16.4 Crown Ethers The strength of this bonding depends on the kind of ether. Simple ethers form relatively weak complexes with metal ions. A major advance in the area came in 1967 when Charles J. Pedersen of Du Pont described the preparation and properties of a class of Pedersen was a corecipient polyethers that form much more stable complexes with metal ions than do simple ethers. of the 1987 Nobel Prize in Pedersen prepared a series of macrocyclic polyethers, cyclic compounds contain- chemistry ing four or more oxygens in a ring of 12 or more atoms. He called these compounds crown ethers, because their molecular models resemble crowns. Systematic nomencla ture of crown ethers is somewhat cumbersome. and so pedersen devised a shorthand description whereby the word"crown"is preceded by the total number of atoms in the ring and is followed by the number of oxygen ator 12-Crown-4 18-Crown-6 he parent i 12-Crc and 18-crown-6 are a cyclic tetramer and hexamer, respectively, of repeat CH2- units; they are polyethers based on ethylene glycol(HOCH2CHrOH) Acohol PROBLEM 16.4 What organic compound mentioned earlier in this chapter is a yclic dimer of -OCH2CH2--units The metal-ion complexing properties of crown ethers are clearly evident in their effects on the solubility and reactivity of ionic compounds in nonpolar media. Potassium fluoride(Kf)is ionic and practically insoluble in benzene alone, but dissolves in it when 18-crown-6 is present. The reason for this has to do with the electron distribution of 18- crown-6 as shown in Figure 16.2a. The electrostatic potential surface consists of essen- tially two regions: an electron-rich interior associated with the oxygens and a hydrocarbon- FIGURE 16.2 like exterior associated with the CH2 groups. When KF is added to a solution of 18- electrostatic potential map crown-6 in benzene, potassium ion(K )interacts with the oxygens of the crown ether of 18-crown-6. The region of to form a Lewis acid-Lewis base complex. As can be seen in the space-filling model of highest electron density and their lone pairs. The outer periphery of the crown ether(blue)is relatively non polar(hydrocarbon-like)and soluble in nonpolar solvent fom o benzene.(b)Aspace crown ether where it is bound Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteThe strength of this bonding depends on the kind of ether. Simple ethers form relatively weak complexes with metal ions. A major advance in the area came in 1967 when Charles J. Pedersen of Du Pont described the preparation and properties of a class of polyethers that form much more stable complexes with metal ions than do simple ethers. Pedersen prepared a series of macrocyclic polyethers, cyclic compounds contain￾ing four or more oxygens in a ring of 12 or more atoms. He called these compounds crown ethers, because their molecular models resemble crowns. Systematic nomencla￾ture of crown ethers is somewhat cumbersome, and so Pedersen devised a shorthand description whereby the word “crown” is preceded by the total number of atoms in the ring and is followed by the number of oxygen atoms. 12-Crown-4 and 18-crown-6 are a cyclic tetramer and hexamer, respectively, of repeat￾ing ±OCH2CH2± units; they are polyethers based on ethylene glycol (HOCH2CH2OH) as the parent alcohol. PROBLEM 16.4 What organic compound mentioned earlier in this chapter is a cyclic dimer of ±OCH2CH2± units? The metal–ion complexing properties of crown ethers are clearly evident in their effects on the solubility and reactivity of ionic compounds in nonpolar media. Potassium fluoride (KF) is ionic and practically insoluble in benzene alone, but dissolves in it when 18-crown-6 is present. The reason for this has to do with the electron distribution of 18- crown-6 as shown in Figure 16.2a. The electrostatic potential surface consists of essen￾tially two regions: an electron-rich interior associated with the oxygens and a hydrocarbon￾like exterior associated with the CH2 groups. When KF is added to a solution of 18- crown-6 in benzene, potassium ion (K) interacts with the oxygens of the crown ether to form a Lewis acid-Lewis base complex. As can be seen in the space-filling model of O O O O 12-Crown-4 O O O O O O 18-Crown-6 16.4 Crown Ethers 623 Pedersen was a corecipient of the 1987 Nobel Prize in chemistry. (a) (b) FIGURE 16.2 (a) An electrostatic potential map of 18-crown-6. The region of highest electron density (red ) is associated with the negatively polarized oxygens and their lone pairs. The outer periphery of the crown ether (blue) is relatively non￾polar (hydrocarbon-like) and causes the molecule to be soluble in nonpolar solvents such as benzene. (b) A space- filling model of the complex formed between 18-crown-6 and potassium ion (K). K fits into the cavity of the crown ether where it is bound by Lewis acid-Lewis base interaction with the oxygens. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website