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Crown Ethers
Crown Ethers
structure
cyclic polyethers derived from repeating
—OCH2CH2— units
properties
form stable complexes with metal ions
applications
synthetic reactions involving anions
18-Crown-6
O
O
O
O
O
O
negative charge concentrated in cavity inside
the molecule
18-Crown-6
O
O
O
O
O
O
negative charge concentrated in cavity inside
the molecule
18-Crown-6
O
O
O
K+
O
O
O
forms stable Lewis acid/Lewis base complex
with K+
18-Crown-6
O
O
O
K+
O
O
O
forms stable Lewis acid/Lewis base complex
with K+
Ion-Complexing and Solubility
K+F–
not soluble in benzene
Ion-Complexing and Solubility
O
O
O
K+F–
O
O
benzene
O
add 18-crown-6
Ion-Complexing and Solubility
O
O
O
O
F–
O
O
K+
O
O
benzene
O
O
18-crown-6 complex of K+ dissolves
in benzene
O
O
Ion-Complexing and Solubility
O
O
O
O
O
O
K+
O
O
benzene
O
F– carried into benzene
to preserve electroneutrality
O
O
O
+ F–
Application to organic synthesis
Complexaton of K+ by 18-crown-6 "solubilizes"
salt in benzene
Anion of salt is in a relatively unsolvated state
in benzene (sometimes referred to as a
"naked anion")
Unsolvated anion is very reactive
Only catalytic quantities of 18-crown-6 are
needed
Polyether Antibiotics
Example
KF
CH3(CH2)6CH2Br
18-crown-6
benzene
CH3(CH2)6CH2F
(92%)
Polyether Antibiotics
Monensin
Polyether Antibiotics
Monensin
Polyether Antibiotics
Monensin B
Monensin A
O
O
O
O
O
O
Na
H
O
HO
H
O
H
O
O
O
O
O
O
O
Na
H
O
HO
H
O
H
O
O
O