ChemInform Abstract: Carbon Acidity. Part 70. Equilibrium Ion Pair Acidities in Tetrahydrofuran of Benzylic Carbon Acids Stabilized by an Adjacent Cyano, Carboalkoxy, and Sulfonyl Substituent. Delocalization and Aggregation of Ion Pairs.

ChemInform Abstract

The pKa values of (I) are determined with the aid of transmetalation equilibria, using indicators such as (IV). The lack of concentration dependence of the pKa values indicates that the benzylic carbanions exist exclusively as monomeric (contact) ion pairs in THF. The degree of association depends on the extent of charge delocalization. Whereas the formation of Li-salts is accompanied by a net entropy increase, for the Cs-salts the opposite effect is observed in most cases. It is shown that contact/solvent separated (indicator) and monomer/aggregate equilibria have different dependencies on the degree of carbanion delocalization.