The stability constants, log K1 (ionized ligand) and log K2 (free acid), for Na+ complexation by four monoionizable crown ethers with a common sym-dibenzo-16-crown-5 polyether ring framework, sym-dibenzo-16-crown-5-oxyacetic acid (1), sym-dibenzo-16-crown-5-oxypropanoic acid (2), sym-dibenzo-16-crown-5-propanesulfonic acid (3), and sym-dibenzo-16-crown-5-oxymethyl phosphonic acid monoethyl ester (4), have been determined in 80% (w/w) methanol-water by calorimetric titration. Acid dissociation constants, pKa, have also been determined for these compounds in the same solvents by potentiometric titration. Similar constants have also been measured for 2,6-dimethylenebenzoic acid-18-crown-5 (5) and its methyl ester analogue 6. Furthermore, log K1, log K2, and pKa values were determined for 1, 5, and 6 in 99% (w/w) methanol-water. Increasing the length of the acidic side arm has a destabilizing effect upon the complexes formed with log K1 for 5 > 1 > 2 > 3 in 80% (w/w) methanol-water. The highest stability is achieved when the negative charge density of the side arm is located near the cavity space to be occupied by the sodium ion. Solvation effects are evident in the magnitude of the stability constants obtained in 5 and 6. For 5 in 99% (w/w) methanol-water, Na+ is apparently competing with an intramolecular hydrogen bond for the cavity space of the crown ether. With crown phosphonic acid monoalkyl ester 4, Na+ is more selectively complexed relative to K+ than with crown carboxylic acid 1. The kinetics of complexation of the crown carboxylate from 1 with Na+ in 99% (w/w) methanol-water were examined with the electric field-jump technique. The rate of formation is nearly diffusion controlled and indicates significant interaction between the side arm and Na+ prior to desolvation by the polyester ring. An additional relaxation, but of opposite amplitude, was also observed.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry