A Conductance Study of 1:1 Complexes of 15-Crown-5, 16-Crown-5, and Benzo-15-crown-5 with Alkali Metal Ions in Nonaqueous Solvents

Abstract

Conductivities of alkali metal salts in the presence of 15-crown-5 (15C5), 16-crown-5 (16C5), and benzo-15-crown-5 (B15C5) were measured at 25 °C in acetonitrile, propylene carbonate, and methanol. Formation constants (KML+) of 1:1 complexes of 15C5 and 16C5 with alkali metal ions and conductance parameters (λ0 and å) of the Na+ complexes of 15C5, 16C5, and B15C5 were determined, λ0 and å being limiting ionic molar conductivity and distance of closest approach of ions, respectively. In general, selectivities of 15C5 and 16C5 for alkali metal ions show the size-fit correlation. In contrast to 15C5, 16C5 forms much the most stable complex with Na+ in every solvent. Generally, KML+-value sequences of a given crown ether complex for the solvents are the reverse of donor-number sequences of the solvents. Value of å increases with an increase in the cationic size (Na+→Na(15C5)+→Na(16C5)+→Na(B15C5)+). Mobility of the Na+–crown ether complex decreases with an increase in the size of the crown ether. From λ0 values and Walden products of these Na(crown ether)+ complexes and a tetrapropylammonium ion, it seems likely that Na(15C5)+ and Na(16C5)+ act as structure breakers in hydrogen-bonding solvents, whereas Na(B15C5)+ as a structure maker.