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In organic chemistry, crown ethers are cyclic chemical compounds that consist of a ring containing several ether groups (). The most common crown ethers are cyclic of ethylene oxide, the repeating unit being ethyleneoxy, i.e., . Important members of this series are the tetramer ( n = 4), the pentamer ( n = 5), and the hexamer ( n = 6). The term "crown" refers to the resemblance between the structure of a crown ether bound to a cation, and a crown sitting on a person's head. The first number in a crown ether's name refers to the number of atoms in the cycle, and the second number refers to the number of those atoms that are oxygen. Crown ethers are much broader than the of ethylene oxide; an important group are derived from catechol.
Crown ethers strongly bind certain cations, forming complexes. The oxygen atoms are well situated to coordinate with a cation located at the interior of the ring, whereas the exterior of the ring is Hydrophobe. The resulting cations often form salts that are soluble in , and for this reason crown ethers are useful in phase transfer catalysis. The ligand of the polyether influences the affinity of the crown ether for various cations. For example, 18-crown-6 has high affinity for the potassium cation, 15-crown-5 for the sodium cation, and 12-crown-4 for the lithium cation. The high affinity of 18-crown-6 for potassium ions contributes to its toxicity. The smallest crown ether still capable of binding cations is 8-crown-4, with the largest experimentally confirmed crown ether being 81-crown-27. Crown ethers are not the only macrocyclic ligands that have affinity for the potassium cation. such as valinomycin also display a marked preference for the potassium cation over other cations.
Crown ethers have been shown to coordinate to Lewis acids through electrostatic, σ-hole (see halogen bond) interactions, between the Lewis basic oxygen atoms of the crown ether and the Electrophile Lewis acid center.
Pedersen shared the 1987 Nobel Prize in Chemistry for the discovery of the synthetic routes to, and binding properties of, crown ethers.
Apart from its high affinity for potassium cations, 18-crown-6 can also bind to protonated and form very stable complexes in both solution and the gas phase. Some amino acids, such as lysine, contain a primary amine on their side chains. Those protonated amino groups can bind to the cavity of 18-crown-6 and form stable complexes in the gas phase. Hydrogen bond are formed between the three hydrogen atoms of protonated amines and three oxygen atoms of 18-crown-6. These hydrogen-bonds make the complex a stable adduct. By incorporating Luminescence substituents into their backbone, these compounds have proved to be sensitive ion probes, as changes in the absorption or fluorescence of the photoactive groups can be measured for very low concentrations of metal present. Some attractive examples include macrocycles, incorporating oxygen and/or nitrogen donors, that are attached to polyaromatic species such as anthracenes (via the 9 and/or 10 positions) or naphthalenes (via the 2 and 3 positions). Some modifications of dye Ionophore by crown ethers exhibit extinction coefficients that are dependent on the chain lengths of chained cations.
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