FROM NEUTRAL TO IONIC SPECIES - AMINE-P-TERT-BUTYLCALIX(N)ARENE (N = 6, 8) INTERACTION - ELECTROCHEMICAL, THERMODYNAMIC AND STRUCTURAL STUDIES IN BENZONITRILE

Solubilities and derived Gibbs energies, enthalpies and entropies of s olution of p-tert-butylcalix(n)arenes (n = 4, 6, 8) in benzonitrile at 298.15 K are reported. An important consequence of amine-calixarene i nteractions in solution is the generation of new electrolytes. Thus, u sing two different calixarenes (cyclic hexamer and octamer) and variou s amines, conductance measurements are performed in order to derive eq uilibria data for the process involving two neutral species to give an electrolyte, as a result of a proton-transfer reaction from the calix arene to the amine. The results obtained by conductivity are checked b y using a combination of pK(a) values for p-tert-butylcalix(n)arene an d various amines in benzonitrile calculated from potentiometric data. Thermodynamic parameters for these processes obtained from titration c alorimetry reflect the distinctive properties of cryptands relative to other cyclic and aliphatic amines, suggesting that cryptand 222 is li kely to host the proton released by the cyclic octamer in its cavity. C-13 NMR studies on these systems revealed that the charge on the cali xarene anion is delocalised as a result of a dynamic interchange of th e remaining protons among the oxygen atoms present in the oligomeric m olecule. Finally, the extraction of amines from aqueous solutions by c alixarenes is analysed by taking into account the individual parameter s that contribute to the overall process. It is shown that the transfe r of amines from water to benzonitrile is favoured in the presence of calixarenes. Although the main contribution to the extraction process results from ion-pair formation in solution, the selective extraction of these amines by calixarenes is largely controlled by the transfer G ibbs energy of the amine from water to the non-aqueous phase.