Organic compounds with molecules containing large rings of carbon and oxygen atoms. The crown ethers are macrocyclic polyethers. The first to be synthesized was the compound 18-crown-6, which consists of a ring of six –CH2–CH2–O– units (i.e. C12H24O6). The general method of naming crown ethers is to use the form n-crown-m, where n is the number of atoms in the ring and m is the number of oxygen atoms. Substituted crown ethers can also be made. The crown ethers are able to form strongly bound complexes with metal ions by coordination through the oxygen atoms. The stability of these complexes depends on the size of the ion relative to the cavity available in the ring of the particular crown ether. Crown ethers also form complexes with ammonium ions (NH4+) and alkyl ammonium ions (RNH3+). They can be used for increasing the solubility of ionic salts in nonpolar solvents. For example, dicyclohexyl-18-crown-6 complexes with the potassium ion of potassium permanganate and allows it to dissolve in benzene, giving a purple neutral solution that can oxidize many organic compounds. They also act as catalysts in certain reactions involving organic salts by complexing with the positive metal cation and thereby increasing its separation from the organic anion, which shows a consequent increase in activity. Some of the uses of crown ethers depend on their selectivity for specific sizes of anions. Thus they can be used to extract specific ions from mixtures and enrich isotope mixtures. Their selectivity also makes them useful analytical reagents. See also cryptands.