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)<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. > 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