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332 CHAPTER 8 • INTRODUCTION TO ALKYL HALIDES, ALCOHOLS, ETHERS, THIOLS, AND SULFIDES

However, most epoxides are named substitutively as derivatives of oxirane. The atoms of the epoxide ring are numbered consecutively, with the oxygen receiving the number 1 regardless of the substituents present.

1 O CH3 H2CC$ $ 3 2 L L CH3 2,2-dimethyloxirane

PROBLEMS 8.6 Draw the structure of each of the following compounds. (a) ethyl propyl ether (b) dicyclohexyl ether (c) dicyclopentyl sulfide (d) tert-butyl isopropyl sulfide (e) allyl benzyl ether (f) phenyl vinyl ether (g) (2R,3R)-2,3-dimethyloxirane (h) 5-(ethylthio)-2-methylheptane 8.7 Give a substitutive name for each of the following compounds.

(a) (CH3)3C O CH3 (b) CH3CH2 O CH2CH2 OH (c) L L (d)L L L CH3OCH2 H S $CCA )

H) $ CH2CH2 OH L 8.8 (a) A chemist used the name 3-butyl-1,4-dioxane in a paper. Although the name unambigu- ously describes a structure, what should the name have been? Explain. (b) Give the structure of 2-butoxyethanol, which is an ingredient in whiteboard cleaner and kitchen cleaning sprays.

8.2 STRUCTURES

In all of the compounds covered in this chapter, the bond angles at carbon are very nearly tetrahedral. For example, in the simple methyl derivatives (the methyl halides, methanol, methanethiol, dimethyl ether, and dimethyl sulfide) the H C H bond angle in the methyl group does not deviate more than a degree or so from 109.5L. InL an alcohol, thiol, ether, or sul- fide, the bond angle at oxygen or sulfur further defines the shape° of the molecule. You learned in Sec. 1.3B that the shapes of such molecules can be predicted by thinking of an unshared electron pair as a bond without an atom at the end. This means that the oxygen or sulfur has four “groups”: two electron pairs and two alkyl groups or hydrogens. These molecules are therefore bent at oxygen and sulfur, as you can see from the structures in Fig. 8.1. The angle at sulfur is generally found to be closer to 90 than the angle at oxygen. One reason for this trend is that the unshared electron pairs on sulfur° occupy orbitals derived from energy level 3 that take up more space than those on oxygen, which are derived from level 2. The repulsion between these unshared pairs and the electrons in the chemical bonds forces the bonds closer together than they are on oxygen. The lengths of bonds between carbon and other atoms follow the trends discussed in Sec. 1.3B. Within a column of the periodic table, bonds to atoms of higher atomic number are longer. Thus, the C S bond of methanethiol is longer than the C O bond of methanol (see Fig. 8.1 and Table 8.1).L Within a row, bond lengths decrease toward Lhigher atomic number (that is, to the right). Thus, the C O bond in methanol is longer than the C F bond in methyl fluoride (see Table 8.1); similarly,L the C S bond in methanethiol is longerL than the C Cl bond in methyl chloride. L L 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 333

8.3 EFFECT OF MOLECULAR POLARITY AND HYDROGEN BONDING ON PHYSICAL PROPERTIES 333

O1 0.96 Å S 1.335 Å 1.426 Å 1 21 H 1.82 Å H3C 109° 96° H H3C

O 1.413 Å S 1.803 Å 21 21 H3C 111.4° CH3 99° H3C CH3

Figure 8.1 Bond lengths and bond angles in a simple alcohol, thiol, ether, and sulfide. Bond angles at sulfur are smaller than those at oxygen, and bonds to sulfur are longer than the corresponding bonds to oxygen.

Increasing electronegativity Increasing atomic radius TABLE 8.1 Bond Lengths (in Angstroms) in Some Methyl Derivatives

H3 C CH3 H3 C NH2 H3 C OHH3 C F 1.536L 1.474L 1.426L 1.391L

H3 C SHH3 C Cl 1.82L 1.781L

H3 C Br 1.939L

H3 C I 2.129L

PROBLEMS 8.9 Using the data in Table 8.1, estimate the carbon–selenium bond length in H3C Se CH3. 8.10 From the data in Fig. 8.1, tell which bonds have the greater amount of p characterL (Sec.L 1.9B): C O bonds or C S bonds. Explain. L L

EFFECT OF MOLECULAR POLARITY AND 8.3 HYDROGEN BONDING ON PHYSICAL PROPERTIES

A. Boiling Points of Ethers and Alkyl Halides Most alkyl halides, alcohols, and ethers are polar molecules; that is, they have permanent di- pole moments (Sec. 1.2D). The following examples are typical.

H3C F H3C Cl H3C OH H3C O CH3 H3C CH2 CH3 methyl fluorideL methyl chlorideL methanolL dimethylL L ether LpropaneL dipole moment 1.82 D 1.94 D 1.7 D 1.31 D 0.08 D