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…
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.
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Q1: What are crown ethers and why are they called crown ethers?
Crown ethers are cyclic compounds derived from ethylene glycol with multiple ether linkages. First discovered by Charles Pederson in 1967, they are named for their crown-like molecular shape. The naming formula x-crown-y indicates the total ring atoms (x) and oxygen atoms (y). For example, 18-crown-6 is an 18-membered ring containing six ether oxygen atoms.
Q2: How do crown ethers bind metal cations?
Crown ethers bind metal cations through lone pairs of electrons on oxygen atoms lining their internal cavity. These oxygen atoms create a polar cavity that coordinates effectively with metal ions. The selectivity depends on cavity size: 18-crown-6 binds potassium ions strongly because the cavity diameter matches the potassium ion diameter, while 12-crown-4 with its smaller cavity binds lithium ions more effectively.
Q3: Why can crown ethers dissolve inorganic salts in nonpolar organic solvents?
Crown ethers have a nonpolar hydrocarbon-like outer surface and a polar inner cavity. This dual nature allows them to solvate metal cations while remaining soluble in nonpolar solvents like benzene. For instance, potassium permanganate does not dissolve in benzene alone, but 18-crown-6 forms a complex with the potassium ion, enabling dissolution and producing characteristic purple benzene used in oxidation reactions.
Q4: How do crown ethers enhance nucleophilicity of anions in nonpolar solvents?
Crown ethers sequester cations, leaving anions free to participate in reactions. In nonpolar solvents, fluoride ions normally interact strongly with polar environments, limiting their reactivity. Crown ethers isolate the cation, making the unsolvated anion available as a better nucleophile for SN2 reactions. This activation allows otherwise inaccessible anions to function effectively in nucleophilic substitution.
Q5: What is the relationship between crown ether cavity size and metal ion selectivity?
Crown ether selectivity for metal cations depends on matching the cavity diameter to the ion diameter. Alkali metal ions whose diameters approximate the ether cavity diameter bind most effectively. The larger 18-crown-6 cavity preferentially binds potassium ions, while the smaller 12-crown-4 cavity binds lithium ions more strongly, demonstrating how structural design controls ion recognition.
Q6: What makes the naming system for crown ethers useful?
The x-crown-y naming formula provides immediate structural information about crown ether composition. The first number (x) indicates total ring atoms, while the second (y) specifies oxygen atom count. This systematic approach allows chemists to quickly identify ring size and oxygen content, facilitating communication about structure and predicting binding properties based on cavity dimensions.
Q7: How do crown ethers function as solvating agents for inorganic salts?
Crown ethers act as solvating agents by complexing with metal cations through their polar inner cavity while maintaining a nonpolar outer surface. This allows inorganic salts to dissolve in nonpolar organic solvents where they normally would not. The cation becomes incorporated into the crown ether complex, enabling the salt to enter solution and making the anion available for subsequent chemical reactions.