SolutionsMolecular Recognition
1. Which crown?
Which crown ether would you choose to complex the Na+ ion? And K+?
For Na+, 15-crown-5 (cavity of 1.7-2.2 Å). For K+, 18-crown-6 (2.6-3.2 Å).
2. “Naked” fluoride
Why does KF hardly react with 1-bromooctane in acetonitrile, but does react when 18-crown-6 is added?
Because the crown ether complexes K+ and separates it from fluoride: the anion is poorly solvated and has no nearby counterion, and its nucleophilicity increases greatly. The complex also carries the salt into the organic solvent.
3. In the cell
How are crown ethers related to the transport of ions across cell membranes?
Some natural molecules (ionophores) resemble crown ethers: they surround the ion with oxygens and wrap it in a lipophilic exterior, which allows its selective passage through the membrane.