On the Acidity of the Hydroxyl Groups in Calix[4]arenes and the Dissociation-dependent Conformational Change

By using a water-soluble calix[4]arene derivative and a p-nitro-substituted calix[4]arene the pKa values of the OH groups were determined for the first time. The first dissociation occurred at pH < 1 while the dissociation of the last proton occurred at pH > 11, indicating that these molecules have a super-acidic proton as well as a super-basic oxy-anion within a molecule. We also found that the calixarene conformation changes as a function of medium pH. These novel findings are rationalized in terms of strong intramolecular hydrogen bonding among the OH groups.