Keto Enol Tautomerism

7 Key excerpts on "Keto Enol Tautomerism"

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  Organic Chemistry as a Second Language

  Second Semester Topics

  • David R. Klein(Author)
  • 2019(Publication Date)
  • Wiley

    (Publisher)

  But when we just focus on the atoms (which atoms are connected to which other atoms), we find that the difference is in the placement of just one proton. We have a special name to describe the relationship between compounds that differ from each other in the placement of just one proton. We call them tautomers. So, the enol above is said to be the tautomer of the ketone, and similarly, the ketone is the tautomer of the enol. The equilibrium shown above is called keto-enol tautomerism. Keto-enol tautomerism is NOT resonance. The two compounds shown above are NOT two representations of the same compound. They are, in fact, different compounds. These two compounds are in equilibrium with each other. In most cases, the equilibrium greatly favors the ketone: O O H This should make sense, because the last two chapters focused on the formation of C===O bonds as a driving force for reactions. A ketone has a C===O bond, but an enol does not. So we should not be surprised that the equilibrium favors the ketone. There are some situations where the equilibrium can favor the enol. For example: O O H In this case, the enol is an aromatic compound, and it is much more stable than the ketone (which is not aromatic). There are many other situations where an enol can be more stable than its tautomer. You will probably find some of these examples in your textbook (such as 1,3-diketones). But in most cases (other than these few exceptional cases), the equilibrium will favor a ketone over an enol. It is very hard (close to impossible) to prevent the equilibrium from being established. Imagine that you are performing a reaction that generates an enol as the product, and you take great efforts to remove all traces of acid or base. Your hope is that you can prevent the equilibrium from being established, so as to avoid the conversion of the enol into a ketone. But you will find that your efforts will likely be unsuccessful.

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  Organic Chemistry

  • David R. Klein(Author)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  This is the case for most ketones. In some cases, the enol tautomer is stabilized and exhibits a more substantial presence at equilib- rium. Consider, for example, the enol form of a beta-diketone, such as 2,4-pentanedione. 10–30% O O 70–90% O O H The equilibrium concentrations of the diketone and the enol depend on the solvent that is used, but the enol generally dominates. Two factors contribute to the remarkable stability of the enol in this case: (1) The enol has a conjugated π system, which is a stabilizing factor (see Section 16.2), and (2) the enol can form an intramolecular H-bonding interaction between the hydroxyl proton and the nearby carbonyl group (shown with a gray dotted line above). Both of these factors serve to stabilize the enol. Phenol is an extreme example in that the concentration of the ketone is practically negligible. In this case, the ketone lacks aromaticity, while the enol is aromatic and significantly more stable. OH > 99.99% O < 0.01% Tautomerization is catalyzed by trace amounts of either acid or base. The acid-catalyzed process is shown in Mechanism 21.1 (seen previously in Mechanism 9.2). MECHANISM 21.1 ACID-CATALYZED TAUTOMERIZATION The carbonyl group is protonated to form a resonance-stabilized cation The cationic intermediate is deprotonated to give the enol Resonance-stabilized cation Proton transfer Proton transfer H O O H H O H O H H H O H H O ⊕ ⊕ ⊕ In the first step, the carbonyl group is protonated to form a resonance-stabilized cation, which is then deprotonated at the α position to give the enol. Notice that none of the reagents or intermediates are strong bases, which is consistent with acidic conditions. LOOKING BACK This requirement was discussed in Section 20.7.

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  Tautomerism

  Methods and Theories

  • Liudmil Antonov(Author)
  • 2013(Publication Date)
  • Wiley-VCH

    (Publisher)

  ). The reaction is initiated by injecting the vapor into the solvent.

  Figure 1.9

  (a) Enol and (b) diketo form of acetylacetone. In the vapour, the enol form is dominant, and in water the diketo form.

  The third type of experiment is photolysis, where the product is one of a tautomer pair [2, 7, 75]. Again, almost all reactions studied are keto–enol tautomerizations where the proton transfer is not direct but in a number of steps via the solvent. Since the first step is often an ionization (proton transfer to solvent molecule), which is thought to be diffusion-controlled [67], it does give some insight into proton transfer reactions, but exact elucidation is hard, since often there are numerous possibilities for reaction mechanisms and roles of solvent molecules and internal vibrations [76, 77]. In view of the lack of understanding of proton transfer reactions, it would be much better to have a simpler and more direct way to initiate intramolecular proton transfer. This possibility is offered by looking at intramolecular proton transfer reactions in the excited state, which can be initiated much faster and followed on a much shorter timescale than ground-state reactions.

  The vast majority of papers devoted to tautomerization dynamics deal with ESIPT reactions. Since Weller's suggestion that the large Stokes shift he measured for salicylic acid fluorescence was caused by rapid proton transfer in the excited state [62], and the development of techniques to study this on a femtosecond timescale, the field has blossomed. Most of the 2000 papers on tautomerization dynamics is on ESIPT, from both an experimental and a theoretical point of view. The number of compounds exhibiting ESIPT is far too large to discuss here. It ranges from molecules as simple as malonaldehyde to systems as complicated as 3-hydroxyflavone or 2-(2′ -hydroxyphenyl)benzothiazole. In particular, substituted salicylic acids and ortho-hydroxybenzaldehydes have attracted much attention from both experimentalists and theoreticians. Weller's idea is depicted in Figure 1.10

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  Organic Reaction Mechanisms 1988

  An annual survey covering the literature dated December 1987 to November 1988

  • A. C. Knipe, W. E. Watts, A. C. Knipe, W. E. Watts(Authors)
  • 2008(Publication Date)
  • Wiley

    (Publisher)

  The only exception to this generalization is when R2 is rneth~1.l~~ The tautomeric equilibrium of Cphenylazo- 1 -naphthol favours the hydrazone form (37) by adding water to organic solvents and by applying pressure. There is not a good correlation of the equilibrium constants using Kirkwood-type equation^.'^.' This tautomeric equilibrium has also been studied theoretically, and spectroscopically.‘” Tautomerism in the dihydropyrimidine system has been investigated as a function of substituents at the 2- and 5-po~itions.’~~ NHPh I H H 1 P- CH?=C ‘Ph (39) 1 Reactions of Aldehydes and Ketones and their Derivatives 15 Keto-enol tautomerism in B-keto-esters has been studied by 'H NMR in dif- ferent solvents. In ortho-substituted benzoyl derivatives the equilibrium varies in a complex manner. For example, halogen substituents cause an increase in enol content but the degree of enolization is not dependent on resonance effects.'36 The photolysis of o-phthalaldehyde in a nitrogen matrix gives the E-enol(38).'37 Intramolecular proton transfer in o-hydroxybenzaldehyde in the gas phase depends on the electronic state such that it occurs in the 'enol' tautomer (39) but not in the ground or other excited states.'38 The rate of the uncatalysed return to the ground state shows an unusual temperature dependence which is attributed to only the hydrogen-bonded tautomer (39) undergoing proton transfer.'3s*i39 The enol form of acetophenone can be generated from the photo-hydration of phenylacetylene and its rate of ketonization studied as a function of buffers and pH. The Brcansted a-value for the general acid-catalysed ketonization of the enol and enolate anion are 0.50 and 0.32, respectively.

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  Organic Chemistry

  A Mechanistic Approach

  • Penny Chaloner(Author)
  • 2014(Publication Date)
  • CRC Press

    (Publisher)

  The process has also been studied in the gas phase, where enols predominate for almost all β-dicarbonyl compounds. In solution, the keto forms are generally some 8–9 kJ mol −1 more stable than in the gas phase. The spectroscopic data of Figure 17.7 are instructive, but may also be puzzling. The first unusual feature is the very high value of δ for the enolic proton. As a general rule, the value of δ for the protons of OH groups increases with the extent of hydrogen bonding. For example, the chemical shift for the OH of phenol is δ 4.35 in a 1 % solution, δ 5.95 in a 5 % solution, and δ 7.45 in neat phenol; as the concentration increases, so does the extent of hydrogen bond- ing. The chemical shift of the OH proton of a carboxylic acid is in the range δ 10–13—and we know that these are extensively hydrogen bonded, even in dilute solutions. But in these enolized β-diketones, there is something quite special happening, over and above a simple hydrogen bond—the chemical shift of the OH proton in 17.16, which is clearly hydrogen bonded, is δ 3.87. The second puzzling feature of the spectrum of pentane-2,4-dione is that the two possible enolic tautomers give rise to only one signal for a methyl group. If we look at either separately, we would expect to see two signals for the methyl groups—one is attached to the ketone and one to the carbon–carbon double bond. But there is only one—and this is not just a coincidence as the same is true of other comparable β-diketones. We have to con- sider whether the two forms are in a true equilibrium or what we see is more like a resonance hybrid of the two. Any energy barrier to an equilibrium would be likely to be low—shifting a proton from one oxygen to the other is easy. All the evidence, both theoretical and experi- mental, however, is that, like other hydrogen-bonded systems, there are two genuine energy minima, but the barrier to interconversion of the two forms is low.

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  Organic Reaction Mechanisms 1990

  An annual survey covering the literature dated December 1989 to November 1990

  • A. C. Knipe, W. E. Watts, A. C. Knipe, W. E. Watts(Authors)
  • 2008(Publication Date)
  • Wiley

    (Publisher)

  1 Reactions of Aldehydes and Ketones and their Derivatives 13 have been reviewed.'48 An extensive set of keto%nol equilibrium constants have been determined. The enol content of ketones is generally smaller than that of corresponding aldehydes, which is attributed to alkyl group stabilization of the keto form. The rates of the acid-catalysed ketonization reaction are much more sensitive to structural changes than are those for en~lization.'~~ A /3-dialkylamino substituent favours the formation of proximal enolates (39) from ketones, which is attributed to an inductive effect.15' Although thermo- dynamically unstable relative to the keto form, the sterically hindered enols (40) ketonize only very slowly presumably owing to the hindered approach of the acid catalyst to /3-carbon of the double bond.15' There are several discrepancies between calculated and observed stabilities in keto-enol eq~i1ibria.I~~ The theor- etical treatment of molecules containing sulphur is a difficult problem, as has been shown by a re-investigation of keto-enol tautomerism in dithiomalondialde- h ~ d e . ' ~ ~ Semi-empirical computational methods have been used to estimate the pK, values of enols. This includes the observation that the (Z)-enol of phenylacet- aldehyde is less acidic than the (E)-enol.' 54 Enolate dianions can be formed by double deprotonating /3-hydroxy ketones.155 The diasteroselectivity of the 0-methylation of sterically hindered /3-keto carbox- ylates and their enols has been in~estigated.'~~ Hydrolysis and Reactions of Vinyl Ethers and Related Compounds The acid-catalysed hydrolysis of simple enol ethers normally proceeds by rate- limiting protonation of the /3-carbon. In conjugated polyenol ethers the electron density at the /3-carbon decreases with increasing length of the polyene and so that rate of hydrolysis decreases.

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  Organic Reaction Mechanisms 1979 (Including Index 1975-1975)

  An annual survey covering the literature dated December 1978 through November 1979

  • A. C. Knipe, W. E. Watts, A. C. Knipe, W. E. Watts(Authors)
  • 2008(Publication Date)
  • Wiley

    (Publisher)

  The two prochiral hydrogens of the former are exchanged at equal rates. In contrast, transcarboxylase converts fluoropyruvate into only 3-~-fluorooxaloacetate.~~~ A steroidal epoxide has been used as an active-site-directed irreversible inhibitor for A5-3-oxosteroidi ~ o m e r a s e . ~ ~ ~ There have also been investigations on the epimerization of (-))-menthone in methanol-O-d,226 the enol-enethiol tautomerism of thioa~etylacetone,~~' proton exchange of the acetyl group Daunomycin,228 the NMR spectra of enolate hydrogen bonding between the enol of pentane-2,4-dione and fluoride the carboxylation of cy~lohexanone,2~~ the relationship between the I3C chemical shift of the carbonyl group and the dissociation constant of B-diketone~,~~~ the effect of pH on the rate of iodination of the enolization and isomerization of monosaccharides in aqueous solution,2a aldose-ketose interconversion on anion- exchange resins, 235 photoenoli~ation,2~~*~~' the asymmetric alkylation of carbonyl compounds with lithium of sodium tetraalkylaluminates modified by chiral amino alcohols,238 and tautomerism of a~etylacetone,2~~* 240 of hexafluoroacetyl- acetone,241 of phosphorylated /3-dicarbonylcompound~,2~~ and of radical cations.% There has been a review on the alkylation and related reactions of ketones and aldehydes via metal enolates.= Molecular orbital calculations on keto-enol tautomerism have been re- ported.245* 246 Homoenolization General acid catalysis of the cleavage of the anions of 1-phenylcyclopropanols to 1 -phenylpropanones has been investigated. With water as the general acid, the 1 Reactions of Aldehydes and Ketones and their Derivatives 15 p values for cis- and trans-1-phenyl-2-arylcyclopropanols are 5.0 and 4.0, re- spectively. The data are consistent with the general acid-catalysed cleavage of the anion, the transition state having a large amount of carbanionic character. No interconversion between the cis- and trans-isomers was r e p ~ r t e d .

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