Keto-enol forms

Fig. 6 The 2-(2-hydroxyphenyl)benzothiazole (HBT) unit that represents the keto-enol equilibrium (tautomerism). Normally, the enol (keto) form is rather stable in the ground state (in the excited state), respectively...

Qq is calculated in the same way as the g-value but relates to the single and double reference CO and CC bonds, theoretically the bonds not involved in the conjugated system of double and single bonds. Qo amounts to 0.320 according to the early reference bonds proposed by Gilli and coworkers [9]. The A-parameter is a descriptor which, similar to the g-parameter, indicates the degree of the ir-electron delocalization. If it is equal to 0, then it corresponds to the enol-keto form A = 1 stands for the keto nol form and A = 1 /2 corresponds to the full equalization of the bond lengths mentioned above. The latter is also applied to the transition state of the C D H- -0=C C=0- -H-O-C proton transfer reaction (Scheme 2). 

Figure 2. The dependence between the A-parameter and the O- O distance (in A). The enol-keto form corresponds to the A-values close to unity and the keto-enol form to that value close to zero. The full equalization of CC and CO bonds is for A-values equal to 0.5. Full circles designate the systems with intramolecular RAHBs while empty circles the intermolecular RAHBs. The g-par-ameter is also indicated. (Reprinted with permission from Ref. [5]. Copyright 1989 American Chemical Society.)...

For malondialdehyde (selected structures 27,28 in Figure 6.8) only experimental studies have been found regarding in-solution equilibria. The structure of the enol-keto form, H0-CH=CH-CH=0 (28), comprises a conjugated doublebond system subject to s-cisis-tmns conformational isomerism about the CH-CH bond [14]. A cis-trans isomerism comes into existence regarding the CH=CH double bond. Considering also the rotational flexibihty of the alcohol hydrogen in the OH group, a number of structurally different species could be present in the equUibrium composition. 

Substitution Derivatives of Ethyl Malonate, Ethyl malonate resembles ethyl acetoacetate in that it gives rise to mono- and di-substituted derivatives in precisely similar circumstances. Thus when ethanolic solutions of ethyl malonate and of sodium ethoxide are mixed, the sodium derivative (A) of the enol form is produced in solution. On boiling this solution with an alkyl halide, e.g, methyl iodide, the methyl derivative (B) of the keto form is obtained. When this is treated again in ethanolic solution with sodium ethoxide, the... 

Acetoacetic ester is the classical example of a tautomeric substance, which at room temperature exists as an equilibrium mixture of the kelo and enol forms containing approximately 93 per cent, of the keto form ... 

The enol form is thus temporarily removed from the solution and the ferric chloride colouration produced by the enol form consequently disappears and the solution becomes colourless. Some of the unchanged keto form of the ester then passes into the enol form in order to restore the original equilibrium and the ferric chloride colouration therefore reappears. 

Reduction of anthraquinone with tin and concentrated hydrochloric acid in the presence of boiling glacial eicetic acid gives anthrone this substance (keto form) under certain conditions passes into the enol form, anthranol ... 

Space does not permit any further detailed discussion except for a brief account of two interesting subjects. The first is concerned with keto-enol tautomerism. The classical example is ethyl acetoacetate, which can exist in the keto form (I) and the enol form (II) ... 

The former exhibits absorption tjrpical of an isolated keto group, whereas the latter shows a high intensity -band associated with the conjugated system HO—C=C—C=0. The proportions of the two forms under various conditions are readily determined from the ultraviolet spectra. The ultraviolet spectra in various solvents are shown in Fig. A, 7, 2. Since the absorption of the keto form is negligible, the percentage of enol present is 100(em/e ), where e is the observed extinction at 245 mp. and that of the pure enol. It was shown that in alcoholic solution is 1900 and the percentage of enol is 12. Thus e is ca. 16000, and use of this value permits the approximate evaluation of the enol content in different solvents. The results are collected in Table XII. 

Polar solvents shift the keto enol equilibrium toward the enol form (174b). Thus the NMR spectrum in DMSO of 2-phenyl-A-2-thiazoline-4-one is composed of three main signals +10.7 ppm (enolic proton). 7.7 ppm (aromatic protons), and 6.2 ppm (olefinic proton) associated with the enol form and a small signal associated with less than 10% of the keto form. In acetone, equal amounts of keto and enol forms were found (104). In general, a-methylene protons of keto forms appear at approximately 3.5 to 4.3 ppm as an AB spectra or a singlet (386, 419). A coupling constant, Jab - 15.5 Hz, has been reported for 2-[(S-carboxymethyl)thioimidyl]-A-2-thiazoline-4-one 175 (Scheme 92) (419). This high J b value could be of some help in the discussion on the structure of 178 (p. 423). 

The aldehyde or ketone is called the keto form and the keto enol equilibration referred to as keto-enol isomerism or keto-enol tautomerism Tautomers are constitu tional isomers that equilibrate by migration of an atom or group and their equilibration IS called tautomerism The mechanism of keto-enol isomerism involves the sequence of proton transfers shown m Figure 9 6... 

In these and numerous other simple cases the keto form is more stable than the enol by some 45-60 kJ/mol (11-14 kcal/mol) The chief reason for this difference is that a carbon-oxygen double bond is stronger than a carbon-carbon double bond... 

The enol form is phenol and the stabilization gamed by forming an aromatic ring is more than enough to overcome the normal preference for the keto form... 

The structure of guanine illustrates an important feature of substituted pyrimidines and purines Oxygen substitution on the ring favors the keto form rather than the enol Ammo substitution does not... 

Pyrazolones, also containing two double bonds, are predominantiy ia the keto form (4), although they can also exist ia the enol form (5). 

In 4-pyrazolones, the enol form predominates, although the keto form has also been observed. 

The compounds 2- (16) and 4-pyridone (38) undergo chlorination with phosphoms oxychloride however, 3-pyridinol (39) is not chlotinated similarly. The product from (38) is 4-chloropyridine [626-61-9]. The 2- (16) and 4-oxo (38) isomers behave like the keto form of the keto—enol tautomers, whereas the 3-oxo (39) isomer is largely phenolic-like, and fails to be chlotinated (38). 

With increa sing pressure at 25°C the <7j -keto form is stabilized vis 6 vis both the enol and trans-k.e. o isomers so that both therm ochromic and photochromic materials exhibit the same type of piezochromism, but in different degrees. The conversion increases continuously with pressure, much like the changing of chemical equiUbrium in Hquid solution with pressure. 

As would be expected oiHhe basis of electronegativity arguments, enols are much more acidic than the corresponding keto forms. It has been possible to determine the pAT of the enol form of acetophenone as being 10.5. The ipK of the keto form is 18.4. Because the enolate is the same for both equilibria, the pAT values are related to the enol keto equilibrium. 

Addition of hydride ion from the catalyst gives the adsorbed dianion (15). The reaction is completed and product stereochemistry determined by protonation of these species from the solution prior to or concurrent with desorption. With the heteroannular enolate, (13a), both cis and trans adsorption can occur with nearly equal facility. When an angular methyl group is present trans adsorption (14b) predominates. Protonation of the latter species from the solution gives the cis product. Since the heteroannular enolate is formed by the reaction of A" -3-keto steroids with strong base " this mechanism satisfactorily accounts for the almost exclusive formation of the isomer on hydrogenation of these steroids in basic media. The optimum concentration of hydroxide ion in this reaction is about two to three times that of the substrate. 

Aldehydes and ketones ( keto forms) normally exist in equilibrium with their enol tautomers. 

The UV spectrum of 5-phenyl-3 hydroxythiophene is very similar to that of its methyl ether in alcoholic solution, indicating that it exists largely in the enol form in this solvent. The same coincidence of the wavelength maxima was also obtained for 5-phenyl-2-hydroxy-thiophene and its methyl ether. In chloroform solution, the maxima were shifted toward longer wavelengths, suggesting that the tautomeric equilibrium in this solvent is displaced more toward the keto form. ... 

Hydroxyisothiazoles can theoretically exhibit keto-enol tauto-merism and methyl derivatives of both the keto (34) and enol (35) forms have been synthesized by ring closure of the appropriate... 

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