CHAPTER 4 ALCOHOLS AND ALKYL HALIDES ur first three chapters established some fundamental principles concerning the structure of organic molecules. In this chapter we begin our discussion of organic chemical reactions by directing attention to alcohols and alkyl halides. These two rank among the most useful classes of organic compounds because they often serve as starting materials for the preparation of numerous other families Two reactions that lead to alkyl halides will be described in this chapter. Both illus- trate functional group transformations. In the first, the hydroxyl group of an alcohol is replaced by halogen on treatment with a hydrogen halide R-OH+ H-X →>R一X+H-OH Alcohol In the second, reaction with chlorine or bromine causes one of the hydrogen substituents of an alkane to be replaced by haloge R一H X? Alkane Halogen Alkyl halide Hydrogen halide Both reactions are classified as substitutions, a term that describes the relationship between reactants and products--one functional group replaces another. In this chapter we go beyond the relationship of reactants and products and consider the mechanism of each reaction. A mechanism attempts to show how starting materials are converted into products during a chemical reaction While developing these themes of reaction and mechanism, we will also use hols and alkyl halides as vehicles to extend the principles of IUPAC nomenclature, 126 Back Forward Main Menu Study Guide ToC Student OLC MHHE Website
CHAPTER 4 ALCOHOLS AND ALKYL HALIDES Our first three chapters established some fundamental principles concerning the structure of organic molecules. In this chapter we begin our discussion of organic chemical reactions by directing attention to alcohols and alkyl halides. These two rank among the most useful classes of organic compounds because they often serve as starting materials for the preparation of numerous other families. Two reactions that lead to alkyl halides will be described in this chapter. Both illustrate functional group transformations. In the first, the hydroxyl group of an alcohol is replaced by halogen on treatment with a hydrogen halide. In the second, reaction with chlorine or bromine causes one of the hydrogen substituents of an alkane to be replaced by halogen. Both reactions are classified as substitutions, a term that describes the relationship between reactants and products—one functional group replaces another. In this chapter we go beyond the relationship of reactants and products and consider the mechanism of each reaction. A mechanism attempts to show how starting materials are converted into products during a chemical reaction. While developing these themes of reaction and mechanism, we will also use alcohols and alkyl halides as vehicles to extend the principles of IUPAC nomenclature, conR±H Alkane X2 Halogen R±X Alkyl halide H±X Hydrogen halide R±OH Alcohol H±X Hydrogen halide R±X Alkyl halide H±OH Water 126 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
4.1 IUPAC Nomenclature of Alkyl Halides tinue to develop concepts of structure and bonding, and see how structure affects prop- erties. A review of acids and bases constitutes an important part of this chapter in which a qualitative approach to proton-transfer equilibria will be developed that will be used throughout the remainder of the text 4. 1 IUPAC NOMENCLATURE OF ALKYL HALIDES The IUPAC rules permit alkyl halides to be named in two different ways, called func- The IUPAC rules permit cer. tional class nomenclature and substitutive nomenclature. In functional class nomencla. ain common alkyl grou ture the alkyl group and the halide (fluoride, chloride, bromide, or iodide)are desig- clude n-propyl, isopropyl nated as separate words. The alkyl group is named on the basis of its longest continuous n-butyl, sec-butyl, isobutyl chain be ng at the carbon to which the halogen is attached CH3F CH3CH, CH, CHCH,CI CH3CH, CHCH, CH, CH Methyl fluoride Pentyl chloride 1-Ethylbutyl bromide Cyclohexyl iodide Substitutive nomenclature of alkyl halides treats the halogen as a halo-(fluoro- chloro bromo, or iodo-)substituent on an alkane chain. The carbon chain is numbered in the direction that gives the substituted carbon the lower locant. CH3CH, CH, CH2 CH,F CH3 CHCH,CH2 CH3 CH3CH,CHCH,CH3 1-Fluoropentane 3-lodopentane When the carbon chain bears both a halogen and an alkyl substituent, the two substituents are considered of equal rank, and the chain is numbered so as to give the lower number to the substituent nearer the end of the chain CH3 CHCH,CH, CHCH,CH3 CH3 CHCH, CH, CHCH-CH3 CH3 5-Chloro-2-methy heptane 2-Chloro-5-methy heptane PROBLEM 4.1 Write structural formulas, and give the functional class and sub stitutive names of all the isomeric alkyl chlorides that have the molecular formula CaHgCI Substitutive names are preferred, but functional class names are sometimes more convenient or more familiar and are frequently encountered in organic chemistry the IUPAC rules. the term 4.2 IUPAC NOMENCLATURE OF ALCOHOLS instead of "functional class Functional class names of alcohols are derived by naming the alkyl group that bears the hydroxyl substituent(OH) and then adding alcohol as a separate word. The chain is always numbered beginning at the carbon to which the hydroxyl group is attached Substitutive names of alcohols are developed by identifying the longest continu- Back Forward Main Menu Study Guide ToC Student OLC MHHE Website
tinue to develop concepts of structure and bonding, and see how structure affects properties. A review of acids and bases constitutes an important part of this chapter in which a qualitative approach to proton-transfer equilibria will be developed that will be used throughout the remainder of the text. 4.1 IUPAC NOMENCLATURE OF ALKYL HALIDES The IUPAC rules permit alkyl halides to be named in two different ways, called functional class nomenclature and substitutive nomenclature. In functional class nomenclature the alkyl group and the halide ( fluoride, chloride, bromide, or iodide) are designated as separate words. The alkyl group is named on the basis of its longest continuous chain beginning at the carbon to which the halogen is attached. Substitutive nomenclature of alkyl halides treats the halogen as a halo- ( fluoro-, chloro-, bromo-, or iodo-) substituent on an alkane chain. The carbon chain is numbered in the direction that gives the substituted carbon the lower locant. When the carbon chain bears both a halogen and an alkyl substituent, the two substituents are considered of equal rank, and the chain is numbered so as to give the lower number to the substituent nearer the end of the chain. PROBLEM 4.1 Write structural formulas, and give the functional class and substitutive names of all the isomeric alkyl chlorides that have the molecular formula C4H9Cl. Substitutive names are preferred, but functional class names are sometimes more convenient or more familiar and are frequently encountered in organic chemistry. 4.2 IUPAC NOMENCLATURE OF ALCOHOLS Functional class names of alcohols are derived by naming the alkyl group that bears the hydroxyl substituent (±OH) and then adding alcohol as a separate word. The chain is always numbered beginning at the carbon to which the hydroxyl group is attached. Substitutive names of alcohols are developed by identifying the longest continuous chain that bears the hydroxyl group and replacing the -e ending of the 5-Chloro-2-methylheptane CH3CHCH2CH2CHCH2CH3 CH3 W Cl W 1 23 4 56 7 2-Chloro-5-methylheptane CH3CHCH2CH2CHCH2CH3 Cl W CH3 W 1 23 4 56 7 CH3CH2CH2CH2CH2F 1-Fluoropentane 2-Bromopentane CH3CHCH2CH2CH3 Br W 5 4 3 2 1 1 23 4 5 3-Iodopentane CH3CH2CHCH2CH3 I W 1 2 34 5 CH3CH2CH2CH2CH2Cl Pentyl chloride CH3F Methyl fluoride CH3CH2CHCH2CH2CH3 Br W 1-Ethylbutyl bromide H I Cyclohexyl iodide 12 3 4 4.1 IUPAC Nomenclature of Alkyl Halides 127 The IUPAC rules permit certain common alkyl group names to be used. These include n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, and neopentyl (Section 2.10). Prior to the 1993 version of the IUPAC rules, the term “radicofunctional” was used instead of “functional class.” Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER FOUR Alcohols and Alkyl Halides Several alcohols are com- corresponding alkane by the suffix -ol. The position of the hydroxyl group is indicated by number, choosing the sequence that assigns the lower locant to the carbon that bears (wood alcohol Coho Wood alcohol is methanol (methyl alcohol, CH CH=CH,OH CH3 CHCH, CH,CH, CH3 CH3 CCH, CHCH3 (ethyl alcohol, CH3CH2OH). Functional class 2-propanol (isopropyl alco- hol, (CH3)2 CHOH] name Ethyl alcohol 1-Methylpentyl alcohol 1, 1-Dimethy l alcoho Hydroxyl groups take precedence over ("outrank")alkyl groups and halogen substituents in determining the direction in which a carbon chain is numbered CH3CHCHCH2CHCH2 CH3 FCH, CH2CH2OH 6-Methyl- trans-2-Mel thylcyclopentanol 3-Fluoro-1-propanol (not 2-methyl PROBLEM 4.2 Write structural formulas, and give the functional class and sub- stitutive names of all the isomeric alcohols that have the molecular formula CaH1oO 4.3 CLASSES OF ALCOHOLS AND ALKYL HALIDES Alcohols and alkyl halides are classified as primary, secondary, or tertiary according to the classification of the carbon that bears the functional group(Section 2.10). Thus, pri- mary alcohols and primary alkyl halides are compounds of the type RCH2G(where G is the functional group), secondary alcohols and secondary alkyl halides are compounds of the type R2CHG, and tertiary alcohols and tertiary alkyl halides are compounds of the type r3CG CH CH3,OH CH3CH2 CHCH3 CH3CCH? CH,CH3 CH3 2, 2-Dimethyl-l-propanol 2-Bromobutane 1-Methylcyclohexanol 2-Chloro-2-methylpentane (a primary alcohol) (a secondary alkyl halide) (a tertiary alcohol (a tertiary alkyl halide) PROBLEM 4.3 Classify the isomeric CaH1o0 alcohols as being primary, secondary, or tertiary Many of the properties of alcohols and alkyl halides are affected by whether their functional groups are attached to primary, secondary, or tertiary carbons. We will see a number of cases in which a functional group attached to a primary carbon is more reac tive than one attached to a secondary or tertiary carbon, as well as other cases in which Back Forward Main Menu Study Guide ToC Student OLC MHHE Website
corresponding alkane by the suffix -ol. The position of the hydroxyl group is indicated by number, choosing the sequence that assigns the lower locant to the carbon that bears the hydroxyl group. Hydroxyl groups take precedence over (“outrank”) alkyl groups and halogen substituents in determining the direction in which a carbon chain is numbered. PROBLEM 4.2 Write structural formulas, and give the functional class and substitutive names of all the isomeric alcohols that have the molecular formula C4H10O. 4.3 CLASSES OF ALCOHOLS AND ALKYL HALIDES Alcohols and alkyl halides are classified as primary, secondary, or tertiary according to the classification of the carbon that bears the functional group (Section 2.10). Thus, primary alcohols and primary alkyl halides are compounds of the type RCH2G (where G is the functional group), secondary alcohols and secondary alkyl halides are compounds of the type R2CHG, and tertiary alcohols and tertiary alkyl halides are compounds of the type R3CG. PROBLEM 4.3 Classify the isomeric C4H10O alcohols as being primary, secondary, or tertiary. Many of the properties of alcohols and alkyl halides are affected by whether their functional groups are attached to primary, secondary, or tertiary carbons. We will see a number of cases in which a functional group attached to a primary carbon is more reactive than one attached to a secondary or tertiary carbon, as well as other cases in which the reverse is true. 6-Methyl-3-heptanol (not 2-methyl-5-heptanol) CH3CHCH2CH2CHCH2CH3 CH3 W OH W 7 65 4 32 1 3-Fluoro-1-propanol FCH2CH2CH2OH 321 OH CH3 1 5 4 3 2 trans-2-Methylcyclopentanol CH3CH2OH Ethyl alcohol Ethanol 1-Methylpentyl alcohol 2-Hexanol CH3CHCH2CH2CH2CH3 OH W 1,1-Dimethylbutyl alcohol 2-Methyl-2-pentanol CH3CCH2CH2CH3 OH CH3 W W Functional class name: Substitutive name: 128 CHAPTER FOUR Alcohols and Alkyl Halides CH3CCH2OH CH3 W W CH3 2,2-Dimethyl-1-propanol (a primary alcohol) CH3CCH2CH2CH3 CH3 W C W Cl 2-Chloro-2-methylpentane (a tertiary alkyl halide) CH3CH2CHCH3 Br W 2-Bromobutane (a secondary alkyl halide) CH3 OH 1-Methylcyclohexanol (a tertiary alcohol) Several alcohols are commonplace substances, well known by common names that reflect their origin (wood alcohol, grain alcohol) or use (rubbing alcohol). Wood alcohol is methanol (methyl alcohol, CH3OH), grain alcohol is ethanol (ethyl alcohol, CH3CH2OH), and rubbing alcohol is 2-propanol [isopropyl alcohol, (CH3)2CHOH]. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
4. 4 Bonding in Alcohols and Alkyl halides Lone-pair orbitals FIGURE 4. I hybrid- used in bonding are the 1s orbitals of hydrogen hybridized orbitals and oxygen. (b)Th gles at carbon and re close to tetrahedral, and the carbon-oxygen o bond is about 10 pm shorter than a arbon-carbon single bond 8.5° -o bond distance 142 pm 4. 4 BONDING IN ALCOHOLS AND ALKYL HALIDES The carbon that bears the functional group is sp-hybridized in alcohols and alkyl halides Figure 4.1 illustrates bonding in methanol. The bond angles at carbon are approximately tetrahedral, as is the C-O-H angle. A similar orbital hybridization model applies to alkyl halides, with the halogen substituent connected to sp-hybridized carbon by a o bond Carbon-halogen bond distances in alkyl halides increase in the order C-F(140 pm) O、xCH3 H H H2C Chloromethane PROBLEM 4.4 Bromine is less electronegative than chlorine, yet methyl bromide and methyl chloride have very similar dipole moments. Why Figure 4.2 shows the distribution of electron density methanol and chloromethane. Both are similar in that the sites of highest electrostatic potential (red) are near the electronegative atoms-oxygen and chlorine. The polarization of the bond FIGURE 4.2 Electro methanol and chloro- methane. The most pos charged ones red. The elec- trostatic potential is most methanol and near chlori Methanol(CH3OH) Chloromethane(CHCl) in chloromethane Back Forward Main Menu Study Guide ToC Student OLC MHHE Website
4.4 BONDING IN ALCOHOLS AND ALKYL HALIDES The carbon that bears the functional group is sp3 -hybridized in alcohols and alkyl halides. Figure 4.1 illustrates bonding in methanol. The bond angles at carbon are approximately tetrahedral, as is the C±O±H angle. A similar orbital hybridization model applies to alkyl halides, with the halogen substituent connected to sp3 -hybridized carbon by a bond. Carbon–halogen bond distances in alkyl halides increase in the order C±F (140 pm) C±Cl (179 pm) C±Br (197 pm) C±I (216 pm). Carbon–oxygen and carbon–halogen bonds are polar covalent bonds, and carbon bears a partial positive charge in alcohols (C±O) and in alkyl halides (C±X). The presence of these polar bonds makes alcohols and alkyl halides polar molecules. The dipole moments of methanol and chloromethane are very similar to each other and to water. PROBLEM 4.4 Bromine is less electronegative than chlorine, yet methyl bromide and methyl chloride have very similar dipole moments. Why? Figure 4.2 shows the distribution of electron density in methanol and chloromethane. Both are similar in that the sites of highest electrostatic potential (red) are near the electronegative atoms—oxygen and chlorine. The polarization of the bonds Water ( 1.8 D) H O H Chloromethane ( 1.9 D) CH3 Cl Methanol ( 1.7 D) O H3C H 4.4 Bonding in Alcohols and Alkyl Halides 129 C H H H C H O O H H H H Lone-pair orbitals (a) (b) σ bond C±O±H angle 108.5 C±O bond distance 142 pm FIGURE 4.1 Orbital hybridization model of bonding in methanol. (a) The orbitals used in bonding are the 1s orbitals of hydrogen and sp3 - hybridized orbitals of carbon and oxygen. (b) The bond angles at carbon and oxygen are close to tetrahedral, and the carbon–oxygen bond is about 10 pm shorter than a carbon–carbon single bond. Methanol (CH3OH) Chloromethane (CH3Cl) FIGURE 4.2 Electrostatic potential maps of methanol and chloromethane. The most positively charged regions are blue, the most negatively charged ones red. The electrostatic potential is most negative near oxygen in methanol and near chlorine in chloromethane. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER FOUR Alcohols and Alkyl Halides ygen and chlorine, as well as their unshared electron pairs, contribute to the con- ation of negative charge on these atoms Relatively simple notions of attractive forces between opposite charges are suffi cient to account for many of the properties of chemical substances. You will find it help- ful to keep the polarity of carbon-oxygen and carbon-halogen bonds in mind as we develop the properties of alcohols and alkyl halides in later sections 4.5 PHYSICAL PROPERTIES OF ALCOHOLS AND ALKYL HALIDES NTERMOLECULAR FORCES Boiling Point. When describing the effect of alkane structure on boiling point in Sec tion 2.14, we pointed out that the forces of attraction between neutral molecules are of three types listed here. The first two of these involve induced dipoles and are often referred to as dispersion forces, or London forces. nduced-dipole/induced-dipole forces 2. Dipole/induced-dipole forces 3. Dipole-dipole forces Induced-dipole/induced-dipole forces are the only intermolecular attractive forces available to nonpolar molecules such as alkanes. In addition to these forces, polar mol ecules engage in dipole-dipole and dipole/induced-dipole attractions. The dipole-di attractive force is easiest to visualize and is illustrated in Figure 4.3. Two molecules of a polar substance experience a mutual attraction between the positively polarized region of one molecule and the negatively polarized region of the other. As its name implies the dipole/induced-dipole force combines features of both the induced-dipole/induced- dipole and dipole-dipole attractive forces. a polar region of one molecule alters the elec- tron distribution in a nonpolar region of another in a direction that produces an attrac tive force between them Because so many factors contribute to the net intermolecular attractive force, it is not always possible to predict which of two compounds will have the higher boiling point. We can, however, use the boiling point behavior of selected molecules to inform us of the relative importance of various intermolecular forces and the structural features hat influence them Consider three compounds similar in size and shape: the alkane propane, the alco- hol ethanol, and the alkyl halide fluoroethane. CH3CH,CH CH3,OH CH3CH,F Propane(u=0 D) Ethanol (u= 1.7 D) Fluoroethane (u= 1.9 D) p:-42C bp:78°C p:-32C Both polar compounds, ethanol and fluoroethane, have higher boiling points than the nonpolar propane. We attribute this to a combination of dipole/induced-dipole and dipole-dipole attractive forces that stabilize the liquid states of ethanol and fluoroethane, FIGURE 4.3 A dipole-dipole out that are absent in propane. attractive force. Two mole The most striking aspect of the data, however, is the much higher boiling point of ules of a polar substance are ethanol compared with both propane and fluoroethane. This suggests that the attractive oriented so that the posi- forces in ethanol must be unusually strong Figure 4.4 shows that this force results from tively polarized region of a dipole-dipole attraction between the positively polarized proton of the -OH group of one and the negatively po arized region of the other one ethanol molecule and negatively polarized oxygen of another. The term attract each other hydrogen bonding is used to describe dipole-dipole attractive forces of this type. The Back Forward Main Menu Study Guide ToC Student OLC MHHE Website
to oxygen and chlorine, as well as their unshared electron pairs, contribute to the concentration of negative charge on these atoms. Relatively simple notions of attractive forces between opposite charges are suffi- cient to account for many of the properties of chemical substances. You will find it helpful to keep the polarity of carbon–oxygen and carbon–halogen bonds in mind as we develop the properties of alcohols and alkyl halides in later sections. 4.5 PHYSICAL PROPERTIES OF ALCOHOLS AND ALKYL HALIDES: INTERMOLECULAR FORCES Boiling Point. When describing the effect of alkane structure on boiling point in Section 2.14, we pointed out that the forces of attraction between neutral molecules are of three types listed here. The first two of these involve induced dipoles and are often referred to as dispersion forces, or London forces. 1. Induced-dipole/induced-dipole forces 2. Dipole/induced-dipole forces 3. Dipole–dipole forces Induced-dipole/induced-dipole forces are the only intermolecular attractive forces available to nonpolar molecules such as alkanes. In addition to these forces, polar molecules engage in dipole–dipole and dipole/induced-dipole attractions. The dipole–dipole attractive force is easiest to visualize and is illustrated in Figure 4.3. Two molecules of a polar substance experience a mutual attraction between the positively polarized region of one molecule and the negatively polarized region of the other. As its name implies, the dipole/induced-dipole force combines features of both the induced-dipole/induceddipole and dipole–dipole attractive forces. A polar region of one molecule alters the electron distribution in a nonpolar region of another in a direction that produces an attractive force between them. Because so many factors contribute to the net intermolecular attractive force, it is not always possible to predict which of two compounds will have the higher boiling point. We can, however, use the boiling point behavior of selected molecules to inform us of the relative importance of various intermolecular forces and the structural features that influence them. Consider three compounds similar in size and shape: the alkane propane, the alcohol ethanol, and the alkyl halide fluoroethane. Both polar compounds, ethanol and fluoroethane, have higher boiling points than the nonpolar propane. We attribute this to a combination of dipole/induced-dipole and dipole–dipole attractive forces that stabilize the liquid states of ethanol and fluoroethane, but that are absent in propane. The most striking aspect of the data, however, is the much higher boiling point of ethanol compared with both propane and fluoroethane. This suggests that the attractive forces in ethanol must be unusually strong. Figure 4.4 shows that this force results from a dipole–dipole attraction between the positively polarized proton of the OH group of one ethanol molecule and the negatively polarized oxygen of another. The term hydrogen bonding is used to describe dipole–dipole attractive forces of this type. The Ethanol ( 1.7 D) bp: 78°C CH3CH2OH Fluoroethane ( 1.9 D) bp: 32°C CH3CH2F Propane ( 0 D) bp: 42°C CH3CH2CH3 130 CHAPTER FOUR Alcohols and Alkyl Halides FIGURE 4.3 A dipole–dipole attractive force. Two molecules of a polar substance are oriented so that the positively polarized region of one and the negatively polarized region of the other attract each other. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
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
