Crown ether

Crown ether is a compound with special complexation properties developed since the 1970s. It is a crown-like compound, also known as a "crown compound". It has a macrocyclic polyether of [CH2CH2Y]n unit in its molecular structure, of which Y Is a hetero atom such as O, N, S or P, n>2. Named for its three-dimensional shape like a crown. The narrowly defined crown ether refers only to macrocyclic polyethers wherein Y is O. Such as 18-crown-6, dicyclohexyl-18-crown-6 and the like. The habitual naming order is: acyclic substituent, number of atoms in the ring, class name (crown), and number of heteroatoms in the ring. Such as dibenzo-18-crown-6. The saturated crown ether is a colorless viscous liquid or a low melting solid, and the aromatic ether containing crown ether is a colorless crystal. The aromatic ring-containing crown ether is hardly soluble in water, alcohol and common organic solvents at room temperature, and is more soluble in dichloromethane, chloroform, pyridine, formic acid, etc., and the cyclohexyl-containing crown ether has a higher solubility than the corresponding benzo-crown ether. Soluble in water, alcohol, aromatics. Also soluble in petroleum ether. Generally, it has good thermal stability, but its ether bonds are easily oxidized at high temperatures and melting. The aromatic ring in the molecule can undergo bromination, nitration, esterification and ozonation, and can also be polycondensed into a high polymer with formaldehyde. A complex called a pocket can be formed with the positive ion, so that many salts are soluble in the non-polar organic solvent, allowing many unfavorable reactions to proceed. The stronger the lipophilicity, the greater the catalytic activity and the selectivity due to the fact that the fixed space in the molecule can only accommodate ions of corresponding size. The stability of the pocket formed by the crown ether and the positive ion mainly depends on the relative size of the polyether ring and the positive ion. Since the cavity is formed by electrostatic force, the stability constant is generally small, so it can be added by water or thin. It is easy to use because it is decomposed by acid or heat decomposition. Method: generally can be prepared by Williamson ether synthesis method, but in order to prevent chain polymerization, high dilution method, step condensation method template reaction can be adopted. Currently the most widely used and most readily available three crown ethers: 18-crown-6, dibenzo-18-crown-6 and dicyclohexyl-18-crown-6. An important characteristic of crown ethers is that they are positive with metals. Ions form complexes and are used as phase transfer catalysts in organic synthesis.