The formation constants, KML+, for 1:1 complexes of 15-crown-5 (15C5), 18-crown-6 (18C6), and dibenzo-24-crown-8 (DB24G8) with alkali metal ions and the limiting ionic molar conductivities, λ°, of the complexed cations in propylene carbonate have been determined at 25 °C conductometrically. The KML+ value series about the same alkali metal ion among the crown ethers are given in the order 18C6>>DB24C8>15C5. The KML+ sequences of the alkali metal ions with 15C5, 18C6, and DB24C8 are Li+>Na+>K+>Rb+>Cs+, K+>Na+>Rb+>>Cs+, and Na+>K+>Rb+≥Cs+, respectively. The indication is that the ability of the alkali metal ions to form complexes with the three crown ethers is not always affected primarily by the relative sizes of the alkali metal ions and the crown ether cavity. In each case of 18C6 and DB24C8 the λ° values of the alkali metal ions are nearly equal, while in the case of 15C5 there are considerably differences, suggesting that in the former case the charge of the alkali metal ion trapped in the crown ether cavity is effectively shielded. This is not true in the latter case.
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Takeda et al. (1980) studied this question.
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