16.5 Preparation of Ethers omplex(Figure 16.2b), K, with an ionic radius of 266 pm, fits comfortably within 260-320 pm internal cavity of 18-crown-6. Nonpolar CH2 groups dominate the outer surface of the complex, mask its polar interior, and permit the complex to dissolve in nonpolar solvents. Every K that is carried into benzene brings a fluoride ion with it, resulting in a solution containing strongly complexed potassium ions and relatively unsolvated fluoride ions 18-Crown-6 18-Crown-6-potassium fluoride fluoride complex (in solution) In media such as water and alcohols, fluoride ion is strongly solvated by hydro- gen bonding and is neither very basic nor very nucleophilic. On the other hand, the poorly solvated, or"naked, " fluoride ions that are present when potassium fluoride dis solves in benzene in the presence of a crown ether are better able to express their anionic reactivity. Thus, alkyl halides react with potassium fluoride in benzene containing 18 crown-6, thereby providing a method for the preparation of otherwise difficultly acces sible alkyl fluorides CH3(CH2)CH,Br CH3(CH2)6CH,F The reaction proceeds in the -Bromooctane I-Fluorooctane(92%) direction indicated because a C-F bond is much stronger No reaction is observed when the process is carried out under comparable conditions but than a C-Br bond with the crown ether omitted Catalysis by crown ethers has been used to advantage to increase the rate of many organic reactions that involve anions as reactants. Just as important, though, is the increased understanding that studies of crown ether catalysis have broug our knowl- edge of biological processes in which metal ions, including Na and K, are transported through the nonpolar interiors of cell membranes. 16.5 PREPARATION OF ETHERS Because they are widely used as solvents, many simple dialkyl ethers are commercially available. Diethyl ether and dibutyl ether, for example, are prepared by acid-catalyzed condensation of the corresponding alcohols, as described earlier in Section 15.7 CHa,O 0H 00>CH; CH,CH,CH,OCH, CH,CH, CH3+H,O 1-Butanol Dibutyl ether(60%o) In general, this method is limited to the preparation of symmetrical ethers in which both alkyl groups are primary Isopropyl alcohol, however, is readily available at low cost and gives high enough yields of disopropyl ether to justify making( CH3)2CHOCH(CH3)2 by this method on an industrial scale Back Forward Main MenuToc Study Guide ToC Student o MHHE Websitethis complex (Figure 16.2b), K, with an ionic radius of 266 pm, fits comfortably within the 260–320 pm internal cavity of 18-crown-6. Nonpolar CH2 groups dominate the outer surface of the complex, mask its polar interior, and permit the complex to dissolve in nonpolar solvents. Every K that is carried into benzene brings a fluoride ion with it, resulting in a solution containing strongly complexed potassium ions and relatively unsolvated fluoride ions. In media such as water and alcohols, fluoride ion is strongly solvated by hydro￾gen bonding and is neither very basic nor very nucleophilic. On the other hand, the poorly solvated, or “naked,” fluoride ions that are present when potassium fluoride dis￾solves in benzene in the presence of a crown ether are better able to express their anionic reactivity. Thus, alkyl halides react with potassium fluoride in benzene containing 18- crown-6, thereby providing a method for the preparation of otherwise difficultly acces￾sible alkyl fluorides. No reaction is observed when the process is carried out under comparable conditions but with the crown ether omitted. Catalysis by crown ethers has been used to advantage to increase the rate of many organic reactions that involve anions as reactants. Just as important, though, is the increased understanding that studies of crown ether catalysis have brought to our knowl￾edge of biological processes in which metal ions, including Na and K, are transported through the nonpolar interiors of cell membranes. 16.5 PREPARATION OF ETHERS Because they are widely used as solvents, many simple dialkyl ethers are commercially available. Diethyl ether and dibutyl ether, for example, are prepared by acid-catalyzed condensation of the corresponding alcohols, as described earlier in Section 15.7. In general, this method is limited to the preparation of symmetrical ethers in which both alkyl groups are primary. Isopropyl alcohol, however, is readily available at low cost and gives high enough yields of diisopropyl ether to justify making (CH3)2CHOCH(CH3)2 by this method on an industrial scale. 2CH3CH2CH2CH2OH 1-Butanol H2SO4 130°C CH3CH2CH2CH2OCH2CH2CH2CH3 Dibutyl ether (60%) H2O Water CH3(CH2)6CH2Br 1-Bromooctane KF, benzene, 90°C 18-crown-6 CH3(CH2)6CH2F 1-Fluorooctane (92%) O O O O O O 18-Crown-6 benzene KF Potassium fluoride (solid) O O O O O O 18-Crown-6-potassium fluoride complex (in solution) F K 16.5 Preparation of Ethers 625 The reaction proceeds in the direction indicated because a C±F bond is much stronger than a C±Br bond. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website