Crown ethers are heterocyclic chemical compounds that consist of a ring containing several ether groups. 18-Crown-6 is an Organic compound with the formula 6 and the IUPAC name of 147101316-hexaoxacyclooctadecane A chemical compound is a substance consisting of two or more different elements chemically bonded together in a fixed proportion by Mass. Ether is a class of Organic compounds which contain an ether group — an Oxygen Atom connected to two (substituted Alkyl The most common crown ethers are oligomers of ethylene oxide, the repeating unit being ethyleneoxy, i. In Chemistry, an oligomer consists of a limited number of Monomer units (ολιγος or oligos is Greek for "a few" in contrast to a e. , -CH2CH2O-. 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 head. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge A crown is the traditional Symbolic form of Headgear worn by a Monarch or by a Deity, for whom the crown traditionally represents power 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. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Crown ethers are much broader than the oligomers of ethylene oxide; an important group are derived from catechol.
Crown ethers strongly bind certain cations, forming complexes. The term complex in Chemistry is usually used to describe molecules or ensembles formed by the combination of Ligands and metal Ions. 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 hydrophobic. The resulting cations often form salts that are soluble in nonpolar solvents, and for this reason crown ethers are useful in phase transfer catalysis. A phase transfer catalyst or PTC in Chemistry is a Catalyst which facilitates the migration of a reactant in a Heterogeneous system from one The denticity of the polyether influences the affinity of the crown ether for various cations. In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally For example, 18-crown-6 has high affinity for potassium cation, 15-crown-5 for sodium cation, and 12-crown-4 for lithium cation. The high affinity of 18-crown-6 for potassium ions contributes towards its toxicity.
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structures of common crown ethers: 12-crown-4, 15-crown-5, 18-crown-6, dibenzo-18-crown-6, and diaza-18-crown-6
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Crown ethers in nature
Crown ethers are not the only macrocyclic ligands that have affinity for the potassium cation. 18-Crown-6 is an Organic compound with the formula 6 and the IUPAC name of 147101316-hexaoxacyclooctadecane 18-Crown-6 is an Organic compound with the formula 6 and the IUPAC name of 147101316-hexaoxacyclooctadecane Ionophores such as nonactin and valinomycin also display a marked preference for the potassium cation over other cations. Nonactin is a member of a family of naturally occurring cyclic Ionophores known as the macrotetrolide Antibiotics. Valinomycin is a dodecadepsipeptide, that is it is made of twelve alternating Amino acids and Esters to form a macrocyclic molecule
History of synthetic crown ethers
In 1967, Charles Pedersen, who was a chemist working at DuPont, discovered a simple method of synthesizing a crown ether when he was trying to prepare a complexing agent for divalent cations [1]. Charles John Pedersen ( October 3, 1904 &ndash October 26, 1989) was an American organic chemist best known for describing A chemist is a Scientist trained in the Science of Chemistry. E I du Pont de Nemours and Company (,) is an American chemical company that was founded in July 1802 as a Gunpowder mill by Eleuthère Irénée Chelation is the binding or complexation of a bi- or multidentate Ligand. His strategy entailed link two catecholate groups through one hydroxyl on each molecule. Pyrocatechol, more commonly known as catechol, is the Organic compound with the formula C6H4(OH2 Hydroxyl in Chemistry stands for a molecule consisting of an Oxygen atom and a Hydrogen atom connected by a Covalent bond. This linking defines a polydentate ligand that could partially envelop the cation and, by ionization of the phenolic hydroxyls, neutralize the bound dication. Ionization is the physical process of converting an Atom or Molecule into an Ion by adding or removing charged particles such as Electrons He was surprised to isolate a by-product that strongly complexed potassium cation. A by-product is a secondary or incidental product deriving from a Manufacturing process a Chemical reaction or a biochemical pathway and is not the primary product Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39 Citing earlier work on the dissolution of potassium in 16-crown-4 [2] [3], he realized that the cyclic polyethers represented a new class of complexing agents that were capable of binding alkali metal cations. Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39 Ether is a class of Organic compounds which contain an ether group — an Oxygen Atom connected to two (substituted Alkyl Trends The alkali metals show a number of trends when moving down the group - for instance decreasing electronegativity increasing reactivity and decreasing melting and boiling He proceeded to report systematic studies of the synthesis and binding properties of crown ethers in a seminal series of papers. The fields of organic synthesis, phase transfer catalysts, and other emerging disciplines benefited from the discovery of crown ethers. Organic synthesis is a special branch of Chemical synthesis and is concerned with the construction of Organic compounds via Organic reactions Organic A phase transfer catalyst or PTC in Chemistry is a Catalyst which facilitates the migration of a reactant in a Heterogeneous system from one Pedersen particularly popularized the dibenzo crown ethers. [4]
Pedersen shared the 1987 Nobel Prize in Chemistry for the discovery of the synthetic routes to and binding properties, of crown ethers. The Nobel Prize in Chemistry (Nobelpriset i kemi is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of Chemistry.
Affinity for cations
Apart from its high affinity for potassium cations, 18-crown-6 can also bind to protonated amines and form very stable complexes in both solution and the gas phase. 18-Crown-6 is an Organic compound with the formula 6 and the IUPAC name of 147101316-hexaoxacyclooctadecane Some amino acids, such as lysine, contain a primary amine on their side chains. In Chemistry, an amino acid is a Molecule containing both Amine and Carboxyl Functional groups In Biochemistry, this Lysine (abbreviated as Lys or K) is an α- Amino acid with the Chemical formula HO2CCH(NH2(CH24NH2 Amines are Organic compounds and Functional groups that contain a basic Nitrogen Atom with a Lone pair. Those protonated amino groups can bind to the cavity of 18-crown-6 and form stable complexes in the gas phase. Hydrogen-bonds 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.
Aza-crowns
"Aza-crowns" consist of crown ethers wherein an ether oxygen has been replaced by an amine group. The prefix aza- is used in Organic chemistry to form names of organic compounds where a Carbon atom is replaced by a Nitrogen atom A well-known tetrazacrown is cyclen. Cyclen or 14710-tetraazacyclododecane is a Macrocycle and the Aza analogue of the Crown ether 12-crown-4. Mixed amine-ether crowns are also known. [5]
References
- ^ C. J. Pedersen, J. Am. Chem. Soc., 1967, 89, 7017. The Journal of the American Chemical Society (usually abbreviated as J
- ^ D. G. Stewart. D. Y. Waddan and E. T. Borrows, British Patent 785,229, Oct. 23, 1957.
- ^ J. L. Down, J. Lewis, B. Moore and G. W. Wilkinson, Proc. Chem. Soc. , 1959, 209; J. Chem. Soc. , 1959, 3767.
- ^ Charles J. Pedersen (1988). "Macrocyclic Polyethers: Dibenzo-18-Crown-6 Polyether and Dicyclohexyl-18-Crown-6 Polyether". Org. Synth.; Coll. Organic Syntheses is a Scientific journal that since 1921 has provided the Chemistry community with annual collections of detailed and checked procedures Vol. 6: 395.
- ^ Vincent J. Gatto, Steven R. Miller, and George W. Gokel (1988). "4,13-Diaza-18-Crown-6". Org. Synth.; Coll. Organic Syntheses is a Scientific journal that since 1921 has provided the Chemistry community with annual collections of detailed and checked procedures Vol. 8: 152.
External links
Dictionary
crown ether
-noun
- (chemistry) any of a class of macrocyclic compounds containing repeat units of -CH2CH2O-
- (chemistry) any similar compound containing atoms of nitrogen, sulphur, phosphorus or silicon instead of, or as well as, oxygen
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