Jun. 25, 2021
Crown ether is a newly developed compound with special complexing properties after the 1970s. Because of its crown-like shape, it is also called "crown compound". It is a type of [CH2CH2Y] n (n>Crown ether in a narrow sense refers to the macrocyclic polyether Y is O, such as 18-crown-6 and dicyclohexyl-18-crown-6. They are named after the order of non-ring substituents, number of ring atoms, class name (crown) and number of ring hetero atoms are named according to common nomenclature, such as dibenzo-18-crown-6. Saturated crown ether is a colorless viscous liquid or low melting solid while aromatic ring-containing crown ether is colorless crystal. Crown ethers containing aromatic rings are insoluble in water, alcohol and common organic solvents at room temperature, but are more soluble in dichloromethane, chloroform, pyridine and formic acid. Crown ethers containing cyclohexyl groups are more soluble in water, alcohols, aromatic hydrocarbons and petroleum ethers than the corresponding benzo crown ethers.
Crown ethers generally have good thermal stability. However, its ether bonds are easily oxidized when melted or at high temperatures. The aromatic ring in the molecule can participate in bromination, nitration, esterification and ozonation, and can also form a polymer with formaldehyde during condensation. Crown ether can form a complex substance called cryptoester, which makes many salts soluble in non-polar organic solvents, so many difficult reactions can be carried out easily. The stronger the lipophilicity, the stronger the catalytic activity. In addition, the reaction is selective because its fixed space can only accommodate ions of the corresponding size. The stability of the polyether ring formed by the crown ether and the positive ion mainly depends on the relative size of the polyether ring and the positive ion; because the compound is formed by electrostatic interaction, the stability constant is generally small. What makes the reaction convenient is that the complex is easily decomposed by adding water, dilute acid or thermal decomposition.
Preparation method: generally use Williamson (Williamson) ether synthesis. But in order to prevent chain polymerization, it is recommended to use high dilution method, hierarchical condensation and template reaction.
At present, the most widely used and easily available crown ethers are 18-crown-6, dibenzo-18-crown-6 and dicyclohexyl-18-crown-6. The formation of complexes between crown ethers and metal cations is an important feature of crown ethers, which can be used for phase transfer catalysis in organic synthesis.
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