Carbon Acidity. 72. Ion Pair Acidities of Phenyl Alkyl Ketones. Aggregation Effects in Ion Pair Acidities

Article abstract of DOI:10.1021/ja00254a031

Equilibrium cesium ion pair acidities of acetophenone, propiophenone, isobutyrophenone, and o-methoxyacetophenone and the lithium ion pair acidity of o-methoxyacetophenone in tetrahydrofuran have been determined by an indicator method.For all of these compounds, the observed pK_a values decrease as the equilibrium enolate concentration is increased.It is proposed that this concentration dependence is a consequence of the aggregation of the enolates, and a method is described whereby average aggregation numbers can be determined from the acidity data.The results indicate that the extent of aggregation of enolate ions is influenced by both electronic and steric factors.In addition, internal solvation is found to be important for the lithium, but not the cesium enolate of o-methoxyacetophenone.