ETHERS, EPOXIDES. AND SULFIDES

Descriptive Passage and Interpretive Problems 16 Epoxide Rearrangements and the NIH Shift 721 

3 Acid-Catalyzed Ring Opening of Ethylene Oxide 704 

IN CONTRAST TO ALCOHOLS with their rich chemical reactivity, ethers (compounds containing a C—O—C unit) undergo relatively few chemical reactions. As you saw when we discussed Grignard reagents in Chapter 14 and lithium aluminum hydride reductions in Chapter 15, this lack of reactivity of ethers makes them valuable as solvents in a number of synthetically important transformations. In the present chapter you will learn of the conditions in which an ether linkage acts as a functional group, as well as the methods by which ethers are prepared. 

Unlike most ethers, epoxides (compounds in which the C—O—C unit forms a three-membered ring) are very reactive substances. The principles of nucleophilic substitution are important in understanding the preparation and properties of epoxides. 

Sulfides (RSR ) are the sulfur analogs of ethers. Just as in the preceding chapter, where we saw that the properties of thiols (RSH) are different from those of alcohols, we will explore differences between sulfides and ethers in this chapter. 

The four solvents decrease in polarity in this order water, ethanol, ethyl ether, and dichloromethane. The three solutes decrease in polarity in this order sodium acetate, 2-naphthol, and naphthalene. The guiding principle in determining solubility is, Like dissolves like. Compounds of similar polarity will dissolve (in) each other. Thus, sodium acetate will dissolve in water, will dissolve only slightly in ethanol, and will be virtually insoluble in ethyl ether and dichloromethane. 2-Naphthol will be insoluble in water, somewhat soluble in ethanol, and soluble in ether and dichloromethane. Naphthalene will be insoluble in water, partially soluble in ethanol, and soluble in ethyl ether and dichloromethane. (Actual solubilities are difficult to predict, but you should be able to predict trends.) 

Oxygen shares one of its electron pairs with aluminum oxygen is the Lewis base, and aluminum is the Lewis acid. An oxygen atom with three bonds and one unshared pair has a positive formal charge. An aluminum atom with four bonds has a negative formal charge. 

The crown ether has two effects on KMn04 first, it makes KMn04 much more soluble in benzene second, it holds the potassium ion tightly, making the permanganate more available for reaction. Chemists call this a naked anion because it is not complexed with solvent molecules. 

Please see the note on p. 13 of this Solutions Manual regarding placement of position numbers. 

14-4 lUPAC name first then common name (see Appendix 1 in this Solutions Manual for a summary of lUPAC nomenclature) 

Ethers are named, in substitutive lUPAC nomenclature, as alkoxy derivatives of alkanes. Functional class lUPAC names of ethers are derived by listing the two alkyl groups in the general structure ROR in alphabetical order as separate words, and then adding the word ether at the end. When both alkyl groups are the same, the prefix di- precedes the name of the alkyl group. 

Substitutive lUPAC name Functionai ciass lUPAC name  

Recall from Section 6.18 that epoxides may be named as -epoxy derivatives of alkanes in substitutive lUPAC nomenclature. 

As with other functional groups, we will discuss how ethers are formed and how they react. Ethers (other than epoxides) are relatively unreactive, however, and they are not frequently used as synthetic intermediates. Because they are stable with many types of reagents, ethers are commonly used as solvents for organic reactions. In this chapter, we consider the properties of ethers and how these properties make ethers such valuable solvents for organic reactions. 

Like water, ethers have a bent structure, with an sp hybrid oxygen atom giving a nearly tetrahedral bond angle. In water, the nonbonding electrons compress the H—O—H bond angle to 104.5°, but in a typical ether, the bulk of the alkyl groups enlarges the 

Although ethers lack the polar hydroxyl group of alcohols, they are still strongly polar compounds. The dipole moment of an ether is the vector sum of two polar C—O bonds, with a substantial contribution from the two lone pairs of electrons. Table 14-1 compares the dipole moments of dimethyl ether, diethyl ether, and tetrahydrofuran (THE) with those of alkanes and alcohols of similar molecular weights. An ether such as THE provides a strongly polar solvent without the reactivity of a hydroxyl group. 

Diethyl ether was found to be a much safer anesthetic than chloroform. Like chloroform, ether is more soluble in fatty tissue than in water, so it passes quickly into the central nervous system and takes effect quickly. Ether is also volatile, making it easy to administer. But ether is much less toxic than chloroform because ether degrades to ethanol, which the body can oxidize. 

An ether inhaler used by William Morton in the first public demonstration of ether anesthesia on October 16, 1846, at Massachusetts General Hospital. 

1 (b) Oxirane is the lUPAC name for ethylene oxide. A chloromethyl group (ClCHj—) is attached 

This compound is more commonly known as epichlorohydrin. 

2-Epoxybutane and tetrahydrofuran both have the molecnlar formula C4H8O—that is, they are constitutional isomers—and so it is appropriate to compare their heats of combustion directly. Angle strain from the three-membered ring of 1,2-epoxybutane canses it to have more internal energy than tetrahydrofnran, and its combustion is more exothermic. 

3 An ether can function only as a proton acceptor in a hydrogen bond, but an alcohol can be either a proton acceptor or a donor. The only hydrogen bond possible between an ether and an alcohol is therefore the one shown  

4 The compound is 1,4-dioxane it has a six-membered ring and two oxygens separated by CHj—CH2 units. 

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