concentration gave the keto–enol equilibrium constant, KE. In the same sense, the analysis of spectral changes measured as a function of pH in aqueous basic media allowed us to determine the acidity equilibrium constant, Ka. The combination of both equilibrium constants gives the acidity constant of the enol ionizing as an oxygen acid, pKEa=7.72 and the acidity constant of the ketone ionizing as a carbon acid, pKKa=8.12. The kinetic study of the nitrosation reaction of ACPE has been realized in aqueous strong acid media under several experimental conditions. As expected, the reaction is first-order with respect to ACPE concentration, but in sharp contrast to other β-dicarbonyl compounds, the dependence of both [H+] or [X−] (X−=Cl−, Br−, or SCN−) is not simple first-order; instead a fractional order which varied from 1 to 0 was observed. The kinetic interpretation of these experimental facts has been done on the basis of a reaction mechanism that considers the formation of an intermediate in steady-state, which has been postulated as a chelate-nitrosyl complex. The quantitative treatment of the experimental data gave the values of every rate constant appearing in the proposed reaction mechanism.

Electronic effects on enol acidity and keto-enol equilibrium constants for ring-substituted 2-tetralones

Equilibrium constants for the ionization of a variety of phenyl-substituted 2-tetralones (pKaK), for the ionization of their enols (pKaE), and for keto-enol tautomerization (pKE) were determined. Hammett plots of pKaK and pKaE vs. σ- are linear with slopes (-ρ) of -1.66 ± 0.06 and -0.90 ± 0.03, respectively, except for deviations of the points corresponding to 6-nitro-2-tetralone (1b) and its enol. We have previously attributed the negative deviation of 1b from the correlation for the acidities of the ketones obtained with the more limited set of data to the lack of a free electron pair on C-1 of the free tetralone (Nevy et al.). The negative deviation of the point for 1b from the correlation for the acidities of the enols suggests that charge transfer from the hydroxyl group of the enol to the nitro group is less important than it is for phenols. This study represents the first systematic study of electronic effects on equilibria among ketone, enol, and enolate in aqueous solution. Key words: enol, acidity, equilibrium, substituent, conjugation.

Electronic effects on enol acidity and keto-enol equilibrium constants for ring-substituted 2-tetralones

Equilibrium constants for the ionization of a variety of phenyl-substituted 2-tetralones (pKaK), for the ionization of their enols (pKaE), and for keto-enol tautomerization (pKE) were determined. Hammett plots of pKaK and pKaE vs. σ- are linear with slopes (-ρ) of -1.66 ± 0.06 and -0.90 ± 0.03, respectively, except for deviations of the points corresponding to 6-nitro-2-tetralone (1b) and its enol. We have previously attributed the negative deviation of 1b from the correlation for the acidities of the ketones obtained with the more limited set of data to the lack of a free electron pair on C-1 of the free tetralone (Nevy et al.). The negative deviation of the point for 1b from the correlation for the acidities of the enols suggests that charge transfer from the hydroxyl group of the enol to the nitro group is less important than it is for phenols. This study represents the first systematic study of electronic effects on equilibria among ketone, enol, and enolate in aqueous solution. Key words: enol, acidity, equilibrium, substituent, conjugation.

Ketonization of (Z)-2-methoxy-1,2-diphenylvinyl alcohol. Quantitative evaluation of the remarkable kinetic stability of this enol

Rates of ketonization of (Z)-2-methoxy-1,2-diphenylvinyl alcohol were measured in aqueous perchloric acid and sodium hydroxide solutions as well as in formic acid and ammonium ion buffers, and the results were used to construct a rate profile for this reaction. These data show this substance to be a remarkably stable enol with a lifetime of 3.6 h at the bottom of its rate profile and a hydrogen ion catalytic coefficient 660 000 times less than that for the enol of acetophenone. Comparison with simple models allows partition of this rate factor into a 440-fold retardation for the β-phenyl substituent and a 1500-fold retardation for the β-methoxy substituent. A global rate of enolization of the keto isomer producing both Z and E enol isomers was also measured, and this leads to pKE > 5.4 as the lower limit of the keto-enol equilibrium constant for the Z enol. This result could be dissected into an enol-content enhancing effect of δ pKE = 3.2 for the β-phenyl group and a surprising enol-content diminishing effect of δ pKE > 0.6 for the β-methoxy group. Key words: enol ketonization, enol stability, keto-enol equilibrium, β-phenyl and β-methoxy substituent effects in keto-enol systems.

Determination of keto–enol equilibrium constants and the kinetic study of the nitrosation reaction of β-dicarbonyl compounds

E. Iglesias, J. Chem. Soc., Perkin Trans. 2, 1997, 431 DOI: 10.1039/A607039F

Isoxazoles. 10. 10. Degradation and Enolization Kinetics of 4-Aminoisoxazolyl-l,2-naphthoquinone in Basic Aqueous Solution

The kinetics of enolization and degradation ofN‐(5‐methyl‐4‐isoxazolyl)‐4‐amino‐1,2‐naphthoquinone (1) was investigated in aqueous solutions over a pH range of 7.30 to 12.25, at 35 °C and at constant ionic strength (μ = 0.5) using reversed‐phase HPLC. Pseudo‐first‐order kinetics was observed throughout the pH range studied. The rate of enolization (ke), the keto–enol equilibrium constant (kt), and specific base catalysis rate constant (kOH) were determined. Good agreement between the theoretical pH–rate profile and the experimental data supports the proposed transformation process. The average recovery for1and its tautomerization product 2‐hydroxy‐N‐(5‐methyl‐4‐isoxazolyl)‐1,4‐naphthoquinone 4‐imine (2) from mixtures of different composition was evaluated.

Characterization of the indan-1-one keto–enol system

3-Hydroxyindene was generated by Norrish type II photoelimination of 2-Methoxyindan-1-one, and its subsequent rate of ketonization was measured in dilute aqueous perhloric acid, sodium hydroxide, acetic acid buffer and ammonia beffer solutions. These data, when combined with acid-catalysed rates of enolization of indan-1-one determined by 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 ionizing as an oxygen acid, and pKaK= 16.96 for the acidity constant of the ketone ionzing 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 present equilibrium constantss refer to wholly aqueous solution at 25 °C and ionic strength = 0.10 mol dm–3.

Kinetics and Mechanism of the Isomerization of 1H-Indene-1-carboxylic Acid to 1H-Indene-3-carboxylic Acid in Aqueous Solution and Determination of Their Keto-Enol Equilibrium Constants and Acid Dissociation Constants of the Keto and Enol Forms. Implication for the Photolysis of Diazonaphthoquinones

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTKinetics and Mechanism of the Isomerization of 1H-Indene-1-carboxylic Acid to 1H-Indene-3-carboxylic Acid in Aqueous Solution and Determination of Their Keto-Enol Equilibrium Constants and Acid Dissociation Constants of the Keto and Enol Forms. Implication for the Photolysis of DiazonaphthoquinonesJ. Andraos, A. J. Kresge, and V. V. PopikCite this: J. Am. Chem. Soc. 1994, 116, 3, 961–967Publication Date (Print):February 1, 1994Publication History Published online1 May 2002Published inissue 1 February 1994https://doi.org/10.1021/ja00082a017RIGHTS & PERMISSIONSArticle Views251Altmetric-Citations21LEARN 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 (834 KB) Get e-AlertsSupporting Info (2)»Supporting Information Supporting Information Get e-Alerts

The .alpha.-cyano-.alpha.-phenylacetic acid keto-enol system. Flash photolytic generation of the enol in aqueous solution and determination of the keto-enol equilibrium constants and acid dissociation constants interrelating all keto and enol forms in that medium

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTThe .alpha.-cyano-.alpha.-phenylacetic acid keto-enol system. Flash photolytic generation of the enol in aqueous solution and determination of the keto-enol equilibrium constants and acid dissociation constants interrelating all keto and enol forms in that mediumJ. Andraos, Y. Chiang, A. J. Kresge, I. G. Pojarlieff, N. P. Schepp, and J. WirzCite this: J. Am. Chem. Soc. 1994, 116, 1, 73–81Publication Date (Print):January 1, 1994Publication History Published online1 May 2002Published inissue 1 January 1994https://doi.org/10.1021/ja00080a009RIGHTS & PERMISSIONSArticle Views433Altmetric-Citations24LEARN 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 (1 MB) Get e-AlertsSupporting Info (2)»Supporting Information Supporting Information Get e-Alerts

A flash photolytic investigation of the photosolvolysis of α-(2,6-dimethoxyphenyl)vinyl chloride. Characterization of the 2,6-dimethoxyacetophenone keto–enol system

Photosolvolysis of α-(2,6-dimethoxyphenyl)vinyl chloride in trifluoroethanol solution at 25 °C was found to produce the α-(2,6-dimethoxyphenyl)vinyl cation as a transient species with the lifetime τ = 1 μs. Addition of water reduced this lifetime markedly, until, at compositions greater than 80% water, it was too short to measure (τ < 20 ns); extrapolation of the data suggests τ = 9 ns in pure water. Hydration of this cation gave the much longer lived enol of 2,6-dimethoxyacetophenone, whose rates of ketonization were measured in dilute perchloric acid and sodium hydroxide solutions. These data, when combined with rates of enolization of the ketone determined by iodine scavenging, give pKE = 6.98 for the keto–enol equilibrium constant in the 2,6-dimethoxyacetophenone system, [Formula: see text] for the acidity constant of the enol ionizing as an oxygen acid, and [Formula: see text] for the acidity constant of the ketone ionizing as a carbon acid; these constants refer to wholly aqueous solution at 25 °C and ionic strength = 0.10 M.