Complex Formation of Oxonium Ion with 18-Crown-6 and 24-Crown-8 Derivatives in a Nitrobenzene Solution Saturated with Water

Abstract

KeywordsStability constant, oxonium ion complex, crown ether, nitrobenzene , ion-transfer polarographyIn liquid chromatography and liquid-membrane transport experiments using crown ethers (L)1, protonated amino acids form stable complexes with L and are stereoselectively transferred from a water (w) to an organic (o) phase. The formation of protonated amino acids in the w-phase requires the presence of a large excess of oxonium ion (>10-2 mol/dm3), because the primary acid-dissociation constants of common amino acids are relatively low (pKa1=2 -3). In such a case, the oxonium ion as well as the protonated amino acids can easily complex with L in an organic solvent, such as nitrobenzene (NB)2-4, 1,2-dichloroethane3 and acetonitrile.5,6 When we examine the behavior of the facilitated ion transfer of protonated amino acids by L across the w/o-interface, we necessarily encounter the problem of competitive facilitated ion transfers between protonated amino acids and oxonium ions. Consequently, we need to examine the facilitation effect of L on oxonium ion transfer at the w/ o-interface in the course of studying the effect of L on the transfer of protonated amino acids. Nevertheless, no systematic study on the facilitated transfer of oxonium ion has been reported so far.In the present paper, we shall determine by an iontransfer polarographic method the stability constants of the oxonium ion complexes with several representative Ls in the NB solution saturated with water, and then examine the facilitation ability of L on the oxonium ion transfer across the w/ NB-interface. Experimental ChemicalsTetrabutylammonium tetraphenylborate (TBA•TPB) and NB were prepared by the methods described before.7'8 Commercial dibenzo-l8-crown-6 (DB18C6, Nisso Co. and Merck, Schuchardt) and dibenzo-24-crown-8 (DB24C8, Merck) were recrystallized from benzene and hexane, respectively. Dicyclohexyl-24-crown-8 (DCH24C8) was purchased from Aldrich Co. and distilled under reduced pressure (bp. 185 -187° C at 2.7X102 Pa). Benzo-l8-crown-6 (B18C6) was synthesized by Pedersen's method9, then recrystallized from hexane and dried in vacuo at room temperature. Commercially available dicyclohexyl-l8-crown-6 (DCH 18C6, Merck, Schuchardt) and all the other chemicals were of analytical grade and were used without further purification. The concentrations of hydrochloric acid were determined by acid-base titration. The NB solution and the aqueous solution employed were saturated with water and NB, respectively, and allowed to stand for 2 h at 25° C prior to measurement.8 Electrochemical measurementsThe procedure and the apparatus were the same as those described before.8 The galvanic cells employed for the control or the measurement of potential difference at the w/NB-interface were the following two cells:Cell ( Here, the test interface is marked by an asterisk. The cells (A) and (B) satisfy the conditions CH>>CL$ and CDB18C6>>>CH, respectively, where Cj denotes the bulk concentration of species j. The superscript "NB" of cj refers to the NB-phase. The ce...

Interaction of Hydronium Ion with Dibenzo-18-crown-6: NMR, IR, and Theoretical Study

“…Somewhat surprisingly, the larger water content has a slight effect on K , giving the value of 1.56 × 10 4 L·mol −1 . This slight difference, which is probably mostly due to polarizing effect of excess water, does not bridge the gap between our value and those reported earlier, , however.…”

“…The value of K in nitrobenzene is found to be 1.16 × 10 4 L·mol −1 , which is 5−10 times lower than that found by electrochemical (half-wave potential and conductance) methods. It is not quite unusual that the values of equilibrium constants established by independent methods differ even by an order of magnitude . Nonetheless, the gap between our and the earlier reported values deserves some discussion.…”