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 Website
proton involved must be bonded to an electronegative element, usually oxygen or nitrogen. 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, intermolecular hydrogen bonds are strong enough to impose a relatively high degree of structural 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 molecules 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
CHAPTER FOUR Alcohols and Alkyl Halides With respect to the halogen in a group of alkyl halides, the boiling point increases as one descends the periodic table; alkyl fluorides have the lowest boiling points, alkyl iodides the highest. This trend matches the order of increasing polarizability of the halo- gens. Polarizability is the ease with which the electron distribution around an atom is distorted by a nearby electric field and is a significant factor in determining the strength of induced-dipole/induced-dipole and dipole/induced-dipole attractions. Forces that depend on induced dipoles are strongest when the halogen is a highly polarizable iodine, and weakest when the halogen is a nonpolarizable fluorine The boiling points of the chlorinated derivatives of methane increase with the num- ber of chlorine atoms because of an increase in the induced-dipole/induced-dipole attrac tive forces CHCI CH,CI ne Dichloromethane Trichloromethane Tetrachloromethane ethyl chloride) ( methylene dichloride)(chloroform)(carbon tetrachloride) boiling -24°C 40°C 61°C 77°C Fluorine is unique among the halogens in that increasing the number of fuorines does not produce higher and higher boiling points CH3 CHF CH CHF CH3CF3 CF,CF Fluoroethane 1. 1-Difluoroethane 11.1-Trifluoroethane Hexafluoroethane -25°C 47°C crease with Thus, although the difluoride CH3 CHF2 boils at a higher temperature than CH3CH,F, the trifluoride CH3CF3 boils at a lower temperature than either of them. Even more striking is the observation that the hexafluoride CF3 CF3 is the lowest boiling of any of the fluo- rinated derivatives of ethane. The boiling point of CF3CF3 is, in fact, only 11 higher than that of ethane itself. The reason for this behavior has to do with the very low polar inability of fluorine and a decrease in induced-dipole/induced-dipole forces that accom- panies the incorporation of fluorine substituents into a molecule. Their weak intermole- cular attractive forces give fluorinated hydrocarbons (fluorocarbons) certain desirable physical properties such as that found in the"no stick"Teflon coating of frying pans Teflon is a polymer(Section 6.21)made up of long chains of -CF2CF Solubility in Water. Alkyl halides and alcohols differ markedly from one another in their solubility in water. All alkyl halides are insoluble in water, but low-molecular- weight alcohols(methyl, ethyl, n-propyl, and isopropyl) are soluble in water in all pro- portions. Their ability to participate in intermolecular hydrogen bonding not only affects the boiling points of alcohols, but also enhances their water solubility. Hydrogen-bonded networks of the type shown in Figure 4.5, in which alcohol and water molecules asso- ciate with one another, replace the alcohol-alcohol and water-water hydrogen-bonded networks present in the pure substances Higher alcohols become more hydrocarbon-like" and less water-soluble 1-Octanol, for example, dissolves to the extent of only 1 mL in 2000 mL of water. As the alkyl chain gets longer, the hydrophobic effect(Section 2. 14)becomes more impor- tant, to the point that it, more than hydrogen bonding, governs the solubility of alcohols Density. Alkyl fluorides and chlorides are less dense, and alkyl bromides and iodides Back Forward Main Menu Study Guide ToC Student OLC MHHE Website
With respect to the halogen in a group of alkyl halides, the boiling point increases as one descends the periodic table; alkyl fluorides have the lowest boiling points, alkyl iodides the highest. This trend matches the order of increasing polarizability of the halogens. Polarizability is the ease with which the electron distribution around an atom is distorted by a nearby electric field and is a significant factor in determining the strength of induced-dipole/induced-dipole and dipole/induced-dipole attractions. Forces that depend on induced dipoles are strongest when the halogen is a highly polarizable iodine, and weakest when the halogen is a nonpolarizable fluorine. The boiling points of the chlorinated derivatives of methane increase with the number of chlorine atoms because of an increase in the induced-dipole/induced-dipole attractive forces. Fluorine is unique among the halogens in that increasing the number of fluorines does not produce higher and higher boiling points. Thus, although the difluoride CH3CHF2 boils at a higher temperature than CH3CH2F, the trifluoride CH3CF3 boils at a lower temperature than either of them. Even more striking is the observation that the hexafluoride CF3CF3 is the lowest boiling of any of the fluorinated derivatives of ethane. The boiling point of CF3CF3 is, in fact, only 11° higher than that of ethane itself. The reason for this behavior has to do with the very low polarizability of fluorine and a decrease in induced-dipole/induced-dipole forces that accompanies the incorporation of fluorine substituents into a molecule. Their weak intermolecular attractive forces give fluorinated hydrocarbons (fluorocarbons) certain desirable physical properties such as that found in the “no stick” Teflon coating of frying pans. Teflon is a polymer (Section 6.21) made up of long chains of ±CF2CF2±units. Solubility in Water. Alkyl halides and alcohols differ markedly from one another in their solubility in water. All alkyl halides are insoluble in water, but low-molecularweight alcohols (methyl, ethyl, n-propyl, and isopropyl) are soluble in water in all proportions. Their ability to participate in intermolecular hydrogen bonding not only affects the boiling points of alcohols, but also enhances their water solubility. Hydrogen-bonded networks of the type shown in Figure 4.5, in which alcohol and water molecules associate with one another, replace the alcohol–alcohol and water–water hydrogen-bonded networks present in the pure substances. Higher alcohols become more “hydrocarbon-like” and less water-soluble. 1-Octanol, for example, dissolves to the extent of only 1 mL in 2000 mL of water. As the alkyl chain gets longer, the hydrophobic effect (Section 2.14) becomes more important, to the point that it, more than hydrogen bonding, governs the solubility of alcohols. Density. Alkyl fluorides and chlorides are less dense, and alkyl bromides and iodides more dense, than water. 1,1-Difluoroethane 25°C CH3CHF2 1,1,1-Trifluoroethane 47°C CH3CF3 Hexafluoroethane 78°C CF3CF3 Fluoroethane 32°C CH3CH2F Boiling point: Dichloromethane (methylene dichloride) 40°C CH2Cl2 Trichloromethane (chloroform) 61°C CHCl3 Tetrachloromethane (carbon tetrachloride) 77°C CCl4 Chloromethane (methyl chloride) 24°C CH3Cl Boiling point: 132 CHAPTER FOUR Alcohols and Alkyl Halides These boiling points illustrate why we should do away with the notion that boiling points always increase with increasing molecular weight. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
4.6 Acids and Bases: General Principles FIGURE 4.5 Hydrogen bonding between molecules 8 CH3(CH2)6CH,F CH3(CH2)CH,CI CH3 (CH2).CH, Br CH3(CH2)CH, Density 0.80g/mL 0. 89 g/mL 1.12g/mL 34 g/mL Because alkyl halides are insoluble in water, a mixture of an alkyl halide and water sep arates into two layers. When the alkyl halide is a fluoride or chloride, it is the upper layer and water is the lower. The situation is reversed when the alkyl halide is a bro- mide or an iodide. In these cases the alkyl halide is the lower layer. Polyhalogenation increases the density. The compounds CH_Cl2, CHCl3, and CCl4, for example, are all more dense than water All liquid alcohols have densities of approximately 0.8 g/mL and are, therefore, less dense than water 4.6 ACIDS AND BASES: GENERAL PRINCIPLES A solid understanding of acid-base chemistry is a big help in understanding chemical reactivity. This and the next section review some principles and properties of acids and bases and examine how these principles apply to alcohols According to the theory proposed by Svante Arrhenius, a Swedish chemist and winner of the 1903 Nobel Prize in chemistry, an acid ionizes in aqueous solution to lib Rre protons(H, hydrogen ions), whereas bases ionize to liberate hydroxide ions lO). A more general theory of acids and bases was devised independently by Johannes Bronsted(Denmark) and Thomas M. Lowry(England) in 1923. In the Bronsted-Lowry approach, an acid is a proton donor, and a base is a proton acceptor. B/+HA、-H used to show the electron air of the base abstracting a m the acid. The Base Acid of electrons in the h-a air in the arrows track electron ment, not atomic movement. Back Forward Main Menu Study Guide ToC Student OLC MHHE Website
Because alkyl halides are insoluble in water, a mixture of an alkyl halide and water separates into two layers. When the alkyl halide is a fluoride or chloride, it is the upper layer and water is the lower. The situation is reversed when the alkyl halide is a bromide or an iodide. In these cases the alkyl halide is the lower layer. Polyhalogenation increases the density. The compounds CH2Cl2, CHCl3, and CCl4, for example, are all more dense than water. All liquid alcohols have densities of approximately 0.8 g/mL and are, therefore, less dense than water. 4.6 ACIDS AND BASES: GENERAL PRINCIPLES A solid understanding of acid–base chemistry is a big help in understanding chemical reactivity. This and the next section review some principles and properties of acids and bases and examine how these principles apply to alcohols. According to the theory proposed by Svante Arrhenius, a Swedish chemist and winner of the 1903 Nobel Prize in chemistry, an acid ionizes in aqueous solution to liberate protons (H, hydrogen ions), whereas bases ionize to liberate hydroxide ions (HO). A more general theory of acids and bases was devised independently by Johannes Brønsted (Denmark) and Thomas M. Lowry (England) in 1923. In the Brønsted–Lowry approach, an acid is a proton donor, and a base is a proton acceptor. B Base B H Conjugate acid A Conjugate base Acid H A 4.6 Acids and Bases: General Principles 133 0.89 g/mL CH3(CH2)6CH2Cl 1.12 g/mL CH3(CH2)6CH2Br 1.34 g/mL CH3(CH2)6CH2I 0.80 g/mL CH3(CH2)6CH2F Density (20°C): FIGURE 4.5 Hydrogen bonding between molecules of ethanol and water. Curved arrow notation is used to show the electron pair of the base abstracting a proton from the acid. The pair of electrons in the H±A bond becomes an unshared pair in the anion :A. Curved arrows track electron movement, not atomic movement. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER FOUR Alcohols and Alkyl Halides The Bronsted-Lowry definitions of acids and bases are widely used in organi emistry. As noted in the preceding equation, the conjugate acid of a substance is formed when it accepts a proton from a suitable donor. Conversely, the proton donor is converted to its conjugate base. A conjugate acid-base pair always differ by a single proton TPROBLEM 4.6 Write an equation for the reaction of ammonia (: NH,)with hydro- gen chloride(HCi). Use curved arrows to track electron movement, and identify the acid, base, conjugate acid, and conjugate base. In aqueous solution, an acid transfers a proton to water. Water acts as a Bronsted base H H Water Acid Conjugate acid of water The systematic name for the conjugate acid of water(H3o)is oxonium ion. Its com- mon name is hydronium ion The strength of an acid is measured by its acid dissociation constant or [H3O’I[A Table 4.2 lists a number of Bronsted acids and their acid dissociation constants Strong acids are characterized by Ka values that are greater than that for hydronium ion (H3O, Ka=55). Essentially every molecule of a strong acid transfers a proton to water in dilute aqueous solution. Weak acids have Ka values less than that of H3o; they ar incompletely ionized in dilute aqueous solution A convenient way to express acid strength is through the use of pka, defined as Thus, water, with Ka =1.8 X 10, has a pKa of 15.7; ammonia, with Ka l0, has a pKa of 36. The stronger the acid, the larger the value of its Ka and the smaller the value of pKa. Water is a very weak acid, but is a far stronger acid than ammo- nia. Table 4.2 includes pKa as well as Ka values for acids. Because both systems are widely used, you should practice converting Ka to pKa and vice versa PROBLEM 4.7 Hydrogen cyanide(HCn)has a pka of 9.1. What is its Ka? ls HCN a strong or a weak acid? An important part of the Bronsted-Lowry picture of acids and bases concerns the relative strengths of an acid and its conjugate base. The stronger the acid, the weaker the conjugate base, and vice versa. Ammonia(NH3) is the second weakest acid in Table 4.2. Its conjugate base, amide ion(H2N ) is therefore the second strongest base Hydroxide(Ho) is a moderately strong base, much stronger than the halide ions F Cl, Br and I, which are very weak bases. Fluoride is the strongest base of the halides but is 10-times less basic than hydroxide ion. Back Forward Main Menu Study Guide ToC Student OLC MHHE Website
The Brønsted–Lowry definitions of acids and bases are widely used in organic chemistry. As noted in the preceding equation, the conjugate acid of a substance is formed when it accepts a proton from a suitable donor. Conversely, the proton donor is converted to its conjugate base. A conjugate acid–base pair always differ by a single proton. PROBLEM 4.6 Write an equation for the reaction of ammonia (:NH3) with hydrogen chloride (HCl). Use curved arrows to track electron movement, and identify the acid, base, conjugate acid, and conjugate base. In aqueous solution, an acid transfers a proton to water. Water acts as a Brønsted base. The systematic name for the conjugate acid of water (H3O) is oxonium ion. Its common name is hydronium ion. The strength of an acid is measured by its acid dissociation constant or ionization constant Ka. Ka Table 4.2 lists a number of Brønsted acids and their acid dissociation constants. Strong acids are characterized by Ka values that are greater than that for hydronium ion (H3O, Ka 55). Essentially every molecule of a strong acid transfers a proton to water in dilute aqueous solution. Weak acids have Ka values less than that of H3O; they are incompletely ionized in dilute aqueous solution. A convenient way to express acid strength is through the use of pKa, defined as follows: pKa log10 Ka Thus, water, with Ka 1.8 1016, has a pKa of 15.7; ammonia, with Ka 1036, has a pKa of 36. The stronger the acid, the larger the value of its Ka and the smaller the value of pKa. Water is a very weak acid, but is a far stronger acid than ammonia. Table 4.2 includes pKa as well as Ka values for acids. Because both systems are widely used, you should practice converting Ka to pKa and vice versa. PROBLEM 4.7 Hydrogen cyanide (HCN) has a pKa of 9.1. What is its Ka? Is HCN a strong or a weak acid? An important part of the Brønsted–Lowry picture of acids and bases concerns the relative strengths of an acid and its conjugate base. The stronger the acid, the weaker the conjugate base, and vice versa. Ammonia (NH3) is the second weakest acid in Table 4.2. Its conjugate base, amide ion (H2N), is therefore the second strongest base. Hydroxide (HO) is a moderately strong base, much stronger than the halide ions F, Cl, Br, and I, which are very weak bases. Fluoride is the strongest base of the halides but is 1012 times less basic than hydroxide ion. [H3O][A] [HA] H H O Water (base) H A Acid A Conjugate base Conjugate acid of water H H O H 134 CHAPTER FOUR Alcohols and Alkyl Halides Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
4.6 Acids and Bases: General Principles TABLE 4. 2 Acid Dissociation Constants Ka and pka Values for Some Bronsted acids* Dissociation Conjugate Ac Formulat constant k base Hydrogen iodide HI 101 10 Hydrogen bromide HB ≈10 Br Hydrogen chloride HCI ≈10 Sulfuric acid HOSO,OH 1.6×10 4.8 HOSO2O Hydronium ion H-OH 55 1.7 Hydrogen fluorid 3.5×10-4 3.5 Acetic acid H3COH 18×10-5 CHCO Ammonium ion H—NH 5.6×10 9.2 Water 1.8×10-16 15.7 Methanol CH3O Ethano CH3CH2OH CH3 CH2O isopropyl alcohol (CH3)2 CHOH tert-Butyl alcohol (CH3)3COH (CH3)3CO Ammonia Dimethylamine (CH3)2NH 36 (CH3)2N .Acid strength decreases from top to bottom of the table. Strength of conjugate base increases from top proton-the one that is lost on ionizatio Thetrue", for water is 1 x 10. Dividing this value by 55.5(the number of moles of water in 1 L of s in he table. a paper in the May 1990 issue of the Journal of Chemical Education(. 386)outlines the or this approach. For a dissenting view, see the March 1992 issue of the Journal of Chemical Education(p. 255) PROBLEM 4.8 As noted in Problem 4.7, hydrogen cyanide(HCn)has a pka of 9.1. Is cyanide ion (CN")a stronger base or a weaker base than hydroxide ion (HO)? In any proton-transfer process the position of equilibrium favors formation of the weaker acid and the weaker base Stronger acid stronger base weaker acid weaker base Table 4.2 is set up so that the strongest acid is at the top of the acid column, with the chemistry strongest base at the bottom of the conjugate base column. An acid will transfer a pro- ton to the conjugate base of any acid that lies below it in the table, and the equilibrium constant for the reaction will be greater than one Table 4.2 contains both inorganic and organic compounds Organic compounds are similar to inorganic ones when the functional groups responsible for their acid-base prop- erties are the same. Thus, alcohols(rod) are similar to water(HOh) in both their bror sted acidity(ability to donate a proton from oxygen) and Bronsted basicity(ability to accept a proton on orygen). Just as proton transfer to a water molecule gives oxonium ion(hydronium ion, H30), proton transfer to an alcohol gives an alkyloxonium ion ROH,) Back Forward Main Menu Study Guide ToC Student OLC MHHE Website
PROBLEM 4.8 As noted in Problem 4.7, hydrogen cyanide (HCN) has a pKa of 9.1. Is cyanide ion (CN) a stronger base or a weaker base than hydroxide ion (HO)? In any proton-transfer process the position of equilibrium favors formation of the weaker acid and the weaker base. Table 4.2 is set up so that the strongest acid is at the top of the acid column, with the strongest base at the bottom of the conjugate base column. An acid will transfer a proton to the conjugate base of any acid that lies below it in the table, and the equilibrium constant for the reaction will be greater than one. Table 4.2 contains both inorganic and organic compounds. Organic compounds are similar to inorganic ones when the functional groups responsible for their acid–base properties are the same. Thus, alcohols (ROH) are similar to water (HOH) in both their Brønsted acidity (ability to donate a proton from oxygen) and Brønsted basicity (ability to accept a proton on oxygen). Just as proton transfer to a water molecule gives oxonium ion (hydronium ion, H3O), proton transfer to an alcohol gives an alkyloxonium ion (ROH2 ). Stronger acid stronger base weaker acid weaker base K 1 4.6 Acids and Bases: General Principles 135 TABLE 4.2 Acid Dissociation Constants Ka and pKa Values for Some Brønsted Acids* HI HBr HCl HOSO2OH H±NH3 HOH CH3OH CH3CH2OH (CH3)2CHOH (CH3)3COH H2NH (CH3)2NH Formula† CH3COH O X H±OH2 HF Acid Hydrogen iodide Hydrogen bromide Hydrogen chloride Sulfuric acid Hydronium ion Hydrogen fluoride Acetic acid Ammonium ion Water Methanol Ethanol Isopropyl alcohol tert-Butyl alcohol Ammonia Dimethylamine 10 9 7 4.8 1.7 3.5 4.7 9.2 15.7 16 16 17 18 36 36 pKa CH3CO O X I Br Cl HOSO2O H2O F NH3 HO CH3O CH3CH2O (CH3)2CHO (CH3)3CO H2N (CH3)2N Conjugate base 1010 109 107 1.6 105 1.8 105 5.6 1010 1.8 1016‡ 1016 1016 1017 1018 1036 1036 Dissociation constant, Ka 55 3.5 104 *Acid strength decreases from top to bottom of the table. Strength of conjugate base increases from top to bottom of the table. † The most acidic proton—the one that is lost on ionization—is highlighted. ‡ The “true” Ka for water is 1 1014. Dividing this value by 55.5 (the number of moles of water in 1 L of water) gives a Ka of 1.8 1016 and puts water on the same concentration basis as the other substances in the table. A paper in the May 1990 issue of the Journal of Chemical Education (p. 386) outlines the justification for this approach. For a dissenting view, see the March 1992 issue of the Journal of Chemical Education (p. 255). This is one of the most important equations in chemistry. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website