11.7: Crown Ethers
Crown ethers are cyclic polyethers that contain multiple oxygen atoms, usually arranged in a regular pattern. The first crown ether was synthesized by Charles Pederson while working at DuPont in 1967. For this work, Pedersen was co-awarded the 1987 Nobel Prize in Chemistry. Crown ethers are named using the formula x-crown-y, where x is the total number of atoms in the ring and y is the number of ether oxygen atoms. The term 'crown' refers to the crown-like shape that these ether molecules take. A significant feature of crown ethers is that they form complexes with specific alkali metal cations. The oxygen atoms of crown ethers together form an internal cavity into which the electron lone pairs effectively coordinate the metal ions. The choice of the metal ion depends on the diameter of the ether's internal cavity compared to the diameter of the metal ion. Consequently, crown ethers serve as effective solvating agents for solubilizing inorganic salts in organic solvents. For example, KF would not dissolve in benzene by itself, but the use of 18-crown-6 generates a complex with potassium ion, which dissolves in benzene.
The result is a solution containing unsolvated fluoride ions, free to participate in nucleophilic substitution reactions. Typically, the strong interaction between fluoride ions and polar solvents makes it challenging to free up fluoride anions in a nonpolar solvent. However, crown ether increases the nucleophilic strength of the fluoride anion by making it available to participate in an SN2 reaction. Overall, the role of crown ether is to sequester the cation, leaving the anion to function as a better nucleophile.