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Crown ether
 
 
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Crown ethers are heterocyclic chemical compound
Chemical compound

A chemical compound is a Chemical substance consisting of two or more different chemical element Chemical bond together in a fixed mass ratio that can be split into simpler substances....
s that consist of a ring containing several ether
Ether

Ether is a class of organic compounds which contain an ether functional group ? an oxygen atom connected to two alkyl or aryl groups ? of general formula R?O?R....
 groups. The most common crown ethers are oligomer
Oligomer

In chemistry, an oligomer consists of a limited number of monomer units , in contrast to a polymer which, at least in principle, consists of an unbounded number of monomers....
s of ethylene oxide, the repeating unit being ethyleneoxy, i.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
Crown (headgear)

A crown is the traditional symbolic form of headgear worn by a monarch or by a deity, for whom the crown traditionally represents Political power, legitimacy, Crown of Immortality, righteousness, victory, Roman triumph, resurrection, honour and glory of life after death....
 sitting on a head.






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Crown ethers are heterocyclic chemical compound
Chemical compound

A chemical compound is a Chemical substance consisting of two or more different chemical element Chemical bond together in a fixed mass ratio that can be split into simpler substances....
s that consist of a ring containing several ether
Ether

Ether is a class of organic compounds which contain an ether functional group ? an oxygen atom connected to two alkyl or aryl groups ? of general formula R?O?R....
 groups. The most common crown ethers are oligomer
Oligomer

In chemistry, an oligomer consists of a limited number of monomer units , in contrast to a polymer which, at least in principle, consists of an unbounded number of monomers....
s of ethylene oxide, the repeating unit being ethyleneoxy, i.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
Crown (headgear)

A crown is the traditional symbolic form of headgear worn by a monarch or by a deity, for whom the crown traditionally represents Political power, legitimacy, Crown of Immortality, righteousness, victory, Roman triumph, resurrection, honour and glory of life after death....
 sitting on a 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
Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
. 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
Complex (chemistry)

In chemistry, a complex, also called a "coordination compound" or "metal complex", is a structure consisting of a central atom or molecule connected to surrounding atoms or molecules....
. 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. The denticity
Ligand

In chemistry, a ligand is either an atom, ion, or molecule that bonds to a central metal, generally involving formal donation of one or more of its electrons....
 of the polyether influences the affinity of the crown ether for various cations. 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.

Crown ethers in nature

Crown ethers are not the only macrocyclic ligands that have affinity for the potassium cation. Ionophore
Ionophore

An ionophore is a lipid-soluble molecule usually synthesized by microorganisms to transport ions across the lipid bilayer of the cell membrane....
s such as nonactin
Nonactin

Nonactin is a member of a family of naturally occurring cyclic ionophores known as the macrotetrolide antibiotics. The other members of this homologous family are monactin, dinactin, trinactin and tetranactin which are all neutral ionophoric substances and higher homologs of nonactin....
 and valinomycin
Valinomycin

'Valinomycin' is a depsipeptide, that is, it is made of twelve alternating amino acids and esters to form a macrocyclic molecule.Valinomycin is obtained from the cells of several Streptomyces strains, one of them "S....
 also display a marked preference for the potassium cation over other cations.

History of synthetic crown ethers

In 1967, Charles Pedersen, who was a chemist
Chemist

A chemist is a scientist trained in the science of chemistry. Chemists study the composition of matter and its properties such as density, acidity, size and shape....
 working at DuPont
DuPont

E. I. du Pont de Nemours and Company is an United States chemical industry that was founded in July 1802 as a gunpowder mill by Eleuth?re Ir?n?e du Pont....
, discovered a simple method of synthesizing a crown ether when he was trying to prepare a complexing agent for divalent cations . His strategy entailed linking two catecholate groups through one hydroxyl
Hydroxyl

Hydroxyl in chemistry stands for a molecule consisting of an oxygen atom and a hydrogen atom connected by a covalent bond. The neutral form is a hydroxyl Radical and the hydroxyl anion is called a hydroxide....
 on each molecule. This linking defines a polydentate ligand that could partially envelop the cation and, by ionization
Ionization

Ionization is the physics process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions....
 of the phenolic hydroxyls, neutralize the bound dication. He was surprised to isolate a by-product
By-product

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 or service being produced....
 that strongly complexed potassium
Potassium

Potassium is a chemical element. It has the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash, hence the name....
 cation. Citing earlier work on the dissolution of potassium
Potassium

Potassium is a chemical element. It has the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash, hence the name....
 in 16-crown-4 , he realized that the cyclic polyethers represented a new class of complexing agents that were capable of binding alkali metal
Alkali metal

The alkali metals are a chemical series of chemical elements comprising Periodic table group of the periodic table: lithium , sodium , potassium , rubidium , caesium , and francium ....
 cations. 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
Organic synthesis

Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic_chemistry molecules can often contain a higher level of complexity compared to purely Inorganic_chemistry compounds, so the synthesis of organic compounds has developed into one of the most im...
, phase transfer catalyst
Phase transfer catalyst

A phase transfer catalyst or PTC in chemistry is a catalyst which facilitates the migration of a reactant in a heterogeneous system from one phase into another phase where reaction can take place....
s, and other emerging disciplines benefited from the discovery of crown ethers. Pedersen particularly popularized the dibenzo crown ethers.

Pedersen shared the 1987 Nobel Prize in Chemistry
Nobel Prize in Chemistry

The Nobel Prize in Chemistry is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of chemistry. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outstanding contributions in chemistry, Nobel Prize in Physics, Nobel Prize in Literature, Nobel Peace Pri...
 for the discovery of the synthetic routes to and binding properties, of crown ethers.

Affinity for cations

Apart from its high affinity for potassium cations, 18-crown-6
18-Crown-6

18-Crown-6 is an organic compound with the formula [C2H4O]6 and the IUPAC name of 1,4,7,10,13,16-hexaoxacyclooctadecane....
 can also bind to protonated amines and form very stable complexes in both solution and the gas phase. Some amino acids, such as lysine
Lysine

Lysine is an a-amino acid with the chemical formula HO2CCH4NH2. This amino acid is an essential amino acid, which means that humans cannot synthesize it....
, contain a primary amine
Amine

Amines are organic compounds and functional groups that contain a base nitrogen atom with a lone pair. Amines are derivative s of ammonia, wherein one or more hydrogen atoms are replaced by organic substituents such as alkyl and aryl groups....
 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-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

Some or all of the oxygen atoms in crown ethers can be replaced by nitrogens to form cryptand
Cryptand

Cryptands are a family of synthetic bi- and polycyclic multidentate ligands for a variety of cations. The Nobel Prize for Chemistry in 1987 was given to Donald J....
s. A well-known tetrazacrown is cyclen
Cyclen

Cyclen or 1,4,7,10-tetraazacyclododecane is a macrocycle and the aza analogue of the crown ether 12-crown-4. Derivatives of cyclen are larger cyclic polyamines but the repeating unit is always the same....
 in which there are no oxygens.

See also

  • Cryptand
    Cryptand

    Cryptands are a family of synthetic bi- and polycyclic multidentate ligands for a variety of cations. The Nobel Prize for Chemistry in 1987 was given to Donald J....


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