Abstract
Abstract The solvent extraction of alkaline earth and heavy metal picrates with a series of ring-contracted and ring-enlarged crown ethers has been carried out in order to reveal the effects of methylene-chain length upon cation-binding ability. Compared with the parent 3m-crown-m, the ring-contracted crown ethers and their sila analogues possess limited extractabilities, often as low as those for the corresponding glymes. On the other hand, the ring-enlarged crown ethers, in general, show gradual, but not drastic, decreases in extractability for most cations with increasing methylene-chain length. Interestingly, the extractabilities for Ag+, Tl+, and Ba2+ do not decrease monotonously with ring expansion; in particular 16-crown-5 is a better extractant for Ag+ than 15-crown-5 and even than 18-crown-6. These results are accounted for in terms of cavity size, conformation, and molecular symmetry of crown ethers. Effects of cation hydration upon extractability and the limitations of solvent extraction technique are also discussed.
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