Solvent effects keto-enol tautomer equilibria

A mechanistic study of acetophenone keto-enol tautomerism has been reported, and intramolecular and external factors determining the enol-enol equilibria in the cw-enol forms of 1,3-dicarbonyl compounds have been analysed. The effects of substituents, solvents, concentration, and temperature on the tautomerization of ethyl 3-oxobutyrate and its 2-alkyl derivatives have been studied, and the keto-enol tautomerism of mono-substituted phenylpyruvic acids has been investigated. Equilibrium constants have been measured for the keto-enol tautomers of 2-, 3- and 4-phenylacetylpyridines in aqueous solution. A procedure has been developed for the acylation of phosphoryl- and thiophosphoryl-acetonitriles under phase-transfer catalysis conditions, and the keto-enol tautomerism of the resulting phosphoryl(thiophosphoryl)-substituted acylacetonitriles has been studied. The equilibrium (388) (389) has been catalysed by acid, base and by iron(III). Whereas... 

The effect of solvent polarity on chemical systems including reaction rates and equilibria can be quite significant. In general, it is necessary to consider the relative polarities of the reactants and products. In equilibria, a polar solvent will favour the more polar species. A good example is the keto-enol tautomerization of ethyl acetoacetate shown in Figure 1.9. The keto tautomer is more polar than the enol tautomer and therefore the equilibrium lies to the left in polar media such as water Table 1.11. 

A polar solvent like water is known to have a relevant influence on the covalent structure of polar molecules. This is clearly illustrated by the effect of hydration on the tautomeric equilibria of molecules. A prototypical example is the keto/enol equilibrium of P-diketones whereas the enol form is the most populated species in the gas phase and in apolar solvents, the keto form is the most stable tautomer in aqueous solution [106,107]. Inspection of Figure 5 allows us to rationalize the solvent-induced change in the topology of this molecule. 

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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