Crown Ethers
Title: Crown Ethers
Additional Names: Crown compounds
Literature References: Macrocyclic polyethers with the repeating unit (-CH2-CH2-O-)n, where n is greater than 2. Crown compounds with other heteroatoms (N, S, P) are known. Prepd by C. J. Pedersen, J. Am. Chem. Soc. 89, 2495, 7017 (1967). Described as "crown" ethers due to appearance of space-filling models and ability to "crown" cations. Proposed nomenclature lists non-ring substituents, number of atoms in ring, the class (crown), and the number of heteroatoms in the ring; e.g. dibenzo-18-crown-6. Bicyclic crowns are called cryptates. Crowns complex with cations and solubilize inorganic reagents in organic solvents. Selectivity results from the definite size of the crown cavity which admits only cations of corresponding ionic radii. The stability of complexes depends on the size of the cation and the size of the polyether ring. Chiral crowns are used in optical resolution of enantiomers. Reviews: D. J. Cram, J. M. Cram, Science 183, 803-809 (1974); eidem, Acc. Chem. Res. 11, 8-14 (1978); J. J. Christensen et al., Chem. Rev. 74, 351-384 (1974); G. W. Gokel, H. D. Durst, Synthesis 1976, 168-184; A. C. Knipe, J. Chem. Educ. 53, 618-622 (1976); V. Prelog, Pure Appl. Chem. 50, 893-904 (1978). Historical overview: C. J. Pedersen, Science 241, 536-540 (1988).
 
Derivative Type: 18-Crown-6
Additional Names: 1,4,7,10,13,16-Hexaoxacyclooctadecane
Molecular Formula: C12H24O6
Molecular Weight: 264.32
Percent Composition: C 54.53%, H 9.15%, O 36.32%
Literature References: Prepn: G. W. Gokel et al., Org. Synth. 57, 30 (1977).
Properties: Crystals from acetonitrile, mp 38-39.5°.
Melting point: mp 38-39.5°
 
Derivative Type: Dibenzo-18-crown-6
Additional Names: 2,3,11,12-Dibenzo-1,4,7,10,13,16-hexaoxacyclooctadeca-2,11-diene
Molecular Formula: C20H24O6
Molecular Weight: 360.40
Percent Composition: C 66.65%, H 6.71%, O 26.64%
Literature References: The first crown ether discovered. Prepn: C. J. Pedersen, US 3687978 (1972 to Du Pont).
Properties: White crystals from acetone, mp 164°, bp679 380-384°.
Melting point: mp 164°
Boiling point: bp679 380-384°
 
Derivative Type: Dicyclohexano-18-crown-6
Additional Names: Dicyclohexyl-18-crown-6
Molecular Formula: C20H36O6
Molecular Weight: 372.50
Percent Composition: C 64.49%, H 9.74%, O 25.77%
Properties: Approx LD in rats (mg/kg): 300 orally (Pedersen, 1967).
Toxicity data: Approx LD in rats (mg/kg): 300 orally (Pedersen, 1967)
 
CAUTION: Direct contact may cause eye and skin irritation (Pedersen, 1967).
Use: In organic synthesis as phase transfer reagents, catalysts and in sepn of enantiomers.

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