CHAPTER 2 ALKANES ow that weve reviewed the various bonding models, we are ready to examine organic compounds in respect to their structure, reactions, properties, and appli cations. Were we to list the physical and chemical properties of each of the more than 8 million organic compounds separately, it would tax the capacity of even a pow erful computer. Yet someone who is trained in organic chemistry can simply look at the structure of a substance and make reasonably confident predictions about its propertie including how it will behave in a chemical reaction Organic chemists associate particular structural units, called functional groups with characteristic patterns of reactivity; they look at large molecules as collections of functional groups attached to nonreactive frameworks. Not only does this"functional group approach"have predictive power, but time and experience have shown that it orga nizes the material in a way that makes learning organic chemistry easier for most student We'll begin the chapter with a brief survey of various kinds of hydrocarbons- compounds that contain only carbon and hydrogen--introduce some functional groups then return to hydrocarbons to discuss alkanes in some detail. The names of alkanes may seem strange at first, but they form the foundation for the most widely accepted system of organic nomenclature. The fundamentals of this nomenclature system, the IUPAC rules, constitute one of the main topics of this chapter. 2.1 CLASSES OF HYDROCARBONS Hydrocarbons are compounds that contain only carbon and hydrogen and are divided into two main classes: aliphatic hydrocarbons and aromatic hydrocarbons. This classification dates from the nineteenth century, when organic chemistry was almost exclusively devoted 3 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
53 CHAPTER 2 ALKANES Now that we’ve reviewed the various bonding models, we are ready to examine organic compounds in respect to their structure, reactions, properties, and applications. Were we to list the physical and chemical properties of each of the more than 8 million organic compounds separately, it would tax the capacity of even a powerful computer. Yet someone who is trained in organic chemistry can simply look at the structure of a substance and make reasonably confident predictions about its properties, including how it will behave in a chemical reaction. Organic chemists associate particular structural units, called functional groups, with characteristic patterns of reactivity; they look at large molecules as collections of functional groups attached to nonreactive frameworks. Not only does this “functional group approach” have predictive power, but time and experience have shown that it organizes the material in a way that makes learning organic chemistry easier for most students. We’ll begin the chapter with a brief survey of various kinds of hydrocarbons— compounds that contain only carbon and hydrogen—introduce some functional groups, then return to hydrocarbons to discuss alkanes in some detail. The names of alkanes may seem strange at first, but they form the foundation for the most widely accepted system of organic nomenclature. The fundamentals of this nomenclature system, the IUPAC rules, constitute one of the main topics of this chapter. 2.1 CLASSES OF HYDROCARBONS Hydrocarbons are compounds that contain only carbon and hydrogen and are divided into two main classes: aliphatic hydrocarbons and aromatic hydrocarbons. This classification dates from the nineteenth century, when organic chemistry was almost exclusively devoted Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TWO Alkanes to the study of materials from natural sources, and terms were coined that reflected a sub- stance's origin. Two sources were fats and oils, and the word aliphatic was derived from the greek word aleiphar (fat). Aromatic hydrocarbons, irrespective of their own odor, were typically obtained by chemical treatment of pleasant-smelling plant extracts Aliphatic hydrocarbons include three major groups: alkanes, alkenes, and alkynes Alkanes are hydrocarbons in which all the bonds are single bonds, alkenes contain a carbon-carbon double bond, and alkynes contain a carbon-carbon triple bond. Exam ples of the three classes of aliphatic hydrocarbons are the two-carbon compounds ethane, ethylene, and acetylene. Bonding in ethane, ethylene, H d acetylene was discussed in Sections 1. 16-1.18 Ethane Ethy lene Acetylene (alkane) (alkene) Another name for aromatic hydrocarbons is arenes. Arenes have properties that are much different from alkanes, alkenes, and alkynes. The most important aromatic hydrocarbon nzene Bonding in benzene will discussed in Section 11.5 Benzene Many of the principles of organic chemistry can be developed by examining the series of hydrocarbons in the order: alkanes, alkenes, alkynes, and arenes. Alkanes are introduced in this chapter, alkenes in Chapters 5 and 6, alkynes in Chapter 9, and arenes in Chapters ll and 12. 2.2 REACTIVE SITES IN HYDROCARBONS A functional group is the structural unit responsible for a given molecule's reactivity under a particular set of conditions. It can be as small as a single hydrogen atom, or it can encompass several atoms. The functional group of an alkane is any one of its hydro- gen substituents. A reaction that we shall discuss in Chapter 4 is one in which an alkane reacts with chlorine. For example CH3CH3 CI CH3,CI+ hlorine Chloroethane Hydrogen chloride One of the hydrogen atoms of ethane is replaced by chlorine. This replacement of hydro- gen by chlorine is a characteristic reaction of all alkanes and can be represented for the R一H+ Alkyl chloride Hydrogen chloride Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
to the study of materials from natural sources, and terms were coined that reflected a substance’s origin. Two sources were fats and oils, and the word aliphatic was derived from the Greek word aleiphar (“fat”). Aromatic hydrocarbons, irrespective of their own odor, were typically obtained by chemical treatment of pleasant-smelling plant extracts. Aliphatic hydrocarbons include three major groups: alkanes, alkenes, and alkynes. Alkanes are hydrocarbons in which all the bonds are single bonds, alkenes contain a carbon–carbon double bond, and alkynes contain a carbon–carbon triple bond. Examples of the three classes of aliphatic hydrocarbons are the two-carbon compounds ethane, ethylene, and acetylene. Another name for aromatic hydrocarbons is arenes. Arenes have properties that are much different from alkanes, alkenes, and alkynes. The most important aromatic hydrocarbon is benzene. Many of the principles of organic chemistry can be developed by examining the series of hydrocarbons in the order: alkanes, alkenes, alkynes, and arenes. Alkanes are introduced in this chapter, alkenes in Chapters 5 and 6, alkynes in Chapter 9, and arenes in Chapters 11 and 12. 2.2 REACTIVE SITES IN HYDROCARBONS A functional group is the structural unit responsible for a given molecule’s reactivity under a particular set of conditions. It can be as small as a single hydrogen atom, or it can encompass several atoms. The functional group of an alkane is any one of its hydrogen substituents. A reaction that we shall discuss in Chapter 4 is one in which an alkane reacts with chlorine. For example: One of the hydrogen atoms of ethane is replaced by chlorine. This replacement of hydrogen by chlorine is a characteristic reaction of all alkanes and can be represented for the general case by the equation: R±H Alkane Cl2 Chlorine R±Cl Alkyl chloride HCl Hydrogen chloride CH3CH3 Ethane Cl2 Chlorine CH3CH2Cl Chloroethane HCl Hydrogen chloride C C C C C C H H H H H H Benzene (arene) Ethane (alkane) H C H H C H H H Ethylene (alkene) H H H H C C Acetylene (alkyne) H C C H 54 CHAPTER TWO Alkanes Bonding in ethane, ethylene, and acetylene was discussed in Sections 1.16–1.18. Bonding in benzene will be discussed in Section 11.5. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
2.3 The Key Functional Groups In the general equation the functional group(H) is shown explicitly while the remain der of the alkane molecule is abbreviated as R. This is a commonly used notation which helps focus our attention on the functional group transformation without being distracted by the parts of the molecule that remain unaffected. A hydrogen atom in one alkane is very much like the hydrogen of any other alkane in its reactivity toward chlorine. Our ability to write general equations such as the one shown illustrates why the functional roup approach is so useful in organic chemistry A hydrogen atom is a functional unit in alkenes and alkynes as well as in alkanes These hydrocarbons, however, contain a second functional group as well. The car- bon-carbon double bond is a functional group in alkenes, and the carbon-carbon triple bond is a functional group in alkynes A hydrogen atom is a functional group in arenes, and we represent arenes as ArH to reflect this. What will become apparent when we discuss the reactions of arenes, how- ever, is that their chemistry is much richer than that of alkanes, and it is therefore more appropriate to consider the ring in its entirety as the functional group 2.3 THE KEY FUNCTIONAL GROUPS As a class, alkanes are not particularly reactive compounds, and the h in rh is not a particularly reactive functional group. Indeed, when a group other than hydrogen is present on an alkane framework, that group is almost always the functional group Table 2.1 lists examples of some compounds of this type. All will be discussed in later chapters Some of the most important families of organic compounds, those that contain the carbonyl group(C=O), deserve separate mention and are listed in Table 2.2 Carbonyl- discussed group chemistry is containing compounds rank among the most abundant and biologically significant classes chapters( Chapters 17-21) of naturally occurring substances PROBLEM 2.1 Many compounds contain more th functional group. The structure of prostaglandin E,, a hormone that regul ne relaxation of smooth muscles, contains two different kinds of carbonyl gre Classify each one(alde- hyde, ketone, carboxylic acid, ester, amide, acyl chloride, or carboxylic acid anhy dride) TABLE 2.1 unctional Groups in Some Important Classes of Organic Compounds Generalized Representative clas exa Name of example* Alcohol CH3CH2OH Ethanol CH3 CH2CI Chloroethane CH3 CH2NH2 Ethanamine R2C—CR2 H2C—CH2 Oxirane Ether CH3 CH Diethyl ether Nitrile CH3CH2C≡N Propanenitrile alkane NO2 CH3 CH2NO itroethane Thiol RSH CH3 CH2SH Ethanethiol Most compounds have more than one acceptabl The example given is a primary amine(RNH2) Secondary amines have the general structure R NH; tertiary Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
In the general equation the functional group (±H) is shown explicitly while the remainder of the alkane molecule is abbreviated as R. This is a commonly used notation which helps focus our attention on the functional group transformation without being distracted by the parts of the molecule that remain unaffected. A hydrogen atom in one alkane is very much like the hydrogen of any other alkane in its reactivity toward chlorine. Our ability to write general equations such as the one shown illustrates why the functional group approach is so useful in organic chemistry. A hydrogen atom is a functional unit in alkenes and alkynes as well as in alkanes. These hydrocarbons, however, contain a second functional group as well. The carbon–carbon double bond is a functional group in alkenes, and the carbon–carbon triple bond is a functional group in alkynes. A hydrogen atom is a functional group in arenes, and we represent arenes as ArH to reflect this. What will become apparent when we discuss the reactions of arenes, however, is that their chemistry is much richer than that of alkanes, and it is therefore more appropriate to consider the ring in its entirety as the functional group. 2.3 THE KEY FUNCTIONAL GROUPS As a class, alkanes are not particularly reactive compounds, and the H in RH is not a particularly reactive functional group. Indeed, when a group other than hydrogen is present on an alkane framework, that group is almost always the functional group. Table 2.1 lists examples of some compounds of this type. All will be discussed in later chapters. Some of the most important families of organic compounds, those that contain the carbonyl group (CœO), deserve separate mention and are listed in Table 2.2 Carbonylcontaining compounds rank among the most abundant and biologically significant classes of naturally occurring substances. PROBLEM 2.1 Many compounds contain more than one functional group. The structure of prostaglandin E1, a hormone that regulates the relaxation of smooth muscles, contains two different kinds of carbonyl groups. Classify each one (aldehyde, ketone, carboxylic acid, ester, amide, acyl chloride, or carboxylic acid anhydride). 2.3 The Key Functional Groups 55 TABLE 2.1 Functional Groups in Some Important Classes of Organic Compounds Class Alcohol Alkyl halide Amine† Epoxide Ether Nitrile Nitroalkane Thiol Name of example* Ethanol Chloroethane Ethanamine Oxirane Diethyl ether Propanenitrile Nitroethane Ethanethiol *Most compounds have more than one acceptable name. † The example given is a primary amine (RNH2). Secondary amines have the general structure R2NH; tertiary amines are R3N. Representative example CH3CH2OH CH3CH2Cl CH3CH2NH2 CH3CH2OCH2CH3 CH3CH2CPN CH3CH2NO2 CH3CH2SH H2C CH2 O Generalized abbreviation ROH RCl RNH2 ROR RCPN RNO2 RSH R2C CR2 O Carbonyl group chemistry is discussed in a block of five chapters (Chapters 17–21). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TWO Alkanes TABLE 2.2 Classes of Compounds That Contain a Carbonyl Group Generalized Representative abbreviation example CHaCH Ethanal Ketone RCR CH3CCH3 2-Propanone Carboxylic acid CH3COH Ethanoic acid Carboxylic acid derivati Acyl halide ROX Ethanoyl chloride Acid anhydride RCOCR CH3 COCCH3 Ethanoic anhydride Ester CH3COCH2CH3 Ethyl ethanoate Amide CH3CNH Ethanamide The reactions of the carbonyl group feature prominently in organic synthesis-the branch of organic chemistry that plans and carries out the preparation of compounds of prescribed structure 2.4 INTRODUCTION TO ALKANES: METHANE, ETHANE, AND PROPANE Alkanes have the general molecular formula CnH2n+2. The simplest one, methane( CH4), "Methane and the bic is also the most abundant. Large amounts are present in our atmosphere, in the ground, sphere"that accompanies and in the oceans. Methane has been found on Jupiter, Saturn, Uranus, Neptune, and Pluto, and even on Halleys Comet. Ethane(C2H: CH3 CH3) and propane(C3 Hg: CH3 CH, CH3) are second and third, respectively, to methane in many ways. Ethane is the alkane next to methane in struc- tural simplicity, followed by propane. Ethane (e 10%) is the second and propane(- 5%0) the third most abundant component of natural gas, which is s 75%o methane. The char acteristic odor of natural gas we use for heating our homes and cooking comes from Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
The reactions of the carbonyl group feature prominently in organic synthesis—the branch of organic chemistry that plans and carries out the preparation of compounds of prescribed structure. 2.4 INTRODUCTION TO ALKANES: METHANE, ETHANE, AND PROPANE Alkanes have the general molecular formula CnH2n2. The simplest one, methane (CH4), is also the most abundant. Large amounts are present in our atmosphere, in the ground, and in the oceans. Methane has been found on Jupiter, Saturn, Uranus, Neptune, and Pluto, and even on Halley’s Comet. Ethane (C2H6: CH3CH3) and propane (C3H8: CH3CH2CH3) are second and third, respectively, to methane in many ways. Ethane is the alkane next to methane in structural simplicity, followed by propane. Ethane ( 10%) is the second and propane ( 5%) the third most abundant component of natural gas, which is 75% methane. The characteristic odor of natural gas we use for heating our homes and cooking comes from OH O OH Prostaglandin E1 HO O 56 CHAPTER TWO Alkanes TABLE 2.2 Classes of Compounds That Contain a Carbonyl Group Class Aldehyde Ketone Carboxylic acid Carboxylic acid derivatives: Acyl halide Acid anhydride Ester Amide Ethanal 2-Propanone Ethanoic acid Ethanoyl chloride Ethanoic anhydride Ethyl ethanoate Ethanamide Name of example Generalized abbreviation RCH O X RCR O X RCOH O X RCX O X RCOCR O X O X RCOR O X RCNR2 O X Representative example CH3CH O X CH3CCH3 O X CH3COH O X CH3CCl O X CH3COCCH3 O X O X CH3COCH2CH3 O X CH3CNH2 O X See the boxed essay: “Methane and the Biosphere” that accompanies this section. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
2.5 omeric alkanes the butane Ill pm 153pm Methane 2.1 Structures of trace amounts of unpleasant-smelling sulfur-containing compounds such as ethanethiol (see Table 2. 1) that are deliberately added to it in order to warn us of potentially dan and bond angles gerous leaks. Natural gas is colorless and nearly odorless, as are methane, ethane, and propane Methane is the lowest boiling alkane, followed by ethane, then propane CHA CH3 CH3 CH3CH, CH Propar text are at 1 atm(760 mm of Boiling point:-160°C mercury)unless otherwise This will generally be true as we proceed to look at other alkanes; as the number of car bon atoms increases, so does the boiling point. All the alkanes with four carbons or less are gases at room temperature and atmospheric pressure. with the highest boiling point of the three, propane is the easiest one to liquefy. We are all familiar with"propane tanks. These are steel containers in which a propane-rich mixture of hydrocarbons called liquefied petroleum gas(LPG) is maintained in a liquid state under high pressure convenient clean-burning fuel The structural features of methane, ethane, and propane are summarized in Figure 2.1. All of the carbon atoms are sp-hybridized, all of the bonds are o bonds, and the Modeling software to reproduce bond angles at carbon are close to tetrahedral so that you can better view their 2.5 ISOMERIC ALKANES: THE BUTANES Methane is the only alkane of molecular formula CH4, ethane the only one that is C2H and propane the only one that is C3Hg. Beginning with C4Hio, however, constitutional isomers(Section 1. 8)are possible; two alkanes have this particular molecular formula In one, called n-butane, four carbons are joined in a continuous chain. The n in n-butane stands for"normal and means that the carbon chain is unbranched. The second isomer has a branched carbon chain and is called isobutane CH:,,CH CHaCHCH or(CH3)3CH △ ake molecular models of the two isomers of Cah Boiling point: 0.4°C 10.2°C Melting poin 139°C 160.9°C As noted earlier(Section 1. 16), CH3 is called a methyl group. In addition to having methyl groups at both ends, n-butane contains two CH2, or methylene groups. Isobutane con- tains three methyl groups bonded to a CH unit. The Ch unit is called a methine group Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
trace amounts of unpleasant-smelling sulfur-containing compounds such as ethanethiol (see Table 2.1) that are deliberately added to it in order to warn us of potentially dangerous leaks. Natural gas is colorless and nearly odorless, as are methane, ethane, and propane. Methane is the lowest boiling alkane, followed by ethane, then propane. This will generally be true as we proceed to look at other alkanes; as the number of carbon atoms increases, so does the boiling point. All the alkanes with four carbons or less are gases at room temperature and atmospheric pressure. With the highest boiling point of the three, propane is the easiest one to liquefy. We are all familiar with “propane tanks.” These are steel containers in which a propane-rich mixture of hydrocarbons called liquefied petroleum gas (LPG) is maintained in a liquid state under high pressure as a convenient clean-burning fuel. The structural features of methane, ethane, and propane are summarized in Figure 2.1. All of the carbon atoms are sp3 -hybridized, all of the bonds are bonds, and the bond angles at carbon are close to tetrahedral. 2.5 ISOMERIC ALKANES: THE BUTANES Methane is the only alkane of molecular formula CH4, ethane the only one that is C2H6, and propane the only one that is C3H8. Beginning with C4H10, however, constitutional isomers (Section 1.8) are possible; two alkanes have this particular molecular formula. In one, called n-butane, four carbons are joined in a continuous chain. The n in n-butane stands for “normal” and means that the carbon chain is unbranched. The second isomer has a branched carbon chain and is called isobutane. As noted earlier (Section 1.16), CH3 is called a methyl group. In addition to having methyl groups at both ends, n-butane contains two CH2, or methylene groups. Isobutane contains three methyl groups bonded to a CH unit. The CH unit is called a methine group. CH3CH2CH2CH3 n-Butane 0.4°C 139°C Boiling point: Melting point: CH3CHCH3 W CH3 (CH3) or 3CH Isobutane 10.2°C 160.9°C CH4 Methane Boiling point: 160°C CH3CH3 Ethane 89°C CH3CH2CH3 Propane 42°C 2.5 Isomeric Alkanes: The Butanes 57 109 pm Methane 109.5 153 pm Ethane 111 111 pm 153 pm Propane 111 pm 112 FIGURE 2.1 Structures of methane, ethane, and propane showing bond distances and bond angles. Boiling points cited in this text are at 1 atm (760 mm of mercury) unless otherwise stated. Use your Learning By Modeling software to reproduce the models shown in Figure 2.1 so that you can better view their three-dimensional shapes. Make molecular models of the two isomers of C4H10. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website