Phenol keto form

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... 

By analogy to phenols we would expect the isomers with —OH groups on benzene like rings to be more stable This turns out not to be true because the keto forms are also aromatic owing to amide resonance... 

The second major route to diarylamiaes is the condensation of an aromatic amine with a phenol. Aniline [62-53-3] phenol [108-95-2] and 3.5% phosphoric acid at 325°C gives a 50% yield of DPA (23). Apparently, this reaction iavolves the addition of aniline to the keto form of the phenol. Thus, naphthols and hydroquiaone are more reactive and give higher yields of product. This is the preferred route to A/-phenyi-2-naphthyiamiQe, 4-hydroxydiphenyiamiQe, and diphenyl- -phenylenediamine (24). 

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). 

For most simple phenols this equilibrium lies well to the side of the phenol, since only on that side is there aromaticity. For phenol itself, there is no evidence for the existence of the keto form. However, the keto form becomes important and may predominate (1) where certain groups, such as a second OH group or an N=0 group, are present (2) in systems of fused aromatic rings and (3) in heterocyclic systems. In many heterocyclic compounds in the liquid phase or in solution, the keto form is more stable, although in the vapor phase the positions of many of these equilibria are reversed. For example, in the equilibrium between 4-pyridone (118) and 4-hydroxypyridine (119), 118 is the only form detectable in ethanolic solution, while 119 predominates in the vapor phase. " In other heterocycles, the hydroxy-form predominates. 2-Hydroxypyridone (120) and pyridone-2-thiol (122) are in equilibrium with their tautomers, 121 and 123, respectively. In both cases, the most stable form is the hydroxy tautomer, 120 and 122. ... 

Keto forms of phenol and some simple derivatives have been generated as intermediates with very short lives, but long enough for spectra to be taken at 77 K. Lasne, M. Ripoll, J. Denis, J. Tetrahedron Lett., 1980, 21, 463. See also Capponi, M. Gut, I. Wirz, J. Angew. Chem., Int. Ed. Engl., 1986, 25, 344. 

Nor can there be any question of real tautomerism in the case of phenol. In its chemical properties phenol resembles the aliphatic enols in all respects. We need only recall the agreement in the acid character, the production of colour with ferric chloride, and the reactions with halogens, nitrous acid, and aromatic diazo-compounds (coupling), caused by the activity of the double bond and proceeding in the same way in phenols and aliphatic enols. The enol nature of phenol provides valuable support for the conception of the constitution of benzene as expressed in the Kekule-Thiele formula, since it is an expression of the tendency of the ring to maintain the aromatic state of lowest energy. In this connexion the hypothetical keto-form of phenol (A)—not yet obtained—would be of interest in comparison with... 

The optical properties of the 8-o-PhOH-purine adducts have provided insight into their ground-state structures at the nucleoside level. These adducts have the ability to phototautomerize, through an excited-state intramolecular proton transfer (ESIPT) process, to generate the keto form. This tautomerization depends on the presence of a intramolecular hydrogen (H)-bond between the phenolic OH and the imine nitrogen (N-7). Figure 14 shows normalized absorption and emission spectra for 8-o-PhOH-dG and 8-o-PhOH-dA in aqueous buffered water and hexane. In water, 8-o-PhOH-dG shows only enol emission at 395 nm, while 8-o-PhOH-dA shows enol emission at 374 nm and phenolate emission at 447 nm. In hexane, both adducts show keto emission at 475 nm 8-o-PhOH-dA also shows a small amount of enol emission and no phenolate emission. These results show that in water, the intramolecular H-bond... 

Cyclohexadienone (keto form not aromatic) Phenol (enol form aromatic) ... 

Table 8.6 shows that the equilibrium mixture consists of almost entirely keto form in the case of simple aliphatic and aromatic ketones, whereas significant amounts of enol tautomer are present in /J-diketones and /J-ketoesters. In these latter cases, the enol contains a conjugated tt electron system and an intramolecular hydrogen bond (30). Phenol exists entirely in the enol form, as the alter-... 

The direct replacement of the hydroxyl group in simple phenols by an amino or substituted amino group requires drastic conditions and the method is not suitable for laboratory preparations. With the polyhydric phenols, and more particularly with the naphthols, such replacements occur more readily. Thus 2-naphthol is converted into 2-naphthylamine by heating with ammoniacal ammonium sulphite solution at 150°C in an autoclave. The reaction (the Bucherer reaction) depends upon the addition of the hydrogen sulphite ion to the keto form of the naphthol and the subsequent reaction with ammonia. 

Decarbonylations useful for preparative work are also observed with some aromatic hydroxy compounds. It is reasonable to assume a mechanism which involves the keto form 23). Phenol, which shows little tendency to tautomerize, reacts mainly by forming benzene and to a small extent cyclopentadiene 14). For... 

The enol form of phenol benefits from the resonance stabilization and aromaticity of the benzene ring, but the keto form, with the single sp3 center in the ring, cannot. 

Phenols do not exist or react in their tautomeric keto forms. 

Ordinarily we do not write the enol form of acetone or the keto form of phenol, although minuscule amounts do exist at equilibrium. But both forms of acetylacetone are seen in the NMR spectrum because equilibration is slow enough on the NMR scale and the enol form is stabilized by intramolecular hydrogen bonding. The enol form of acetone and the keto form of phenol are not thus stabilized furthermore, the aro-... 

Phenol is deuterated in exactly the same way as any other conjugated enol except that the final product remains as the very stable enol form of phenol rather than the keto form. The enol form of phenol is so stable because of the aromaticity of the benzene ring. The first step is addition of DjO to th enol . 

In thermochromic crystals, on the other hand, the planar form of N-5-chlorosalicylideneaniline [34 X = 5-Cl, Y = H] (Bregman et al., 1964b) is packed tightly with an interplanar distance of 3.4 A (Fig. 17). Consequently, in the planar anils, the proton transfer from the phenolic OH to the imino nitrogen can take place thermally. The two forms, the OH form and the NH one, are thermally equilibrated, and as the temperature increases, the contribution of the cd-keto form becomes larger (Scheme 4). 

The naphthylamines may be prepared by reduction of the corresponding nitro compound, but they are readily accessible from naphthois by the Bucherer reaction The naphthol is heated, preferably under pressure in an autoclave, with ammonia and aqueous sodium hydrogen sulfite solution, when an addition-elimination sequence occurs. The detailed mechanism is not completely elucidated, but the Bucherer reaction is restricted to those phenols that show a tendency to tautomerize to the keto form, such as the naphthois and 1,3-dihydroxybenzene (resorcinol). Using 1-naphthol for illustration, the first step is addition of the hydrosulfite across the 3,4-double bond of either the enol or keto tautomer (Scheme 12.9). Nucleophilic attack by ammonia at the carbonyl group... 

Sodium borohydride is a weak reducing agent and fragmentation (push by N and pull by C=0) is faster. The borohydride reduces the iminium salt to a methyl group and, at the other end of the molecule, the product is the keto form of a phenol. 

Oxidations by dioxygenases cw-hydroxylation of aromatic double bonds. Oxidations catalyzed by oxidases regio- and stereoselective oxidations of polyols oxidations of carbohydrates oxidations of hydroxy steroids oxidations of alkyl phenols to form chiral /7-hydroxybenzylic alcohols hydroxylation of phenols oxidation of amino acids to keto acids. 

Phenol-imine and keto-amine forms do also exist in the solid state. X-ray structural analyses have shown that when the phenol-imine form is transformed into the keto-amine form an appreciable increase in the C=N distance is observed. In the solid state the shortening in the C—O bond distance from 1.279 A to 2.263 A and the lengthening of the C=N distance from 1.317 A to 1.330 A is due to the dominance of the quinoidal structure (keto-amine form).11... 

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