11.7:
Crown Ethers
Crown ethers are cyclic compounds derived from ethylene glycol and its substituents, with multiple ether linkages.
First discovered by Charles Pederson in 1967, crown ethers are so-called because their molecular shape resembles a crown.
The naming of crown ethers uses the general formula x-crown-y, where x indicates the total number of atoms in the ring and y represents the number of oxygen atoms. For example, 18-crown-6 is an 18-membered ring containing six ether oxygen atoms.
Crown ethers are known to bind metal cations in their internal cavity lined by regularly spaced oxygen atoms. For example, 18-crown-6 strongly binds potassium ion.
The lone pairs of electrons on the oxygen atoms lining the inner cavity, make the cavity polar and coordinate with the metal ion effectively.
The selectivity for the cation varies with the size of the cavity. Crown ethers primarily bind alkali metal ions whose diameters are comparable to the diameter of the ether cavity.
For example, 18-crown-6 binds potassium ion effectively but not lithium ion because the diameter of the ether cavity approximates the diameter of the potassium ion. In contrast, 12-crown-4, with a smaller cavity, binds lithium ion more strongly.
The outer surface of crown ethers resembles a hydrocarbon and is nonpolar. As a result, crown ethers can solubilize inorganic salts in nonpolar organic solvents by solvating the cation.
For example, potassium permanganate, an inorganic salt, does not dissolve in benzene, an organic solvent, by itself. However, upon the addition of 18-crown-6, the salt dissolves, giving benzene a purple color.
18-crown-6 forms a complex with potassium ion, which dissolves in benzene. The unsolvated permanganate ion imparts a characteristic purple color to the benzene solution. The resulting "purple benzene" is a useful reagent for oxidation reactions.
Similarly, crown ethers facilitate the availability of otherwise inaccessible anions in a nonpolar solvent and enhance their nucleophilicity.
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.