Strong hydrogen bonding in p-diketones

The keto/enol equilibrium (15) has been a spur to much research. In the absence of catalysts the equilibrium is established slowly and is very sensitive to a variety of influences, both internal, such as the nature of a- and (3-substituents, and external, such as temperature and solvent. The discovery that the equilibrium was established sufficiently slowly to permit both keto and enol tautomers to be observed by H-nmr spectroscopy allowed these several influences to be easily investigated (see Kol tsov and Kheifets, 1971, for a review of the early work, and Emsley, 1984, for later work). 

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 Although, as a method of studying the equilibrium, this approach has been replaced by non-intrusive methods, work on the physical separation of the tautomers has continued (e.g. Regitz and Schafer, 1981 Vogt and Gompper, 1981).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

A comprehensive study of the keto/enol equilibrium for pentane-2,4- 

14 Structure of the pentane-2,4-dione-diethylamine adduct (taken with permission from Emsley et al., 1986b). 

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The bifluoride ion, HFj 297 Strong hydrogen bonding in p-diketones 309 Keto/enol equilibrium 310 Structures 312 Hydrogen-bond energies 314 Vibrational modes 315 Nmr spectroscopy 317... 

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