16.4 Crown Ethers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

structure cyclic polyethers derived from repeating —OCH 2 CH 2 — units properties form stable complexes with metal ions applications synthetic reactions involving anions Crown Ethers

18-Crown-6 negative charge concentrated in cavity inside the molecule O OO O O O

O OO O O O 18-Crown-6 forms stable Lewis acid/Lewis base complex with K + K+K+

not soluble in benzene Ion-Complexing and Solubility K+F–K+F–

K+F–K+F– add 18-crown-6 benzene O OO O O O

Ion-Complexing and Solubility 18-crown-6 complex of K + dissolves in benzene benzene F – O OO O O O O OO O O O K+K+

Ion-Complexing and Solubility + F–F– F – carried into benzene to preserve electroneutrality benzene O OO O O O O OO O O O K+K+

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.

Example CH 3 (CH 2 ) 6 CH 2 Br KF 18-crown-6 benzene CH 3 (CH 2 ) 6 CH 2 F (92%)

16.5 Preparation of Ethers

Acid-Catalyzed Condensation of Alcohols* 2 CH 3 CH 2 CH 2 CH 2 OH H 2 SO 4, 130°C CH 3 CH 2 CH 2 CH 2 OCH 2 CH 2 CH 2 CH 3 (60%) *Discussed earlier in Section 15.7

H+H+ (CH 3 ) 2 C=CH 2 + CH 3 OH(CH 3 ) 3 COCH 3 tert-Butyl methyl ether tert-Butyl methyl ether (MTBE) was produced on a scale exceeding 15 billion pounds per year in the U.S. during the 1990s. It is an effective octane booster in gasoline, but contaminates ground water if allowed to leak from storage tanks. Further use of MTBE is unlikely. Addition of Alcohols to Alkenes

Think S N 2! primary alkyl halide + alkoxide nucleophile 16.6 The Williamson Ether Synthesis

(71%) CH 3 CH 2 CH 2 CH 2 ONa + CH 3 CH 2 I CH 3 CH 2 CH 2 CH 2 OCH 2 CH 3 + NaI Example

Another Example + CH 3 CHCH 3 ONa CH 2 Cl (84%) CH 2 OCHCH 3 CH 3 Alkyl halide must be primary or methyl Alkoxide ion can be derived from methyl, primary, secondary, or tertiary alcohol.

CH 3 CHCH 3 OHOH Na CH 2 OH HCl CH 2 OCHCH 3 CH 3 CH 2 Cl+ CH 3 CHCH 3 ONa (84%) Origin of Reactants

What happens if the alkyl halide is not primary? CH 2 ONa + CH 3 CHCH 3 Br CH 2 OH + H2CH2C CHCH 3 Elimination by the E2 mechanism becomes the major reaction pathway.

16.7 Reactions of Ethers: A Review and a Preview

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. Summary of reactions of ethers

16.8 Acid-Catalyzed Cleavage of Ethers

HBr CH 3 CHCH 2 CH 3 OCH 3 CH 3 Br + (81%) CH 3 CHCH 2 CH 3 Br heat Example

Br – CH 3 CH 3 CHCH 2 CH 3 O H Br CH 3 CH 3 CHCH 2 CH 3 O H + Mechanism CH 3 CHCH 2 CH 3 Br HBr CH 3 Br CH 3 CHCH 2 CH 3 O H

HIHI 150°C ICH 2 CH 2 CH 2 CH 2 I (65%) O Cleavage of Cyclic Ethers

O HIHI H O + I – ICH 2 CH 2 CH 2 CH 2 I HI H O I Mechanism