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
CHAPTER TWO Alkanes METHANE AND THE BIOSPHERE* ne of the things that environmental scientists the atmosphere, but most of the rest simply ends up do is to keep track of important elements in completing the carbon cycle. It exits the anaerobic the biosphere-in what form do these ele- environment where it was formed and enters the ments normally occur, to what are they transformed, aerobic world where it is eventually converted to car and how are they returned to their normal state? bon dioxide by a variety of processes Careful studies have given clear, although compli- When we consider of methane we have cated, pictures of the"nitrogen cycle, the"sulfur cy- to add" old"methane, methane that was formed carbon ge.cycle, "begins ano ycle, " for example. The millions of years ago but became trapped beneath cle, "and the"phosphorus ends with atmospheric the earth's surface, to the"new"methane just d oxide. It can be represented in an abbrevi- scribed Firedamp, an explosion hazard to miners, oc- in layers of coal and is mostly methane. Petro- leum deposits, formed by microbial decomposition of CO2+ H2o+ energy carbohydrates plant material under anaerobic conditions, are al ways accompanied by pockets of natural gas, which is respiration An interesting thing happens when trapped ubstances of methane leaks from sites under the deep ocean floor. If the pres eno h (50 atm) and th cold enough(4oC), the methane doesn't simply bub Methane is one of literally millions of com- ble to the surface. Individual methane molecules be pounds in the carbon cycle, but one of the most come trapped inside clusters of 6-18 water molecules abundant. It is formed when carbon-containing com- forming methane clathrates or methane hydrates pounds decompose in the absence of air (anaerobic Aggregates of these clathrates stay at the bottom of conditions). The organisms that bring this about are the ocean in what looks like a lump of dirty ice.Ice called methanoarchaea. Cells can be divided into that burns. Far from being mere curiosities, methane three types: archaea, bacteria, and eukarya. clathrates are potential sources of energy on a scale Methanoarchaea are one kind of archaea and may greater than that of all known oil reserves combined rank among the oldest living things on earth. They At present, it is not economically practical to extract can convert a number of carbon-containing com- the methane however pounds, including carbon dioxide and acetic acid, to Methane clathrates have received recent atten- methane tion from a different segment of the scientific com- Virtually anywhere water contacts organic mat- munity. While diving in the Gulf of Mexico in 1997, a ter in the absence of air is a suitable place for research team of biologists and environmental scien- methanoarchaea to thrive--at the bottom of ponds, tists were surprised to find a new species of worm bogs, and rice fields, for example. Marsh gas (swamp grazing on the mound of a methane clathrate.What gas) is mostly methane Methanoarchaea live inside were these worms feeding on? Methane? Bacteria termites and grass-eating animals. One source quotes that live on the methane? A host of questions having 20 L/day as the methane output of a large cow. to do with deep-ocean ecosystems suddenly The scale on which methanoarchaea churn out emerged. Stay tuned methane, estimated to be 10 1-10'2 lb/year, is enor "The biosphere is the part of the earth where life is; it includes the mous. About 10% of this amount makes its way into surface, the oceans, and the lower atmospher n-Butane and isobutane have the same molecular formula but differ in the order in which their atoms are connected. They are constitutional isomers of each other (Section 1.8). Because they are different in structure, they can have different properties. Both are gases at room temperature, but n-butane boils almost 10C higher than isobutane and has a melting point that is over 20C higher Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
58 CHAPTER TWO Alkanes METHANE AND THE BIOSPHERE* One of the things that environmental scientists do is to keep track of important elements in the biosphere—in what form do these elements normally occur, to what are they transformed, and how are they returned to their normal state? Careful studies have given clear, although complicated, pictures of the “nitrogen cycle,” the “sulfur cycle,” and the “phosphorus cycle,” for example. The “carbon cycle,” begins and ends with atmospheric carbon dioxide. It can be represented in an abbreviated form as: Methane is one of literally millions of compounds in the carbon cycle, but one of the most abundant. It is formed when carbon-containing compounds decompose in the absence of air (anaerobic conditions). The organisms that bring this about are called methanoarchaea. Cells can be divided into three types: archaea, bacteria, and eukarya. Methanoarchaea are one kind of archaea and may rank among the oldest living things on earth. They can convert a number of carbon-containing compounds, including carbon dioxide and acetic acid, to methane. Virtually anywhere water contacts organic matter in the absence of air is a suitable place for methanoarchaea to thrive—at the bottom of ponds, bogs, and rice fields, for example. Marsh gas (swamp gas) is mostly methane. Methanoarchaea live inside termites and grass-eating animals. One source quotes 20 L/day as the methane output of a large cow. The scale on which methanoarchaea churn out methane, estimated to be 1011–1012 lb/year, is enormous. About 10% of this amount makes its way into CO2 H2O energy carbohydrates naturally occurring substances of numerous types photosynthesis respiration respiration the atmosphere, but most of the rest simply ends up completing the carbon cycle. It exits the anaerobic environment where it was formed and enters the aerobic world where it is eventually converted to carbon dioxide by a variety of processes. When we consider sources of methane we have to add “old” methane, methane that was formed millions of years ago but became trapped beneath the earth’s surface, to the “new” methane just described. Firedamp, an explosion hazard to miners, occurs in layers of coal and is mostly methane. Petroleum deposits, formed by microbial decomposition of plant material under anaerobic conditions, are always accompanied by pockets of natural gas, which is mostly methane. An interesting thing happens when trapped methane leaks from sites under the deep ocean floor. If the pressure is high enough (50 atm) and the water cold enough (4°C), the methane doesn’t simply bubble to the surface. Individual methane molecules become trapped inside clusters of 6–18 water molecules forming methane clathrates or methane hydrates. Aggregates of these clathrates stay at the bottom of the ocean in what looks like a lump of dirty ice. Ice that burns. Far from being mere curiosities, methane clathrates are potential sources of energy on a scale greater than that of all known oil reserves combined. At present, it is not economically practical to extract the methane, however. Methane clathrates have received recent attention from a different segment of the scientific community. While diving in the Gulf of Mexico in 1997, a research team of biologists and environmental scientists were surprised to find a new species of worm grazing on the mound of a methane clathrate. What were these worms feeding on? Methane? Bacteria that live on the methane? A host of questions having to do with deep-ocean ecosystems suddenly emerged. Stay tuned. *The biosphere is the part of the earth where life is; it includes the surface, the oceans, and the lower atmosphere. n-Butane and isobutane have the same molecular formula but differ in the order in which their atoms are connected. They are constitutional isomers of each other (Section 1.8). Because they are different in structure, they can have different properties. Both are gases at room temperature, but n-butane boils almost 10°C higher than isobutane and has a melting point that is over 20°C higher. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
2.6 Higher n-Alkanes The bonding in n-butane and isobutane continues the theme begun with methane ethane, and propane. All of the carbon atoms are sp-hybridized, all of the bonds are o about 5% n-butane and 95% bonds, and the bond angles at carbon are close to tetrahedral. This generalization holds isobutane in a sealed con- for all alkanes regardless of the number of carbons they have duced by the two compounds 2.6 HIGHER n-ALKANES eep them in the n-Alkanes are alkanes that have an unbranched carbon chain n-Pentane and n-hexane emits a fine stream of the va. prized mixture across a are n-alkanes possessing five and six carbon atoms, respectively k which ignites it. CH3CH, CH,CH, CH3 CH;CH, CH,CH,,CH I-Hexane Their condensed structural formulas can be abbreviated even more by indicating within parentheses the number of methylene groups in the chain. Thus, n-pentane may be writ- ten as CH3 (CH2)3 CH3 and n-hexane as CH3(CH2)4CH3. This shortcut is especially con- venient with longer-chain alkanes. The laboratory synthesis of the"ultralong "alkane CH3(CH2)388 CH3 was achieved in 1985; imagine trying to write a structural formula for this compound in anything other than an abbreviated way! PROBLEM 2.2 An n-alkane of molecular formula C28Hs has been isolated from a certain fossil plant. Write a condensed structural formula for this alkane. n-Alkanes have the general formula CH3(CH2),CH3 and are said to belong to a homologous series of compounds. A homologous series is one in which successive mem- bers differ by a-CH2- group Unbranched alkanes are sometimes referred to as"straight-chain alkanes but, as we'lI see in Chapter 3, their chains are not straight but instead tend to adopt the"zigzag shape portrayed in the bond-line formulas introduced in Section 1.7. Bond-line formula of n-hexane PROBLEM 2.3 Much of the communication between insects involves chemical messengers called pheromones. A species of cockroach secretes a substance from its mandibular glands that alerts other cockroaches to its presence and causes them to congregate. One of the principal components of this aggregation hermone is the alkane shown in the bond-line formula that follows Give the molecular formula of this substance and represent it by a condensed formula 2.7 THE CsH, ISOMERS Three isomeric alkanes have the molecular formula CsHiz. The unbranched isomer is, as we have seen, n-pentane. The isomer with a single methyl branch is called isopen- tane. The third isomer has a three-carbon chain with two methyl branches. It is called n-Pentane: CH: CH,CH? CH2 CH3 or CH=(CH?)3CH3 or (CH3) CHCH,CH3 or Make molecular models pentane: CH: CHCH,CH of the th mers of CsH12. CH Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
The bonding in n-butane and isobutane continues the theme begun with methane, ethane, and propane. All of the carbon atoms are sp3 -hybridized, all of the bonds are bonds, and the bond angles at carbon are close to tetrahedral. This generalization holds for all alkanes regardless of the number of carbons they have. 2.6 HIGHER n-ALKANES n-Alkanes are alkanes that have an unbranched carbon chain. n-Pentane and n-hexane are n-alkanes possessing five and six carbon atoms, respectively. Their condensed structural formulas can be abbreviated even more by indicating within parentheses the number of methylene groups in the chain. Thus, n-pentane may be written as CH3(CH2)3CH3 and n-hexane as CH3(CH2)4CH3. This shortcut is especially convenient with longer-chain alkanes. The laboratory synthesis of the “ultralong” alkane CH3(CH2)388CH3 was achieved in 1985; imagine trying to write a structural formula for this compound in anything other than an abbreviated way! PROBLEM 2.2 An n-alkane of molecular formula C28H58 has been isolated from a certain fossil plant. Write a condensed structural formula for this alkane. n-Alkanes have the general formula CH3(CH2)xCH3 and are said to belong to a homologous series of compounds. A homologous series is one in which successive members differ by a ±CH2± group. Unbranched alkanes are sometimes referred to as “straight-chain alkanes,” but, as we’ll see in Chapter 3, their chains are not straight but instead tend to adopt the “zigzag” shape portrayed in the bond-line formulas introduced in Section 1.7. PROBLEM 2.3 Much of the communication between insects involves chemical messengers called pheromones. A species of cockroach secretes a substance from its mandibular glands that alerts other cockroaches to its presence and causes them to congregate. One of the principal components of this aggregation pheromone is the alkane shown in the bond-line formula that follows. Give the molecular formula of this substance, and represent it by a condensed formula. 2.7 THE C5H12 ISOMERS Three isomeric alkanes have the molecular formula C5H12. The unbranched isomer is, as we have seen, n-pentane. The isomer with a single methyl branch is called isopentane. The third isomer has a three-carbon chain with two methyl branches. It is called neopentane. CH3CHCH2CH3 CH3 n-Pentane: Isopentane: CH3CH2CH2CH2CH3 or or CH3(CH2)3CH3 (CH3)2CHCH2CH3 or or Bond-line formula of n-pentane Bond-line formula of n-hexane CH3CH2CH2CH2CH3 n-Pentane CH3CH2CH2CH2CH2CH3 n-Hexane 2.6 Higher n-Alkanes 59 “Butane” lighters contain about 5% n-butane and 95% isobutane in a sealed container. The pressure produced by the two compounds (about 3 atm) is enough to keep them in the liquid state until opening a small valve emits a fine stream of the vaporized mixture across a spark which ignites it. Make molecular models of the three isomers of C5H12. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TWO Alkanes CH3 Neopentane: CH3CCH (CH3)C he number of C,H2n+z iso. Table 2.3 presents the number of possible alkane isomers as a function of the num- mers has been calculated for ber of carbon atoms they contain. As the table shows, the number of isomers increases alues of n from 1 to 40 enormously with the number of carbon atoms and raises two important questions and the comment ma l. How can we tell when we have written all the possible isomers corresponding to C157H33s exceeds the number a particular molecular formula? niverse(10). These obser 2. How can we name alkanes so that each one has a unique name? ations and the historical The answer to the first question is that you cannot easily calculate the number of April 1989 issue of isomers. The data in Table 2.3 were determined by a mathematician who concluded that the Journal of Chemical Edu- there was no simple expression from which to calculate the number of isomers. The best cation(pp. 278-281). way to ensure that you have written all the isomers of a particular molecular formula is to work systematically, beginning with the unbranched chain and then shortening it while dding branches one by one. It is essential that you be able to recognize when two different looking structural formulas are actually the same molecule written in different ways. The key point is the connectivity of the carbon chain. For example, the following group of struc tural formulas do not represent different compounds; they are just a portion of the many ways we could write a structural formula for isopentane. Each one has a continuous chain of four carbons with a methyl branch located one carbon from the end of the chain CH The fact that all of these CH3,CI CH3CHCH2CH3 CH3 CHCHCH3 ame substance can be clearly CH3 CH3CH,CHCH3 CHCH, CH3 H3 TABLE 2. 3 The Number of Constitutionally Isomeric Alkanes of Particular Molecular formulas Molecular formula Number of constitutional isomers CHa CaH 1235 C6H14 C7H16 cC 855 15n32 4,347 366,319 62491,178805831 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
60 CHAPTER TWO Alkanes Table 2.3 presents the number of possible alkane isomers as a function of the number of carbon atoms they contain. As the table shows, the number of isomers increases enormously with the number of carbon atoms and raises two important questions: 1. How can we tell when we have written all the possible isomers corresponding to a particular molecular formula? 2. How can we name alkanes so that each one has a unique name? The answer to the first question is that you cannot easily calculate the number of isomers. The data in Table 2.3 were determined by a mathematician who concluded that there was no simple expression from which to calculate the number of isomers. The best way to ensure that you have written all the isomers of a particular molecular formula is to work systematically, beginning with the unbranched chain and then shortening it while adding branches one by one. It is essential that you be able to recognize when two differentlooking structural formulas are actually the same molecule written in different ways. The key point is the connectivity of the carbon chain. For example, the following group of structural formulas do not represent different compounds; they are just a portion of the many ways we could write a structural formula for isopentane. Each one has a continuous chain of four carbons with a methyl branch located one carbon from the end of the chain. CH3CHCH2CH3 W CH3 CH3CHCH2CH3 W CH3 CH3CH2CHCH3 W CH3 CH3CH2CHCH3 W CH3 CHCH2CH3 W W CH3 CH3 CH3 CH3CCH3 CH3 Neopentane: or (CH3)4C or TABLE 2.3 The Number of Constitutionally Isomeric Alkanes of Particular Molecular Formulas Molecular formula CH4 C2H6 C3H8 C4H10 C5H12 C6H14 C7H16 C8H18 C9H20 C10H22 C15H32 C20H42 C40H82 Number of constitutional isomers 1 1 1 2 3 5 9 18 35 75 4,347 366,319 62,491,178,805,831 The number of CnH2n2 isomers has been calculated for values of n from 1 to 400 and the comment made that the number of isomers of C167H336 exceeds the number of particles in the known universe (1080 ). These observations and the historical background of isomer calculation are described in a paper in the April 1989 issue of the Journal of Chemical Education (pp. 278–281). The fact that all of these structural formulas represent the same substance can be clearly seen by making molecular models. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
2.8 IUPAC Nomenclature of unbranched alkanes PROBLEM 2. 4 Write condensed and bond-line formulas for the five isomeric C6H14 alkanes. SAMPLE SOLUTION When writing isomeric alkanes, it is best to begin with the CHa CH2 CH2CI Next, remove a carbon from the chain and use it as a one-carbon(methyl)branch at the carbon atom next to the end of the chain CH3CHCH2 CH2 CH3 or Now, write structural formulas for the remaining three isomers. Be sure that each one is a unique compound and not simply a different representation of one writ- ten previously. The answer to the second question--how to provide a name that is unique to a particular structure--is presented in the following section. It is worth noting, however, that being able to name compounds in a systematic way is a great help in deciding whether two structural formulas represent isomeric substances or are the same compound represented in two different ways. By following a precise set of rules, one will always get the same systematic name for a compound, regardless of how it is written. Con versely, two different compounds will always have different names. 2.8 IUPAC NOMENCLATURE OF UNBRANCHED ALKANES Nomenclature in organic chemistry is of two types: common(or""trivial")and system atic. Some common names existed long before organic chemistry became an organized branch of chemical science. Methane, ethane, propane, n-butane, isobutane, n-pentane, isopentane, and neopentane are common names. One simply memorizes the name that goes with a compound in just the same way that one matches names with faces. So long as there are only a few names and a few compounds, the task is manageable. But there are millions of organic compounds already known, and the list continues to grow! A sys- tem built on common names is not adequate to the task of communicating structural information. Beginning in 1892, chemists developed a set of rules for naming organic compounds based on their structures, which we now call the IUPAC rules, in which IUPAC stands for the"International Union of Pure and Applied Chemistry. " (See the accompanying box, "A Brief History of Systematic Organic Nomenclature.) A more detailed account of The IUPAC rules assign names to unbranched alkanes as shown in Table 2. 4. nomenclature may be found Methane, ethane, propane, and butane are retained for CH4, CH3 CH3, CH3CH2CH3, and in the article"The Centen- CH3 CH2 CH2CH3, respectively. Thereafter, the number of carbon atoms in the chain is Nomenclature"in the n specified by a Latin or Greek prefix preceding the suffix -ane, which identifies the com- vember 1992 issue of the pound as a member of the alkane family. Notice that the prefix n- is not part of the Journal of chemical Educa- IUPAC system. The IuPAC name for CH3 CH2CH2CH3 is butane, not n-butane. PROBLEM 2.5 Refer to Table 2.4 as needed to answer the following questions (a) Beeswax contains 8-9% hentriacontane write a condensed structural formula for hentriacontane (b)Octacosane has been found to be present in a certain fossil plant. write a con- densed structural formula for octacosane Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
PROBLEM 2.4 Write condensed and bond-line formulas for the five isomeric C6H14 alkanes. SAMPLE SOLUTION When writing isomeric alkanes, it is best to begin with the unbranched isomer. Next, remove a carbon from the chain and use it as a one-carbon (methyl) branch at the carbon atom next to the end of the chain. Now, write structural formulas for the remaining three isomers. Be sure that each one is a unique compound and not simply a different representation of one written previously. The answer to the second question—how to provide a name that is unique to a particular structure—is presented in the following section. It is worth noting, however, that being able to name compounds in a systematic way is a great help in deciding whether two structural formulas represent isomeric substances or are the same compound represented in two different ways. By following a precise set of rules, one will always get the same systematic name for a compound, regardless of how it is written. Conversely, two different compounds will always have different names. 2.8 IUPAC NOMENCLATURE OF UNBRANCHED ALKANES Nomenclature in organic chemistry is of two types: common (or “trivial”) and systematic. Some common names existed long before organic chemistry became an organized branch of chemical science. Methane, ethane, propane, n-butane, isobutane, n-pentane, isopentane, and neopentane are common names. One simply memorizes the name that goes with a compound in just the same way that one matches names with faces. So long as there are only a few names and a few compounds, the task is manageable. But there are millions of organic compounds already known, and the list continues to grow! A system built on common names is not adequate to the task of communicating structural information. Beginning in 1892, chemists developed a set of rules for naming organic compounds based on their structures, which we now call the IUPAC rules, in which IUPAC stands for the “International Union of Pure and Applied Chemistry.” (See the accompanying box, “A Brief History of Systematic Organic Nomenclature.”) The IUPAC rules assign names to unbranched alkanes as shown in Table 2.4. Methane, ethane, propane, and butane are retained for CH4, CH3CH3, CH3CH2CH3, and CH3CH2CH2CH3, respectively. Thereafter, the number of carbon atoms in the chain is specified by a Latin or Greek prefix preceding the suffix -ane, which identifies the compound as a member of the alkane family. Notice that the prefix n- is not part of the IUPAC system. The IUPAC name for CH3CH2CH2CH3 is butane, not n-butane. PROBLEM 2.5 Refer to Table 2.4 as needed to answer the following questions: (a) Beeswax contains 8–9% hentriacontane. Write a condensed structural formula for hentriacontane. (b) Octacosane has been found to be present in a certain fossil plant. Write a condensed structural formula for octacosane. CH3CHCH2CH2CH3 or CH3 CH3CH2CH2CH2CH2CH3 or 2.8 IUPAC Nomenclature of Unbranched Alkanes 61 A more detailed account of the history of organic nomenclature may be found in the article “The Centennial of Systematic Organic Nomenclature” in the November 1992 issue of the Journal of Chemical Education (pp. 863–865). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TWO Alkanes TABLE 2.4 IUPAC Names of Unbranched Alkanes Number Number Number of carbon of carbon Name of carbon Name Methane Undecane Henicosane Ethane 123456789 Dodecane 1234012 Docosane 23456 3 Propane Tridecane Tricosane Butane Tetradecane Tetracosane Pentane Pentadecane Triacontane Hexane Hexadecane Hentriacontane Heptane Heptadecane Dotriacontane 8 Octane Octadecane Tetracontane Nonane Nonadecane 60 Pentacontane Decane Icosane 100 Hectare Spelled"eicosane"prior to 1979 version of IUPAC rules. (c)What is the IUPAC name of the alkane described in Problem 2.3 as a compo- nent of the cockroach aggregation pheromone? SAMPLE SoLUTION(a) Note in Table 2.4 that hentriacontane has 31 carbon atoms. All the alkanes in Table 2,4 have unbranched carbon chains. Hentriacon tane has the condensed structural formula CH3(CH2)29CH3 In Problem 2. 4 you were asked to write structural formulas for the five alkanes of molecular formula C6Hi4. In the next section you will see how the IUPAC rules generate a unique name for each isomer 2.9 APPLYING THE IUPAC RULES: THE NAMES OF THE C6H14 ISOMERS We can present and illustrate the most important of the IUPAC rules for alkane nomen You might find it helpful clature by naming the five C6Hi4 isomers. By definition (Table 2.4), the unbranc C6H14 isomer is the ChIa isomers CH3CH, CH,CH, CH,CH3 IUPAC name: hexane The IUPAC rules name branched alkanes as substituted derivatives of the unbranched alkanes listed in Table 2. 4. Consider the C6H14 isomer represented by the structure CH3CHCH, CH,CH Step I Pick out the longest continuous carbon chain, and find the IupAc name in Table 2.4 that corresponds to the unbranched alkane having that number of carbons. This is the parent alkane from which the IUPAC name is to be derived. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
62 CHAPTER TWO Alkanes (c) What is the IUPAC name of the alkane described in Problem 2.3 as a component of the cockroach aggregation pheromone? SAMPLE SOLUTION (a) Note in Table 2.4 that hentriacontane has 31 carbon atoms. All the alkanes in Table 2.4 have unbranched carbon chains. Hentriacontane has the condensed structural formula CH3(CH2)29CH3. In Problem 2.4 you were asked to write structural formulas for the five isomeric alkanes of molecular formula C6H14. In the next section you will see how the IUPAC rules generate a unique name for each isomer. 2.9 APPLYING THE IUPAC RULES: THE NAMES OF THE C6H14 ISOMERS We can present and illustrate the most important of the IUPAC rules for alkane nomenclature by naming the five C6H14 isomers. By definition (Table 2.4), the unbranched C6H14 isomer is hexane. The IUPAC rules name branched alkanes as substituted derivatives of the unbranched alkanes listed in Table 2.4. Consider the C6H14 isomer represented by the structure Step 1 Pick out the longest continuous carbon chain, and find the IUPAC name in Table 2.4 that corresponds to the unbranched alkane having that number of carbons. This is the parent alkane from which the IUPAC name is to be derived. CH3CHCH2CH2CH3 W CH3 CH3CH2CH2CH2CH2CH3 IUPAC name: hexane (common name: n-hexane) TABLE 2.4 IUPAC Names of Unbranched Alkanes Number of carbon atoms 1 2 3 4 5 6 7 8 9 10 Name Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane Name Undecane Dodecane Tridecane Tetradecane Pentadecane Hexadecane Heptadecane Octadecane Nonadecane Icosane* Number of carbon atoms 11 12 13 14 15 16 17 18 19 20 Name Henicosane Docosane Tricosane Tetracosane Triacontane Hentriacontane Dotriacontane Tetracontane Pentacontane Hectane Number of carbon atoms 21 22 23 24 30 31 32 40 50 100 *Spelled “eicosane” prior to 1979 version of IUPAC rules. You might find it helpful to make molecular models of all the C6H14 isomers. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
