Crown Ethers: Nomenclature, Chemistry, Application

Crown ethers are organic compounds-ring structures-containing a number of ether linkages (-C-O-C-) that have the appearance (based upon their structural formulas drawn on paper) of royal crowns. The most common varieties sport ethyleneoxy groups (-CH₂-CH₂-O-) joined end-to-end to form the ring. The space or opening inside the ring is of special significance.

Nomenclature

Generally, at least for the most common representatives, an easy form of identification-a common name-is used to identify crown ethers. A number is used representing the number of ring atoms only, followed by a dash and the word "crown," followed by another dash and the number of ether linkages (or oxygen atoms). Thus the crown ether 8-crown-4 has the chemical formula, C₈H₁₆O₄. -See illustration associated with article.

Chemistry

For a moment, consider the chemical compound, soap. Even though dissolved in water, soap can attach a droplet of oil and dissolve it, even though oil is not water-soluble. A crown ether molecule possesses both an appropriately sized opening, and has spaced oxygen atoms, thus enabling the ring to ensnare a cation (a positively charge particle). The cation would ordinarily be water-soluble, but now, encased in the crown ether ring, can be "pulled" into other solvents. Modified crown ethers that contain nitrogen or other features can be used to extend the boundaries of application, but discussing those is beyond the scope of this article.

Applications

Oftentimes, the cation that one wishes to encapsulate is either an alkali metal ion, such as sodium, potassium, or lithium-or it may be an alkaline earth metal ion, such as calcium or magnesium-or an ammonium ion. The size of the opening in the ring must be adjusted to match the size of the ion for maximum effect.¹ Thus, 12-crown-4 works best for lithium, whereas the larger opening sizes of 15-crown-5 and 18-crown-6 work best for sodium and potassium, respectively. The crown ethers used to encapsulate calcium and magnesium often add nitrogen atoms to the ring. Crown ethers are useful in phase-transfer catalysis. For the advanced student, there are a number of articles concerning this application available online.²

¹ An interesting modified crown ether has been developed, with two ethereal rings, attached to each other by a nitrogen linkage. One of the rings can attach somewhat to a cation. Light then applied causes the nitrogen linkage to become modified, closing the other ethereal ring to attach to the other end of the cation. This has been likened to catching a ball (the ion) in a baseball glove (the ether structure). Various sized ions can be "caught" by such a structure.

² See, for instance, "Application of Crown Ethers as Phase Transfer Catalysts in the Electron Transfer Reactions of Coal," by Ramani Narayan and George T. Tsao.

References and Resources:

Nobel Lecture 8 Dec 1987: "The Discovery of Crown Ethers," by Charles J. Pedersen.

Org-Chem.org - Molecular Crowns - Crown Ethers