Propanone resonance structures

Propanone, (CH3)2CO, the simplest ketone, can be described as a resonance hybrid of two resonance structures (Figure 14-51). The first resonance structure is known as the major resonance form and makes the largest contribution to the resonance hybrid the second resonance structure, with the separation of charge, makes a smaller contribution to the resonance hybrid. 

Compare electrostatic potential maps of enolates derived from 2-butanone, 4,4-dimethyl-2-pentanone, 4,4,4-trifluoro-2-butanone and l-phenyl-2-propanone with those of acetone. Which substituents cause significant changes in the electronic structure of these enolates and what are the nature of these changes Justify your answers by making drawings of any important resonance contributors. 

The a-pyridoin ene-diol structure cannot be completely planar (13). This indicates that the initating radical derived from the ene-diol or rather from the ketohydroperoxide form of this compound cannot be so strongly resonance stabilized as to cause inhibition as was postulated for 1 [2-pyridyl] 2-propanone. 

The importance of the dipolar resonance forms is reflected in the stabihties of isomeric carbonyl compounds. Propanal is approximately 27 kj mole" less stable than propanone. For addition reactions, two isomeric products with different stabihties also form. Therefore, we also have to consider the relative stabihties of the products. Hydrogenation of propanal and propanone gives isomeric alcohols. 1-Propanol is approximately 16 kJ mole" less stable than 2-propanol. Since the difference in the stabihties of the reactants is greater than the difference in the stabilities of the products, the equihbrium constants for the addition reactions of carbonyl compounds depend on more differences in the structure of the carbonyl compound than on the differences in the structure of the addition product. Thus, because ketones are more stable than aldehydes, the addition reactions of ketones are less favorable (have smaller equihbrium constants) than addition reactions of aldehydes. 

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