Keto tautomer proportion

The enol rather than the keto tautomer is the intermediate leading to the ketohydroperoxide form (through autoxidation). The catalytic effectiveness of carbonyl compounds would therefore be expected to be directly proportional to their degree of enolization and the reactivity of their enol forms. 

The proportion of enols in simple aliphatic acylphosphonates is too small to observe by NMR spectroscopy. Dialkyl arylacetylphosphonates exist as predominantly enol tau-tomers , which resonate at 13-16 ppm in the NMR spectrum (in one study the keto tautomer was found in the aqueous extract and showed a signal at P -3 ppm) The enol tautomer of diethyl / -anisylacetylphosphonate (29) has been shown by X-ray crystallography to possess the E structure The keto-enol tautomer ratios of a large number of dialkyl aryl- and pyridylacetylphosphonates have been determined by NMR The keto to enol ratio of dimethyl phenylacetylphosphonate was found to depend on the medium. Examination of the P NMR spectrum of this compound in acetone showed a keto to enol ratio of 17 83, whereas in deuteriochloroform a ratio of 7 93 was found. ... 

NaClO the proportion ofthe keto tautomer decreases without fully disappearing. 

Since polar solvents would be expected to stabilize polar forms, a retreat towards the hydroxy tautomer (71) would be predicted in solvents less polar than water, and in the vapour phase. This is borne out in practice at equilibrium both 2- and 4-hydroxypyridine (as well as the 3-hydroxy compound, which even in water exists as an approximate 1 1 mixture of OH and NH forms) exist as such, rather than as the pyridinones. However, the 2- and 4-quinolinones remain in the NH (keto) forms, even in the vapour phase. Hydrocarbon or other solvents of very low polarity would be expected to give results similar to those in the vapour phase, but intermolecular association by hydrogen bonding often leads to a considerably greater proportion of polar tautomers being present than would otherwise have been predicted (77ACR186, 78JOC177). 

Thus, while the 3-hydroxyisoquinoline-3-isoquinolinone equilibrium (80 81) and that of the cinnoline derivatives (82 83) favour the keto forms (81, 83), the proportion of hydroxy tautomers is considerably greater than in the corresponding unfused systems. The benzo-fusion in 2- and 4-quinolinone and in 1-isoquinolinone has the effect of reducing the aromaticity of the heterocyclic ring, and consequently of lowering the proportion of the hydroxy tautomers. 

In contrast to kinetic data, thermodynamic data on enol and enolate formation are far more scarce. This results from the usually very low enol and enolate stabilities which, at equilibrium, make it difficult to measure their proportions relative to the carbonyl compound. This section deals with available data on keto-enol equilibria as well as on acidity constants of the two tautomers. 

When enantiomerically pure (either R or S) 3-phenyl-2-butanone is dissolved in ethanol, no change occurs in the optical activity of the solution over time. If, however, a trace of either acid (e.g., aqueous or gaseous HCl) or base (e.g., sodium ethoxide) is added, the optical activity of the solution begins to decrease gradually and eventually drops to zero. When 3-phenyl-2-butanone is isolated from this solution, it is found to be a racemic mixture. Furthermore, the rate of racemization is proportional to the concentration of acid or base. These observations can be explained by a rate-determining acid- or base-catalyzed formation of an achiral enol intermediate. Tautomer-ism of the achiral enol to the chiral keto form generates the R and S enantiomers with equal probability. 

Solvents and temperature also affect the proportions of enol. Thus pentane-2,4-dione contains 80 % enol as a neat liquid, but in water, it is only 15 % enolized. The equilibrium is shifted because water hydrogen bonds so strongly to the keto form. An increase in temperature also favors the keto form, but this is not a large effect at the temperatures at which ordinary organic reactions are carried out. The rate of ketone/enol interconversion depends strongly on pH, and the individual tautomers of pentane-2,4-dione have been isolated at low temperature, under strictly neutral conditions. Other carbonyl compounds, including esters and amides, can and do enolize, but with much more difficulty. For example, neat diethylmalonate (diethyl propanedioate, 17.13) contains only 0.01 % enol at equilibrium, in contrast to the 80 % for pentane-2,4-dione. 

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