18-Crown-6

February 03, 2025
I form stable complexes with alkali metals.
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18-Crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) is a cyclic polyether and a strong complexing agent. The 18 in its name indicates the number of atoms in the ring; the 6 denotes the number of oxygen atoms.

In 1967, Charles J. Pedersen at DuPont (Wilmington, DE) reported the preparation of a series of 33 crown ethers, including 18-crown-6. To synthesize almost all of the crown ethers, he began with vicinal aromatic diols, which he oligomerized by treating them with sodium hydroxide and a dichloroether compound.

The exception was the synthesis of unsubstituted 18-crown-6, which Pedersen made via the reaction of 17-chloro-3,6,9,12,15-pentaoxaheptadecanol, 1,2-dimethoxyethane1, and potassium t-butoxide2, albeit with a yield of only ≈2%. The author did not provide much information about 18-crown-6, concentrating predominantly on dibenzo-18-crown-63 and its hydrogenated derivative, dicyclohexyl-18-crown-64. Pedersen shared the 1987 Nobel Prize in Chemistry for his pioneering work in crown ethers.

In 1974, two reviews of crown ether chemistry appeared. The first was by Pedersen’s fellow Nobel Prize winner Donald J. Cram* and Jane M. Cram at the University of California, Los Angeles, titled “Host–Guest Chemistry”. In the second, James M. Christensen*, Delbert J. Eatough, and Reed M. Izatt at Brigham Young University (Provo, UT) concentrated on the synthesis of crown ethers and their ability to bind inorganic cations.

In 1972, R. N. Greene at DuPont reported synthetic methods for making 18-crown-6 in yields as high as 93%. Greene also demonstrated that 18-crown-6 is a strong complexing agent for alkali metal cations, with stability constants for its sodium and potassium complexes greater than those measured by Pedersen for dicyclohexyl-18-crown-6.

In 1987, Jerry L. Atwood and co-workers at the University of Alabama (Tuscaloosa) reported the encapsulation of the hydronium cation (H3O+) by 18-crown-6 via its reaction with water and hydrogen chloride. When the reaction takes place in the presence of aromatic solvents such as benzene or toluene, the complex forms a liquid clathrate with the solvent.

And in 2021, Timofey P. Martyanov at the Russian Academy of Sciences (Chernogolovka), Sergey P. Gromov at the Russian Academy of Sciences (Moscow) and M. V. Lomonosov Moscow State University, and colleagues at these and other Russian institutions described more intricate complexes of 18-crown-6. They synthesized a stilbene derivative of 18-crown-6, complexed it with styryl dyes that contained an alkylammonium group, and then irradiated the complex to produce a 2 + 2 cross-photocycloaddition reaction that formed an unusual combination of a crown ether attached to a tetraarylcyclobutane moiety.

1. CAS Reg. No. 110-71-4.
2. CAS Reg. No. 865-47-4.
3. CAS Reg. No. 14187-32-7.
4. CAS Reg. No. 16069-36-6.

18-Crown-6 hazard information*

Hazard class**GHS code and hazard statement
Acute toxicity, oral, category 4H302—Harmful if swallowedChemical Safety Warning
Acute toxicity, dermal, category 5H313—May be harmful in contact with skinChemical Safety Warning
Skin corrosion/irritation, category 2H315—Causes skin irritationChemical Safety Warning
Serious eye damage, category 2AH319—Causes serious eye irritationChemical Safety Warning
Specific target organ toxicity, single exposure, respiratory tract irritation, category 3H335—May cause respiratory irritationChemical Safety Warning

*Compilation of multiple safety data sheets.
**Globally Harmonized System (GHS) of Classification and Labeling of Chemicals. Explanation of pictograms.

Molecule in the News

Eosin Y1 is a triarylmethane dye that is used widely in cosmetics, inks, woolen fabrics, and histology stains. Its first appearance in the chemical literature came in 1880, when P. Phillips Bedson at Owens College (now Victoria University of Manchester, UK) described derivatives of phenylacetic acid.

In recent years, eosin Y was at the center of the mystery surrounding the fading of violet colors to blue in Vincent van Gogh’s classic 1889 painting Irises. Eosin Y is used to make a red pigment called geranium lake, which van Gogh mixed with blue pigments to make his violet paints. In 2020, Aldo Romani at the University of Perugia (Italy) and his colleagues there and at other European institutions reported that photo-oxidation causes geranium lake to degrade over time. The loss of the red pigment left the flowers in van Gogh’s painting looking blue.

Irises is in the permanent collection of the J. Paul Getty Museum (Los Angeles). In 2024, museum personnel created a version of the painting in which they restored the original violet colors.

1. CAS Reg. No. 17372-87-1; SciFindern name: spiro{isobenzofuran-1(3H),9′-[9H]xanthen}-3-one, 2′,4′,5′,7′-tetrabromo-3′,6′-dihydroxy-, sodium salt.

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18-Crown-6 fast facts

CAS Reg. No.17455-13-9
SciFindern name1,4,7,10,13,16-Hexaoxacyclooctadecane
Empirical
formula
C12H24O6
Molar mass264.32 g/mol
AppearanceWhite hygroscopic crystals
Melting point42–45 ºC
Boiling point116 ºC (0.2 Torr)
Water
solubility
75 g/L
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