Electron delocalization enolates

Enolate ion (Section 18 6) The conjugate base of an enol Enolate ions are stabilized by electron delocalization... 

Because acidity depends on the difference in energy of the acid and its conjugate base, we must be sure that the stabilization of the enolate anion by electron delocalization represented by 14a and 14b is greater than the analogous stabilization of the neutral enol represented by 15a and 15b ... 

The enol form of 2,4-pentanedione (and of related dicarbonyl compounds of the type 0=C—CH—C=0) not only is stabilized by electron-delocalization,... 

The liquid ketone exists 85% in the enol form and is moderately acidic. The Ka in water is = 10 9. The enol form is stabilized significantly by both electron delocalization and hydrogen bonding. The amount of enol present at equilibrium depends on the solvent, and is smallest in hydrogen-bonding solvents and largest in nonpolar solvents such as carbon tetrachloride. 

Enols are stronger acids than alcohols because of the increase in electron delocalization in enolate anions as compared to the neutral enols (see Section 15-8A). The stabilization energy of benzenol (Table 21-1) is 48 kcal mole-1, 5 kcal greater than that of benzene. We can ascribe this increase to delocalization of an unshared electron pair from oxygen ... 

Hiberty, P. C. Byrman, C. P. Role of x-electron delocalization in the enhanced acidity of carboxylic acids and enols relative to alcohols, 7. Am. Chem. Soc. 1995, 777, 9875-9880. 

Qq is calculated in the same way as the g-value but relates to the single and double reference CO and CC bonds, theoretically the bonds not involved in the conjugated system of double and single bonds. Qo amounts to 0.320 according to the early reference bonds proposed by Gilli and coworkers [9]. The A-parameter is a descriptor which, similar to the g-parameter, indicates the degree of the ir-electron delocalization. If it is equal to 0, then it corresponds to the enol-keto form A = 1 stands for the keto nol form and A = 1 /2 corresponds to the full equalization of the bond lengths mentioned above. The latter is also applied to the transition state of the C D H- -0=C C=0- -H-O-C proton transfer reaction (Scheme 2). 

There is no difference between the enol-keto and keto-enol 0-H- 0 H-bridge and no difference for the double-single conjugated bonds, especially if R1 and R3 substituents are equivalent. Hence Buemi and Zuccarello [13] proposed to use A = (1 — Q/Qo) since in such a case 0 corresponds to the system with the lack of TT-electron delocalization and A = 1 to the full delocalization with the movement of H-atom to the middle of O- O distance or nearly so. It is worth mentioning that, similar for intramolecular H-bonds, an enhancement of H-bond strength may be observed for intermolecular interactions [3, 5]. Scheme 3 presents an example of a system often found in crystal structures. 

By comparison, decarboxylation is largely a kinetic problem. Enzymes have developed a variety of strategies for stabilizing the anionic intermediate that is produced in the decarboxylation step. Metal ion stabilization of enolates is a common theme, particularly for decarboxylation of /8-keto acids. The most elegant solutions are perhaps the extensive electron delocalizations seen in pyri-doxal phosphate and thiamin pyrophosphate. 

Stability of the conjugated base arises from the electron delocalization that can be identified by using the resonance structure method. Acids in which the proton is removed from the carbon atom are called carbon acids and their conjugate base is called an enolate ion. 

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