16.4 Crown Ethers

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16.4Crown Ethers
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Crown Ethers

• structure cyclic polyethers derived from
  repeating OCH2CH2 units
• properties form stable complexes with metal ions
  
• applications synthetic reactions involving
  anions

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18-Crown-6

• negative charge concentrated in cavity inside
  the molecule

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18-Crown-6
K

• forms stable Lewis acid/Lewis base complex with
  K

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Ion-Complexing and Solubility
KF
not soluble in benzene
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Ion-Complexing and Solubility
KF
benzene
add 18-crown-6
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Ion-Complexing and Solubility
F
K
benzene
18-crown-6 complex of K dissolves in benzene
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Ion-Complexing and Solubility
K
benzene
F carried into benzene to preserve
electroneutrality
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Application to organic synthesis

• Complexation of K by 18-crown-6 "solubilizes"
  potassium salts 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

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Example
KF
CH3(CH2)6CH2Br
CH3(CH2)6CH2F
18-crown-6
(92)
benzene
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16.5Preparation of Ethers

• Acid Catalyzed Dehydration of Alcohols
• Addition of Alcohols to Alkenes
• Williamson Ether Synthesis

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Symmetrical EthersAcid-Catalyzed Condensation
of Alcohols
2CH3CH2CH2CH2OH
CH3CH2CH2CH2OCH2CH2CH2CH3
(60)
Discussed earlier in Section 15.7
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Addition of Alcohols to Alkenes
H
(CH3)2CCH2 CH3OH
(CH3)3COCH3
tert-Butyl methyl ether
tert-Butyl methyl ether (MTBE) was produced on
ascale exceeding 15 billion pounds per year in
the U.S.during the 1990s. It is an effective
octane booster ingasoline, but contaminates
ground water if allowed toleak from storage
tanks. Further use of MTBE is unlikely.
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16.6The Williamson Ether Synthesis

• Think SN2!
• primary alkyl halide alkoxide nucleophile

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Example
CH3CH2CH2CH2ONa CH3CH2I
CH3CH2CH2CH2OCH2CH3 NaI
(71)
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Another Example
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Origin of Reactants
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What happens if the alkyl halide is not primary?
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16.7Reactions of EthersA Review and a Preview
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Summary of reactions of ethers

• No reactions of ethers encountered to this
  point.
• Ethers are relatively unreactive.
• Their low level of reactivity is one reason why
  ethers are often used as solvents in chemical
  reactions.
• Ethers oxidize in air to form explosive
  hydroperoxides and peroxides.

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Summary of Ether Reactions

• Ethers are unreactive to dilute acid, base, or
  reducing agents
• Ethers undergo oxidation only under extreme
  conditions
• combustion
• peroxide formation
• Ethers can be cleaved by strong acids (HI, HBr)

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16.8Acid-Catalyzed Cleavage of Ethers
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Example
HBr
CH3CHCH2CH3
CH3CHCH2CH3

CH3Br
heat
OCH3
Br
(81)
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Mechanism
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Cleavage of Cyclic Ethers
HI
ICH2CH2CH2CH2I
150C
(65)
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Mechanism