Template synthesis and ionophorous properties of substituted crown ethers towards alkali-metal ions

Abstract

A number of alkyl-substituted crown ethers of the type 2-R-12-crown-4, 2-R-15-crown-5, and 2-R-18-crown-6 (R = Me, Et, octyl, decyl, tetradecyl, icosyl, or Ph) have been prepared in relatively good yields by a novel synthetic route employing the metal-ion template effect. The ionophorous properties of these crown ethers with respect to the transport of lithium, sodium, and potassium picrates across a bulk liquid chloroform membrane were measured and have been compared to those of crown ethers already used as ionophores. Substitution of alkyl and phenyl groups on the crown ethers, making them more lipophilic, does not seem to assist the transport of Na⁺ and K⁺ as their picrate salts, but rather appears to make the ionophore more susceptible to remain in the chloroform phase, as the chain length increases, and thereby leads to a reduction in transport. Introduction of alkyl and other groups, however, does seem to assist the transport of Li⁺ as lithium picrate, although the overall rate of transport is considerably slower for Li⁺ than the rate for Na⁺.