4.5 Physical Properties of Alcohols and Alkyl Halides: Intermolecular Forces FIGURE 4. 4 Hydrogen ding in ethanol involves he oxygen of one molecule and the proton of an-OH forces proton involved must be bonded to an electronegative element, usually oxygen or nitro- gen. Protons in C-H bonds do not participate in hydrogen bonding. Thus fluoroethane, even though it is a polar molecule and engages in dipole-dipole attractions, does not form hydrogen bonds and, therefore, has a lower boiling point than ethanol Hydrogen bonding can be expected in molecules that have -OH or -NH groups ydrogen bonds between but their effects can be significant. More than other dipole-dipole attractive forces, inter- than those between -NH molecular hydrogen bonds are strong enough to impose a relatively high degree of struc- the boiling points of water tural order on systems in which they are possible. As will be seen in Chapter 27, the (H, 0, 1000 and ammonia three-dimensional structures adopted by proteins and nucleic acids, the organic mole-(NH3, -33C)demonstrates cules of life, are dictated by patterns of hydrogen bond PROBLEM 4.5 The constitutional isomer of ethanol, dimethyl ether(CH3OCH3) is a gas at room temperature. Suggest an explanation for this observation Table 4. 1 lists the boiling points of some representative alkyl halides and For a discussion co nen comparing the boiling points of related compounds as a function of the alkyl the boiling point behavior of ue of does with alkanes p.62-64. TABLE 4. 1 Boiling Points of Some Alkyl Halides and Alcohols Functional group X and boiling point, C(1 atm) Name of alkyl group Formula X=F X=C X=Br X= X= Oh Methyl CHEX 78 24 42 Ethi CH3 CH2X 38 CH3CH2 CH2X 47 71 103 CH3(CH2)3CH2X Hexu CH3(CH2)4CH2X92134155180 157 Back Forward Main Menu Study Guide ToC Student OLC MHHE Websiteproton involved must be bonded to an electronegative element, usually oxygen or nitro￾gen. Protons in C±H bonds do not participate in hydrogen bonding. Thus fluoroethane, even though it is a polar molecule and engages in dipole–dipole attractions, does not form hydrogen bonds and, therefore, has a lower boiling point than ethanol. Hydrogen bonding can be expected in molecules that have ±OH or ±NH groups. Individual hydrogen bonds are about 10–50 times weaker than typical covalent bonds, but their effects can be significant. More than other dipole–dipole attractive forces, inter￾molecular hydrogen bonds are strong enough to impose a relatively high degree of struc￾tural order on systems in which they are possible. As will be seen in Chapter 27, the three-dimensional structures adopted by proteins and nucleic acids, the organic mole￾cules of life, are dictated by patterns of hydrogen bonds. PROBLEM 4.5 The constitutional isomer of ethanol, dimethyl ether (CH3OCH3), is a gas at room temperature. Suggest an explanation for this observation. Table 4.1 lists the boiling points of some representative alkyl halides and alcohols. When comparing the boiling points of related compounds as a function of the alkyl group, we find that the boiling point increases with the number of carbon atoms, as it does with alkanes. 4.5 Physical Properties of Alcohols and Alkyl Halides: Intermolecular Forces 131 TABLE 4.1 Boiling Points of Some Alkyl Halides and Alcohols Name of alkyl group Methyl Ethyl Propyl Pentyl Hexyl Formula CH3X CH3CH2X CH3CH2CH2X CH3(CH2)3CH2X CH3(CH2)4CH2X Functional group X and boiling point, C (1 atm) X F 78 32 3 65 92 X Cl 24 12 47 108 134 X Br 3 38 71 129 155 X I 42 72 103 157 180 X OH 65 78 97 138 157 FIGURE 4.4 Hydrogen bonding in ethanol involves the oxygen of one molecule and the proton of an ±OH group of another. Hydrogen bonding is much stronger than most other types of dipole–dipole attractive forces. Hydrogen bonds between ±OH groups are stronger than those between ±NH groups, as a comparison of the boiling points of water (H2O, 100°C) and ammonia (NH3, 33°C) demonstrates. For a discussion concerning the boiling point behavior of alkyl halides, see the January 1988 issue of the Journal of Chemical Education, pp. 62–64. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website