Enol content

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. 

The amount of enol present at equilibrium the enol content is quite small for sim pie aldehydes and ketones The equilibrium constants for enolization as shown by the following examples are much less than 1... 

Enolization (Sections 18 4 through 18 6) Aldehydes and ke tones having at least one a hydro gen exist in equilibrium with their enol forms The rate at which equilibrium is achieved is in creased by acidic or basic cata lysts The enol content of simple aldehydes and ketones is quite small p diketones however are extensively enolized... 

The enol content of a carboxylic acid is far less than that of an aldehyde or ketone and introduction of a halogen substituent at the a carbon atom requires a different set... 

The chemical properties of cycHc ketones also vary with ring size. Lower members (addition reactions, than corresponding acycHc ketones. The Cg—C 2 ketones are unreactive, reflecting the strain and high enol content of medium-sized ring systems. Lactones are prepared from cycHc ketones by the Bayer-ViUiger oxidation reaction with peracids. S-Caprolactone is manufactured from cyclohexane by this process ... 

In each of the following pairs of compounds, choose the one that has the greater enol content, and write the structure of its enol form ... 

Unlike most /3-diketones, the following /3-diketone has no detectable enol content and is about as acidic as acetone. Explain. 

The liquid ester is an equilibrium mixture, the enol content of which is increased by distillation and falls slowly, on standing, to 30 per cent.1 The boiling point of i45-i47°/n mm., recorded in the literature, is higher than any noted in the present work. 

Molecules in which the enolic double bond is in conjugation with another double bond. Some of these are shown in Table 2.1. As the table shows, carboxylic esters have a much smaller enolic content than ketones. In molecules... 

TABLE 2.1 The Enol Content of Some Carbonyl Compounds... 

Molecules that contain two or three bulky aryl groups. An example is 2,2-dimesitylethenol (112). In this case, the keto content at equilibrium is only In cases such as this steric hindrance (p. 189) destabilizes the keto form. In 112 the two aryl groups are 120° apart, but in 113 they must move closer together (w 109.5°). Such compounds are often called Fuson-type enols ° There is one example of an amide with a bulky aryl group, A-methyl l w(2,4,6-triisopropylphenyl)acetamide, that has a measurable enol content, in sharp contrast to most amides. 

Frequently, when the enol content is high, both forms can be isolated. The pure keto form of acetoacetic ester melts at — 39°C, while the enol is a liquid even at — 78°C. Each can be kept at room temperature for days if catalysts such as acids or bases are rigorously excluded.Even the simplest enol, vinyl alcohol (CH2= CHOH), has been prepared in the gas phase at room temperature, where it has a half-life of 30min. " The enol Me2C=CCHOH is indefinitely stable in the solid state at —78°C and has a half-life of 24h in the liquid state at 25°C. When both forms cannot be isolated, the extent of enolization is often measured by NMR. 

The enol content of the product at this point is less than 2%. If the unenolized enedione is desired, the following distillation should be omitted and the product used without purification to avoid further isomerization. 

The product is contaminated by 5-15% of 2-acetylcyclo-hexanone, which was present in the crude selenide. This imputity may be avoided by purifying the selenide as described in Note 5. The enol content of the product obtained by the submitters varied... 

Table II indicates those compounds which, on reaction with D-glucose, have not yielded any crystalline product. A large enol content in the ketonic...

K. H. Meyer s bromine method 1 makes it possible to determine the enol content in almost all solutions of tautomeric substances. 

The method detailed here uses cheap starting materials and the sequence proceeds in high-yielding steps. Finally, it is noted that the title compound can exhibit high enol content depending on solvent.2... 

The earliest estimates of the enol content of various p-diketones was made by careful distillation (Meyer and Schoeller, 1920 Meyer and Hopflf, 1921). Although, as a method of studying the equilibrium, this approach has been replaced by non-intrusive methods, work on the physical separation of the tautomers has continued (e.g. Regitzand Schafer, 1981 Vogt and Gompper, 1981). 

Traditionally, the position of equilibrium of (15) has been expressed as the percentage of enol, and for neat pentane-2,4-dione this is 79%. The equilibrium constant is especially sensitive to solvents for pentane-2,4-dione the enol content varies from 13% in water to 98% in cyclohexane, corresponding to a ca. 300-fold change in the value of the equilibrium constant. 

Rate and equilibrium constant measurements for the enolization of 3-phenylcoumaran-2-one (82) in aqueous dioxane indicate an enol content of ca 0.1%, a pKg, of 8.9 (6.0 for the enol tautomer), and a fairly symmetrical transition state for enolate anion formation the Brpnsted Pb = 0.52 Below pH 5, enolization is independent of pH, occurring via O-protonation of the enolate. 

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