有机化学（英文原版）：Chapt02

53 CHAPTER 2 ALKANES N ow that we’ve 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 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 orga- nizes 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 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. 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 hydro- gen 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 hydro- gen by chlorine is a characteristic reaction of all alkanes and can be represented for the general case by the equation: H11001H11001R±H Alkane Cl 2 Chlorine R±Cl Alkyl chloride HCl Hydrogen chloride H11001H11001CH 3 CH 3 Ethane Cl 2 Chlorine CH 3 CH 2 Cl Chloroethane HCl Hydrogen chloride C C C C C C HH H H H H Benzene (arene) Ethane (alkane) HC H H C H H H Ethylene (alkene) HH HH CC Acetylene (alkyne) H CCH 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 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 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 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- containing 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 E 1 , a hormone that regulates the relaxation of smooth muscles, contains two different kinds of carbonyl groups. Classify each one (alde- hyde, ketone, carboxylic acid, ester, amide, acyl chloride, or carboxylic acid anhy- dride). 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 (RNH 2 ). Secondary amines have the general structure R 2 NH; tertiary amines are R 3 N. Representative example CH 3 CH 2 OH CH 3 CH 2 Cl CH 3 CH 2 NH 2 CH 3 CH 2 OCH 2 CH 3 CH 3 CH 2 CPN CH 3 CH 2 NO 2 CH 3 CH 2 SH H 2 CCH 2 O Generalized abbreviation ROH RCl RNH 2 ROR RCPN RNO 2 RSH R 2 CCR 2 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 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 C n H 2nH110012 . The simplest one, methane (CH 4 ), 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 (C 2 H 6 : CH 3 CH 3 ) and propane (C 3 H 8 : CH 3 CH 2 CH 3 ) 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 (H11015 10%) is the second and propane (H11015 5%) the third most abundant component of natural gas, which is H11015 75% methane. The char- acteristic odor of natural gas we use for heating our homes and cooking comes from OH O OH Prostaglandin E 1 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 RCNR 2 O X Representative example CH 3 CH O X CH 3 CCH 3 O X CH 3 COH O X CH 3 CCl O X CH 3 COCCH 3 O X O X CH 3 COCH 2 CH 3 O X CH 3 CNH 2 O X See the boxed essay: “Methane and the Bio- sphere” that accompanies this section. Back Forward Main Menu TOC Study Guide TOC Student OLC 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 dan- 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. 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 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 sp 3 -hybridized, all of the bonds are H9268 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 CH 4 , ethane the only one that is C 2 H 6 , and propane the only one that is C 3 H 8 . Beginning with C 4 H 10 , 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), CH 3 is called a methyl group. In addition to having methyl groups at both ends, n-butane contains two CH 2 , or methylene groups. Isobutane con- tains three methyl groups bonded to a CH unit. The CH unit is called a methine group. CH 3 CH 2 CH 2 CH 3 n-Butane H110020.4°C H11002139°C Boiling point: Melting point: CH 3 CHCH 3 W CH 3 (CH 3 ) 3 CHor Isobutane H1100210.2°C H11002160.9°C CH 4 Methane H11002160°CBoiling point: CH 3 CH 3 Ethane H1100289°C CH 3 CH 2 CH 3 Propane H1100242°C 2.5 Isomeric Alkanes: The Butanes 57 109 pm Methane 109.5H11034 153 pm Ethane 111H11034 111 pm 153 pm Propane 111 pm 112H11034 FIGURE 2.1 Structures of methane, ethane, and propane showing bond dis- tances 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 C 4 H 10 . Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 58 CHAPTER TWO Alkanes METHANE AND THE BIOSPHERE* O ne of the things that environmental scientists do is to keep track of important elements in the biosphere—in what form do these ele- ments normally occur, to what are they transformed, and how are they returned to their normal state? Careful studies have given clear, although compli- cated, pictures of the “nitrogen cycle,” the “sulfur cy- cle,” and the “phosphorus cycle,” for example. The “carbon cycle,” begins and ends with atmospheric carbon dioxide. It can be represented in an abbrevi- ated form as: Methane is one of literally millions of com- pounds in the carbon cycle, but one of the most abundant. It is formed when carbon-containing com- pounds 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 com- pounds, including carbon dioxide and acetic acid, to methane. Virtually anywhere water contacts organic mat- ter 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 10 11 –10 12 lb/year, is enor- mous. About 10% of this amount makes its way into CO 2 H 2 O energy carbohydrates naturally occurring substances of numerous types H11001H11001 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 car- bon 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 de- scribed. Firedamp, an explosion hazard to miners, oc- curs in layers of coal and is mostly methane. Petro- leum deposits, formed by microbial decomposition of plant material under anaerobic conditions, are al- ways 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 bub- ble to the surface. Individual methane molecules be- come 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 atten- tion from a different segment of the scientific com- munity. While diving in the Gulf of Mexico in 1997, a research team of biologists and environmental scien- tists 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 The bonding in n-butane and isobutane continues the theme begun with methane, ethane, and propane. All of the carbon atoms are sp 3 -hybridized, all of the bonds are H9268 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 writ- ten as CH 3 (CH 2 ) 3 CH 3 and n-hexane as CH 3 (CH 2 ) 4 CH 3 . This shortcut is especially con- venient with longer-chain alkanes. The laboratory synthesis of the “ultralong” alkane CH 3 (CH 2 ) 388 CH 3 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 C 28 H 58 has been isolated from a certain fossil plant. Write a condensed structural formula for this alkane. n-Alkanes have the general formula CH 3 (CH 2 ) x CH 3 and are said to belong to a homologous series of compounds. A homologous series is one in which successive mem- bers differ by a ±CH 2 ± 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 C 5 H 12 ISOMERS Three isomeric alkanes have the molecular formula C 5 H 12 . 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 neopentane. CH 3 CHCH 2 CH 3 CH 3 n-Pentane: Isopentane: CH 3 CH 2 CH 2 CH 2 CH 3 or or CH 3 (CH 2 ) 3 CH 3 (CH 3 ) 2 CHCH 2 CH 3 or or Bond-line formula of n-pentane Bond-line formula of n-hexane CH 3 CH 2 CH 2 CH 2 CH 3 n-Pentane CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 n-Hexane 2.6 Higher n-Alkanes 59 “Butane” lighters contain about 5% n-butane and 95% isobutane in a sealed con- tainer. The pressure pro- duced 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 va- porized mixture across a spark which ignites it. Make molecular models of the three isomers of C 5 H 12 . Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 60 CHAPTER TWO Alkanes Table 2.3 presents the number of possible alkane isomers as a function of the num- ber 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 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 3 CHCH 2 CH 3 W CH 3 CH 3 CHCH 2 CH 3 W CH 3 CH 3 CH 2 CHCH 3 W CH 3 CH 3 CH 2 CHCH 3 W CH 3 CHCH 2 CH 3 W W CH 3 CH 3 CH 3 CH 3 CCH 3 CH 3 Neopentane: or (CH 3 ) 4 C or TABLE 2.3 The Number of Constitutionally Isomeric Alkanes of Particular Molecular Formulas Molecular formula CH 4 C 2 H 6 C 3 H 8 C 4 H 10 C 5 H 12 C 6 H 14 C 7 H 16 C 8 H 18 C 9 H 20 C 10 H 22 C 15 H 32 C 20 H 42 C 40 H 82 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 C n H 2nH110012 iso- mers has been calculated for values of n from 1 to 400 and the comment made that the number of isomers of C 167 H 336 exceeds the number of particles in the known universe (10 80 ). These obser- vations and the historical background of isomer calcu- lation are described in a pa- per in the April 1989 issue of the Journal of Chemical Edu- cation (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 PROBLEM 2.4 Write condensed and bond-line formulas for the five isomeric C 6 H 14 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 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.”) The IUPAC rules assign names to unbranched alkanes as shown in Table 2.4. Methane, ethane, propane, and butane are retained for CH 4 , CH 3 CH 3 , CH 3 CH 2 CH 3 , and CH 3 CH 2 CH 2 CH 3 , 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 com- pound as a member of the alkane family. Notice that the prefix n- is not part of the IUPAC system. The IUPAC name for CH 3 CH 2 CH 2 CH 3 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. orCH 3 CHCH 2 CH 2 CH 3 CH 3 CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 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 Centen- nial of Systematic Organic Nomenclature” in the No- vember 1992 issue of the Journal of Chemical Educa- tion (pp. 863–865). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 62 CHAPTER TWO Alkanes (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 CH 3 (CH 2 ) 29 CH 3 . In Problem 2.4 you were asked to write structural formulas for the five isomeric alkanes of molecular formula C 6 H 14 . 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 C 6 H 14 ISOMERS We can present and illustrate the most important of the IUPAC rules for alkane nomen- clature by naming the five C 6 H 14 isomers. By definition (Table 2.4), the unbranched C 6 H 14 isomer is hexane. The IUPAC rules name branched alkanes as substituted derivatives of the unbranched alkanes listed in Table 2.4. Consider the C 6 H 14 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. CH 3 CHCH 2 CH 2 CH 3 W CH 3 CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 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 C 6 H 14 isomers. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website In this case, the longest continuous chain has five carbon atoms; the compound is named as a derivative of pentane. The key word here is continuous. It does not matter whether the carbon skeleton is drawn in an extended straight-chain form or in one with many bends and turns. All that matters is the number of carbons linked together in an uninterrupted sequence. Step 2 Identify the substituent groups attached to the parent chain. The parent pentane chain bears a methyl (CH 3 ) group as a substituent. 2.9 Applying the IUPAC Rules: The Names of the C 6 H 14 Isomers 63 A BRIEF HISTORY OF SYSTEMATIC ORGANIC NOMENCLATURE T he first successful formal system of chemical nomenclature was advanced in France in 1787 to replace the babel of common names which then plagued the science. Hydrogen (instead of “in- flammable air”) and oxygen (instead of “vital air”) are just two of the substances that owe their modern names to the proposals described in the Méthode de nomenclature chimique. It was then that important compounds such as sulfuric, phosphoric, and carbonic acid and their salts were named. The guidelines were more appropriate to inorganic compounds; it was not until the 1830s that names reflecting chemical com- position began to appear in organic chemistry. In 1889, a group with the imposing title of the International Commission for the Reform of Chemical Nomenclature was organized, and this group, in turn, sponsored a meeting of 34 prominent European chemists in Switzerland in 1892. Out of this meeting arose a system of organic nomenclature known as the Geneva rules. The principles on which the Geneva rules were based are the forerunners of our present system. A second international conference was held in 1911, but the intrusion of World War I prevented any substantive revisions of the Geneva rules. The Inter- national Union of Chemistry was established in 1930 and undertook the necessary revision leading to pub- lication in 1930 of what came to be known as the Liège rules. After World War II, the International Union of Chemistry became the International Union of Pure and Applied Chemistry (known in the chemical com- munity as the IUPAC). Since 1949, the IUPAC has is- sued reports on chemical nomenclature on a regular basis. The most recent IUPAC rules for organic chem- istry were published in 1993. The IUPAC rules often offer several different ways to name a single com- pound. Thus although it is true that no two com- pounds can have the same name, it is incorrect to be- lieve that there is only a single IUPAC name for a par- ticular compound. The 1993 IUPAC recommendations and their more widely used 1979 predecessors may both be accessed at the same web site: www.acdlabs.com/iupac/nomenclature The IUPAC rules are not the only nomenclature system in use today. Chemical Abstracts Service sur- veys all the world’s leading scientific journals that publish papers relating to chemistry and publishes brief abstracts of those papers. The publication Chemical Abstracts and its indexes are absolutely es- sential to the practice of chemistry. For many years Chemical Abstracts nomenclature was very similar to IUPAC nomenclature, but the tremendous explosion of chemical knowledge in recent years has required Chemical Abstracts to modify its nomenclature so that its indexes are better adapted to computerized searching. This means that whenever feasible, a com- pound has a single Chemical Abstracts name. Unfor- tunately, this Chemical Abstracts name may be differ- ent from any of the several IUPAC names. In general, it is easier to make the mental connection between a chemical structure and its IUPAC name than its Chem- ical Abstracts name. It is worth noting that the generic name of a drug is not directly derived from systematic nomen- clature. Furthermore, different pharmaceutical com- panies will call the same drug by their own trade name, which is different from its generic name. Generic names are invented on request (for a fee) by the U.S. Adopted Names Council, a private organiza- tion founded by the American Medical Association, the American Pharmaceutical Association, and the U.S. Pharmacopeial Convention. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 64 CHAPTER TWO Alkanes Step 3 Number the longest continuous chain in the direction that gives the lowest number to the substituent group at the first point of branching. The numbering scheme Both schemes count five carbon atoms in their longest continuous chain and bear a methyl group as a substituent at the second carbon. An alternative numbering sequence that begins at the other end of the chain is incorrect: Step 4 Write the name of the compound. The parent alkane is the last part of the name and is preceded by the names of the substituent groups and their numerical locations (locants). Hyphens separate the locants from the words. The same sequence of four steps gives the IUPAC name for the isomer that has its methyl group attached to the middle carbon of the five-carbon chain. Both remaining C 6 H 14 isomers have two methyl groups as substituents on a four- carbon chain. Thus the parent chain is butane. When the same substituent appears more than once, use the multiplying prefixes di-, tri-, tetra-, and so on. A separate locant is used for each substituent, and the locants are separated from each other by commas and from the words by hyphens. PROBLEM 2.6 Phytane is a naturally occurring alkane produced by the alga Spirogyra and is a constituent of petroleum. The IUPAC name for phytane is 2,6,10,14-tetramethylhexadecane. Write a structural formula for phytane. PROBLEM 2.7 Derive the IUPAC names for (a) The isomers of C 4 H 10 (c) (CH 3 ) 3 CCH 2 CH(CH 3 ) 2 (b) The isomers of C 5 H 12 (d) (CH 3 ) 3 CC(CH 3 ) 3 IUPAC name: 2,2-dimethylbutane CH 3 CCH 2 CH 3 W W CH 3 CH 3 IUPAC name: 2,3-dimethylbutane CH 3 CHCHCH 3 W CH 3 W CH 3 IUPAC name: 3-methylpentaneCH 3 CH 2 CHCH 2 CH 3 W CH 3 IUPAC name: 2-methylpentane CH 3 CHCH 2 CH 2 CH 3 W CH 3 CH 3 CHCH 2 CH 2 CH 3 W CH 3 (methyl group attached to C-4) 54321 CH 3 CHCH 2 CH 2 CH 3 W CH 3 CH 3 CHCH 2 CH 2 CH 3 W CH 3 is equivalent to 12345 1 2345 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website SAMPLE SOLUTION (a) There are two C 4 H 10 isomers. Butane (see Table 2.4) is the IUPAC name for the isomer that has an unbranched carbon chain. The other isomer has three carbons in its longest continuous chain with a methyl branch at the central carbon; its IUPAC name is 2-methylpropane. So far, the only branched alkanes that we’ve named have methyl groups attached to the main chain. What about groups other than CH 3 ? What do we call these groups, and how do we name alkanes that contain them? 2.10 ALKYL GROUPS An alkyl group lacks one of the hydrogen substituents of an alkane. A methyl group (CH 3 ±) is an alkyl group derived from methane (CH 4 ). Unbranched alkyl groups in which the point of attachment is at the end of the chain are named in IUPAC nomen- clature by replacing the -ane endings of Table 2.4 by -yl. The dash at the end of the chain represents a potential point of attachment for some other atom or group. Carbon atoms are classified according to their degree of substitution by other car- bons. A primary carbon is one that is directly attached to one other carbon. Similarly, a secondary carbon is directly attached to two other carbons, a tertiary carbon to three, and a quaternary carbon to four. Alkyl groups are designated as primary, secondary, or tertiary according to the degree of substitution of the carbon at the potential point of attachment. Ethyl (CH 3 CH 2 ±), heptyl [CH 3 (CH 2 ) 5 CH 2 ±], and octadecyl [CH 3 (CH 2 ) 16 CH 2 ±] are examples of primary alkyl groups. Branched alkyl groups are named by using the longest continuous chain that begins at the point of attachment as the base name. Thus, the systematic names of the two C 3 H 7 alkyl groups are propyl and 1-methylethyl. Both are better known by their common names, n-propyl and isopropyl, respectively. Propyl group (common name: n-propyl) CH 3 CH 2 CH 2 ± 1-Methylethyl group (common name: isopropyl) (CH 3 ) 2 CH±orCH 3 CH± W CH 3 12 Primary carbon Primary alkyl group C H H C Secondary carbon Secondary alkyl group C H C C Tertiary carbon Tertiary alkyl group CC C C CH 3 CH 2 ± Ethyl group CH 3 (CH 2 ) 5 CH 2 ± Heptyl group CH 3 (CH 2 ) 16 CH 2 ± Octadecyl group CH 3 CH 2 CH 2 CH 3 IUPAC name: butane (common name: n-butane) IUPAC name: 2-methylpropane (common name: isobutane) CH 3 CHCH 3 CH 3 2.10 Alkyl Groups 65 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website An isopropyl group is a secondary alkyl group. Its point of attachment is to a secondary carbon atom, one that is directly bonded to two other carbons. The C 4 H 9 alkyl groups may be derived either from the unbranched carbon skele- ton of butane or from the branched carbon skeleton of isobutane. Those derived from butane are the butyl (n-butyl) group and the 1-methylpropyl (sec-butyl) group. Those derived from isobutane are the 2-methylpropyl (isobutyl) group and the 1,1-dimethylethyl (tert-butyl) group. Isobutyl is a primary alkyl group because its poten- tial point of attachment is to a primary carbon. tert-Butyl is a tertiary alkyl group because its potential point of attachment is to a tertiary carbon. PROBLEM 2.8 Give the structures and IUPAC names of all the C 5 H 11 alkyl groups, and identify them as primary, secondary, or tertiary alkyl groups, as appropriate. SAMPLE SOLUTION Consider the alkyl group having the same carbon skeleton as (CH 3 ) 4 C. All the hydrogens are equivalent, so that replacing any one of them by a potential point of attachment is the same as replacing any of the others. Numbering always begins at the point of attachment and continues through the longest continuous chain. In this case the chain is three carbons and there are two methyl groups at C-2. The IUPAC name of this alkyl group is 2,2-dimethylpropyl. (The common name for this group is neopentyl.) It is a primary alkyl group because the carbon that bears the potential point of attachment (C-1) is itself directly bonded to one other carbon. In addition to methyl and ethyl groups, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, and neopentyl groups will appear often throughout this text. Although these are common names, they have been integrated into the IUPAC system and are an acceptable adjunct to systematic nomenclature. You should be able to recognize these groups on sight and to give their structures when needed. 2.11 IUPAC NAMES OF HIGHLY BRANCHED ALKANES By combining the basic principles of IUPAC notation with the names of the various alkyl groups, we can develop systematic names for highly branched alkanes. We’ll start with the following alkane, name it, then increase its complexity by successively adding methyl groups at various positions. or (CH 3 ) 3 CCH 2 CH 3 1 32 C CH 3 CH 2 CH 3 2-Methylpropyl group (common name: isobutyl) or (CH 3 ) 2 CHCH 2 ±CH 3 CHCH 2 ± 23 W 1 CH 3 1,1-Dimethylethyl group (common name: tert-butyl) (CH 3 ) 3 C±orCH 3 C± 2 W W1 CH 3 CH 3 Butyl group (common name: n-butyl) CH 3 CH 2 CH 2 CH 2 ± 1-Methylpropyl group (common name: sec-butyl) CH 3 CH 2 CH± W CH 3 213 66 CHAPTER TWO Alkanes The names and structures of the most frequently encoun- tered alkyl groups are given on the inside back cover. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website As numbered on the structural formula, the longest continuous chain contains eight car- bons, and so the compound is named as a derivative of octane. Numbering begins at the end nearest the branch, and so the ethyl substituent is located at C-4, and the name of the alkane is 4-ethyloctane. What happens to the IUPAC name when a methyl replaces one of the hydrogens at C-3? The compound becomes an octane derivative that bears a C-3 methyl group and a C-4 ethyl group. When two or more different substituents are present, they are listed in alphabetical order in the name. The IUPAC name for this compound is 4-ethyl-3-methyloctane. Replicating prefixes such as di-, tri-, and tetra- (see Section 2.9) are used as needed but are ignored when alphabetizing. Adding a second methyl group to the original struc- ture, at C-5, for example, converts it to 4-ethyl-3,5-dimethyloctane. Italicized prefixes such as sec- and tert- are ignored when alphabetizing except when they are compared with each other. tert-Butyl precedes isobutyl, and sec-butyl precedes tert-butyl. PROBLEM 2.9 Give an acceptable IUPAC name for each of the following alkanes: (a) (b) (CH 3 CH 2 ) 2 CHCH 2 CH(CH 3 ) 2 (c) SAMPLE SOLUTION (a) This problem extends the preceding discussion by adding a third methyl group to 4-ethyl-3,5-dimethyloctane, the compound just described. It is, therefore, an ethyltrimethyloctane. Notice, however, that the num- bering sequence needs to be changed in order to adhere to the rule of number- ing from the end of the chain nearest the first branch. When numbered properly, this compound has a methyl group at C-2 as its first-appearing substituent. CH 3 CH 2 CHCH 2 CHCH 2 CHCH(CH 3 ) 2 CH 2 CH 3 CH 2 CH(CH 3 ) 2 CH 3 CH 3 CH 2 CHCHCHCH 2 CHCH 3 CH 3 CH 3 CH 3 CH 2 CH 3 CH 3 CH 2 CHCHCHCH 2 CH 2 CH 3 23 45678 1 W CH 2 CH 3 W CH 3 W CH 3 CH 3 CH 2 CHCHCH 2 CH 2 CH 2 CH 3 23 45678 1 W CH 2 CH 3 W CH 3 CH 3 CH 2 CH 2 CHCH 2 CH 2 CH 2 CH 3 23456781 W CH 2 CH 3 2.11 IUPAC Names of Highly Branched Alkanes 67 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 68 CHAPTER TWO Alkanes Cycloalkanes are one class of alicyclic (aliphatic cyclic) hy- drocarbons. An additional feature of IUPAC nomenclature that concerns the direction of num- bering is called the “first point of difference” rule. Consider the two directions in which the following alkane may be numbered: When deciding on the proper direction, a point of difference occurs when one order gives a lower locant than another. Thus, while 2 is the first locant in both numbering schemes, the tie is broken at the second locant, and the rule favors 2,2,6,6,7, which has 2 as its second locant, whereas 3 is the second locant in 2,3,3,7,7. Notice that locants are not added together, but examined one by one. Finally, when equal locants are generated from two different numbering directions, the direction is chosen which gives the lower number to the substituent that appears first in the name. (Remember, substituents are listed alphabetically.) The IUPAC nomenclature system is inherently logical and incorporates healthy elements of common sense into its rules. Granted, some long, funny-looking, hard- to-pronounce names are generated. Once one knows the code (rules of grammar) though, it becomes a simple matter to convert those long names to unique structural formulas. 2.12 CYCLOALKANE NOMENCLATURE Cycloalkanes are alkanes that contain a ring of three or more carbons. They are fre- quently encountered in organic chemistry and are characterized by the molecular formula C n H 2n . Some examples include: As you can see, cycloalkanes are named, under the IUPAC system, by adding the prefix cyclo- to the name of the unbranched alkane with the same number of carbons as Cyclopropane H 2 CCH 2 CH 2 Cyclohexane H 2 C H 2 C CH 2 CH 2 C H 2 H 2 C usually represented as usually represented as 1 2 3 5 7 46 8 2,3,3,7,7-Pentamethyloctane (incorrect!) 8 7 642 53 1 2,2,6,6,7-Pentamethyloctane (correct) CH 3 CH 2 CHCHCHCH 2 CHCH 3 CH 3 CH 3 CH 3 CH 2 CH 3 5-Ethyl-2,4,6-trimethyloctane 5 876 4321 Tabular summaries of the IUPAC rules for alkane and alkyl group nomenclature appear on pages 81–83. If you make a molecular model of cyclohexane, you will find its shape to be very differ- ent from a planar hexagon. We’ll discuss the reasons why in Chapter 3. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website the ring. Substituent groups are identified in the usual way. Their positions are specified by numbering the carbon atoms of the ring in the direction that gives the lowest num- ber to the substituents at the first point of difference. When the ring contains fewer carbon atoms than an alkyl group attached to it, the com- pound is named as an alkane, and the ring is treated as a cycloalkyl substituent: PROBLEM 2.10 Name each of the following compounds: (a) (c) (b) SAMPLE SOLUTION (a) The molecule has a tert-butyl group bonded to a nine- membered cycloalkane. It is tert-butylcyclononane. Alternatively, the tert-butyl group could be named systematically as a 1,1-dimethylethyl group, and the com- pound would then be named (1,1-dimethylethyl)cyclononane. (Parentheses are used when necessary to avoid ambiguity. In this case the parentheses alert the reader that the locants 1,1 refer to substituents on the alkyl group and not to ring positions.) 2.13 SOURCES OF ALKANES AND CYCLOALKANES As noted earlier, natural gas is especially rich in methane and also contains ethane and propane, along with smaller amounts of other low-molecular-weight alkanes. Natural gas is often found associated with petroleum deposits. Petroleum is a liquid mixture con- taining hundreds of substances, including approximately 150 hydrocarbons, roughly half of which are alkanes or cycloalkanes. Distillation of crude oil gives a number of frac- tions, which by custom are described by the names given in Figure 2.2. High-boiling fractions such as kerosene and gas oil find wide use as fuels for diesel engines and fur- naces, and the nonvolatile residue can be processed to give lubricating oil, greases, petro- leum jelly, paraffin wax, and asphalt. CH 3 H 3 C (CH 3 ) 2 CH C(CH 3 ) 3 CH 3 CH 2 CHCH 2 CH 3 3-Cyclobutylpentane Ethylcyclopentane CH 2 CH 3 H 3 C 4 1 35 62 CH 2 CH 3 CH 3 3-Ethyl-1,1-dimethylcyclohexane (not 1-ethyl-3,3-dimethylcyclohexane, because first point of difference rule requires 1,1,3 substitution pattern rather than 1,3,3) 2.12 Cycloalkane Nomenclature 69 The word petroleum is de- rived from the Latin words for “rock” (petra) and “oil” (oleum). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 70 CHAPTER TWO Alkanes Although both are closely linked in our minds and by our own experience, the petroleum industry predated the automobile industry by half a century. The first oil well, drilled in Titusville, Pennsylvania, by Edwin Drake in 1859, provided “rock oil,” as it was then called, on a large scale. This was quickly followed by the development of a process to “refine” it so as to produce kerosene. As a fuel for oil lamps, kerosene burned with a bright, clean flame and soon replaced the more expensive whale oil then in use. Other oil fields were discovered, and uses for other petroleum products were found—illuminating gas lit city streets, and oil heated homes and powered locomo- tives. There were oil refineries long before there were automobiles. By the time the first Model T rolled off Henry Ford’s assembly line in 1908, John D. Rockefeller’s Standard Oil holdings had already made him one of the half-dozen wealthiest people in the world. Modern petroleum refining involves more than distillation, however, and includes two major additional operations: 1. Cracking. It is the more volatile, lower-molecular-weight hydrocarbons that are useful as automotive fuels and as a source of petrochemicals. Cracking increases the proportion of these hydrocarbons at the expense of higher molecular-weight ones by processes that involve the cleavage of carbon–carbon bonds induced by heat (thermal cracking) or with the aid of certain catalysts (catalytic cracking). 2. Reforming. The physical properties of the crude oil fractions known as light gaso- line and naphtha (Figure 2.2) are appropriate for use as a motor fuel, but their igni- tion characteristics in high-compression automobile engines are poor and give rise to preignition, or “knocking.” Reforming converts the hydrocarbons in petroleum to aromatic hydrocarbons and highly branched alkanes, both of which show less tendency for knocking than unbranched alkanes and cycloalkanes. The leaves and fruit of many plants bear a waxy coating made up of alkanes that prevents loss of water. In addition to being present in beeswax (see Problem 2.5), hen- triacontane, CH 3 (CH 2 ) 29 CH 3 , is a component of the wax of tobacco leaves. Cyclopentane and cyclohexane are present in petroleum, but as a rule, unsubsti- C 1 –C 4 C 5 –C 12 C 12 –C 15 C 15 –C 25 Gas oil H1102125H11034C 25–95H11034C 95–150H11034C 150–230H11034C 230–340H11034C Refinery gas Light gasoline Naphtha Kerosene Distill Crude oil Residue FIGURE 2.2 Distillation of crude oil yields a series of volatile fractions having the names indi- cated, along wih a nonvolatile residue. The number of carbon atoms that characterize the hydrocarbons in each fraction is approximate. The tendency of a gasoline to cause “knocking” in an engine is given by its octane number. The lower the oc- tane number, the greater the tendency. The two stan- dards are heptane (assigned a value of 0) and 2,2,4- trimethylpentane (assigned a value of 100). The octane number of a gasoline is equal to the percentage of 2,2,4-trimethylpentane in a mixture of 2,2,4- trimethylpentane and hep- tane that has the same tendency to cause knocking as that sample of gasoline. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website tuted cycloalkanes are rarely found in natural sources. Compounds that contain rings of various types, however, are quite abundant. 2.14 PHYSICAL PROPERTIES OF ALKANES AND CYCLOALKANES Boiling Point. As we have seen earlier in this chapter, methane, ethane, propane, and butane are gases at room temperature. The unbranched alkanes pentane (C 5 H 12 ) through heptadecane (C 17 H 36 ) are liquids, whereas higher homologs are solids. As shown in Fig- ure 2.3, the boiling points of unbranched alkanes increase with the number of carbon atoms. Figure 2.3 also shows that the boiling points for 2-methyl-branched alkanes are lower than those of the unbranched isomer. By exploring at the molecular level the rea- sons for the increase in boiling point with the number of carbons and the difference in boiling point between branched and unbranched alkanes, we can begin to connect struc- ture with properties. A substance exists as a liquid rather than a gas because attractive forces between H 3 C CH 2 CH 3 C Limonene (present in lemons and oranges) CH 3 O Muscone (responsible for odor of musk; used in perfumery) CH 3 CH 3 COH O CC H H 3 C H 3 C Chrysanthemic acid (obtained from chrysanthemum flowers) 2.14 Physical Properties of Alkanes and Cycloalkanes 71 Number of carbon atoms in alkane = Unbranched alkane = 2-Methyl-branched alkane 180 160 140 120 100 80 60 40 20 0 4 5 678910 H1100220 Boiling point, H11034 C (1 atm) FIGURE 2.3 Boiling points of unbranched alkanes and their 2-methyl-branched isomers. (Tem- peratures in this text are expressed in degrees Celsius, °C. The SI unit of temperature is the kelvin, K. To convert degrees Celsius to kelvins add 273.15.) Appendix 1 lists selected physical properties for repre- sentative alkanes as well as members of other families of organic compounds. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 72 CHAPTER TWO Alkanes molecules (intermolecular attractive forces) are greater in the liquid than in the gas phase. Attractive forces between neutral species (atoms or molecules, but not ions) are referred to as van der Waals forces and may be of three types: 1. dipole–dipole 2. dipole/induced-dipole 3. induced-dipole/induced-dipole These forces are electrical in nature, and in order to vaporize a substance, enough energy must be added to overcome them. Most alkanes have no measurable dipole moment, and therefore the only van der Waals force to be considered is the induced- dipole/induced- dipole attractive force. It might seem that two nearby molecules A and B of the same nonpolar substance would be unaffected by each other. The electric field of a molecule, however, is not static, but fluctuates rapidly. Although, on average, the centers of positive and negative charge of an alkane nearly coincide, at any instant they may not, and molecule A can be considered to have a temporary dipole moment. The neighboring molecule B “feels” the dipolar electric field of A and undergoes a spon- taneous adjustment in its electron positions, giving it a temporary dipole moment that is complementary to that of A. The electric fields of both A and B fluctuate, but always in a way that results in a weak attraction between them. Extended assemblies of induced-dipole/induced-dipole attractions can accumulate to give substantial intermolecular attractive forces. An alkane with a higher molecular weight has more atoms and electrons and, therefore, more opportunities for intermolec- ular attractions and a higher boiling point than one with a lower molecular weight. As noted earlier in this section, branched alkanes have lower boiling points than their unbranched isomers. Isomers have, of course, the same number of atoms and elec- trons, but a molecule of a branched alkane has a smaller surface area than an unbranched AB H11546H11545 H11546H11545 AB H11546H11545 H11546H11545 AB H11546H11545 H11546H11545 A B H11546H11545 H11546 H11545 A B H11546 H11545 H11546 H11545 Van der Waals forces involv- ing induced dipoles are of- ten called London forces, or dispersion forces. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website one. The extended shape of an unbranched alkane permits more points of contact for intermolecular associations. Compare the boiling points of pentane and its isomers: The shapes of these isomers are clearly evident in the space-filling models depicted in Figure 2.4. Pentane has the most extended structure and the largest surface area avail- able for “sticking” to other molecules by way of induced-dipole/induced-dipole attrac- tive forces; it has the highest boiling point. 2,2-Dimethylpropane has the most compact structure, engages in the fewest induced-dipole/induced-dipole attractions, and has the lowest boiling point. Induced-dipole/induced-dipole attractions are very weak forces individually, but a typical organic substance can participate in so many of them that they are collectively the most important of all the contributors to intermolecular attraction in the liquid state. They are the only forces of attraction possible between nonpolar molecules such as alkanes. PROBLEM 2.11 Match the boiling points with the appropriate alkanes. Alkanes: octane, 2-methylheptane, 2,2,3,3-tetramethylbutane, nonane Boiling points (°C, 1 atm): 106, 116, 126, 151 Melting Point. Solid alkanes are soft, generally low-melting materials. The forces responsible for holding the crystal together are the same induced-dipole/induced-dipole interactions that operate between molecules in the liquid, but the degree of organization Pentane (bp 36°C) CH 3 CH 2 CH 2 CH 2 CH 3 2-Methylbutane (bp 28°C) CH 3 CHCH 2 CH 3 W CH 3 2,2-Dimethylpropane (bp 9°C) CH 3 CCH 3 W W CH 3 CH 3 2.14 Physical Properties of Alkanes and Cycloalkanes 73 (a) Pentane: CH 3 CH 2 CH 2 CH 2 CH 3 (c) 2,2-Dimethylpropane: (CH 3 ) 4 C (b) 2-Methylbutane: (CH 3 ) 2 CHCH 2 CH 3 FIGURE 2.4 Space-filling models of (a) pentane, (b) 2-methylbutane, and (c) 2,2-dimethyl- propane. The most branched isomer, 2,2-dimethylpropane, has the most compact, most spher- ical, three-dimensional shape. If you haven’t already made models of the C 5 H 12 iso- mers, this would be a good time to do so. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 74 CHAPTER TWO Alkanes is greater in the solid phase. By measuring the distances between the atoms of one mol- ecule and its neighbor in the crystal, it is possible to specify a distance of closest approach characteristic of an atom called its van der Waals radius. In space-filling molecular models, such as those of pentane, 2-methylbutane, and 2,2-dimethylpropane shown in Figure 2.4, the radius of each sphere corresponds to the van der Waals radius of the atom it represents. The van der Waals radius for hydrogen is 120 pm. When two alkane molecules are brought together so that a hydrogen of one molecule is within 240 pm of a hydrogen of the other, the balance between electron–nucleus attractions versus electron–electron and nucleus–nucleus repulsions is most favorable. Closer approach is resisted by a strong increase in repulsive forces. Solubility in Water. A familiar physical property of alkanes is contained in the adage “oil and water don’t mix.” Alkanes—indeed all hydrocarbons—are virtually insoluble in water. When a hydrocarbon dissolves in water, the framework of hydrogen bonds between water molecules becomes more ordered in the region around each molecule of the dissolved hydrocarbon. This increase in order, which corresponds to a decrease in entropy, signals a process that can be favorable only if it is reasonably exothermic. Such is not the case here. Being insoluble, and with densities in the 0.6–0.8 g/mL range, alkanes float on the surface of water (as the Alaskan oil spill of 1989 and the even larger Persian Gulf spill of 1991 remind us). The exclusion of nonpolar molecules, such as alkanes, from water is called the hydrophobic effect. We will encounter it again at sev- eral points later in the text. 2.15 CHEMICAL PROPERTIES. COMBUSTION OF ALKANES An older name for alkanes is paraffin hydrocarbons. Paraffin is derived from the Latin words parum affinis (“with little affinity”) and testifies to the low level of reactivity of alkanes. Like most other organic compounds, however, alkanes burn readily in air. This combination with oxygen is known as combustion and is quite exothermic. All hydro- carbons yield carbon dioxide and water as the products of their combustion. PROBLEM 2.12 Write a balanced chemical equation for the combustion of cyclo- hexane. The heat released on combustion of a substance is called its heat of combustion. The heat of combustion is equal to H11002H9004H° for the reaction written in the direction shown. By convention H9004H° H11005 H° products H11002 H° reactants where H° is the heat content, or enthalpy, of a compound in its standard state, that is, the gas, pure liquid, or crystalline solid at a pressure of 1 atm. In an exothermic process the enthalpy of the products is less than that of the starting materials, and H9004H° is a neg- ative number. H11001H11001CH 4 Methane 2O 2 Oxygen CO 2 Carbon dioxide 2H 2 O Water H9004H° H11005 H11002890 kJ (H11002212.8 kcal) H11001H11001(CH 3 ) 2 CHCH 2 CH 3 2-Methylbutane 8O 2 Oxygen 5CO 2 Carbon dioxide 6H 2 O Water H9004H° H11005 H110023529 kJ (H11002843.4 kcal) Alkanes are so unreactive that George A. Olah of the University of Southern Cali- fornia was awarded the 1994 Nobel Prize in chemistry in part for developing novel substances that do react with alkanes. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website Table 2.5 lists the heats of combustion of several alkanes. Unbranched alkanes have slightly higher heats of combustion than their 2-methyl-branched isomers, but the most important factor is the number of carbons. The unbranched alkanes and the 2-methyl- branched alkanes constitute two separate homologous series (see Section 2.6) in which there is a regular increase of about 653 kJ/mol (156 kcal/mol) in the heat of combustion for each additional CH 2 group. PROBLEM 2.13 Using the data in Table 2.5, estimate the heat of combustion of (a) 2-Methylnonane (in kcal/mol) (b) Icosane (in kJ/mol) SAMPLE SOLUTION (a) The last entry for the group of 2-methylalkanes in the table is 2-methylheptane. Its heat of combustion is 1306 kcal/mol. Since 2-methyl- nonane has two more methylene groups than 2-methylheptane, its heat of com- bustion is 2 H11003 156 kcal/mol higher. Heat of combustion of 2-methylnonane H11005 1306 H11001 2(156) H11005 1618 kcal/mol Heats of combustion can be used to measure the relative stability of isomeric hydrocarbons. They tell us not only which isomer is more stable than another, but by how much. Consider a group of C 8 H 18 alkanes: Figure 2.5 compares the heats of combustion of these C 8 H 18 isomers on a potential energy diagram. Potential energy is comparable with enthalpy; it is the energy a mol- ecule has exclusive of its kinetic energy. A molecule with more potential energy is less CH 3 (CH 2 ) 6 CH 3 Octane (CH 3 ) 2 CHCH 2 CH 2 CH 2 CH 2 CH 3 2-Methylheptane (CH 3 ) 3 CCH 2 CH 2 CH 2 CH 3 2,2-Dimethylhexane (CH 3 ) 3 CC(CH 3 ) 3 2,2,3,3-Tetramethylbutane 2.15 Chemical Properties. Combustion of Alkanes 75 TABLE 2.5 Heats of Combustion (H11002H9004H°) of Representative Alkanes Formula CH 3 (CH 2 ) 4 CH 3 CH 3 (CH 2 ) 5 CH 3 CH 3 (CH 2 ) 6 CH 3 CH 3 (CH 2 ) 7 CH 3 CH 3 (CH 2 ) 8 CH 3 CH 3 (CH 2 ) 9 CH 3 CH 3 (CH 2 ) 10 CH 3 CH 3 (CH 2 ) 14 CH 3 (CH 3 ) 2 CHCH 2 CH 2 CH 3 (CH 3 ) 2 CH(CH 2 ) 3 CH 3 (CH 3 ) 2 CH(CH 2 ) 4 CH 3 kcal/mol 995.0 1151.3 1307.5 1463.9 1620.1 1776.1 1932.7 2557.6 993.6 1150.0 1306.3 kJ/mol 4,163 4,817 5,471 6,125 6,778 7,431 8,086 10,701 4,157 4,812 5,466 H11546H9004HH11543 Compound Hexane Heptane Octane Nonane Decane Undecane Dodecane Hexadecane 2-Methylpentane 2-Methylhexane 2-Methylheptane Unbranched alkanes 2-Methyl-branched alkanes Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 76 CHAPTER TWO Alkanes stable than an isomer with less potential energy. Since these C 8 H 18 isomers all undergo combustion to the same final state according to the equation C 8 H 18 H11001 25 2 O 2 ±￡ 8CO 2 H11001 9H 2 O the differences in their heats of combustion translate directly to differences in their poten- tial energies. When comparing isomers, the one with the lowest potential energy (in this case, the lowest heat of combustion) is the most stable. Among the C 8 H 18 alkanes, the most highly branched isomer, 2,2,3,3-tetramethylbutane, is the most stable, and the unbranched isomer octane is the least stable. It is generally true for alkanes that a more branched isomer is more stable than a less branched one. The small differences in stability between branched and unbranched alkanes result from an interplay between attractive and repulsive forces within a molecule (intramo- lecular forces). These forces are nucleus–nucleus repulsions, electron–electron repul- sions, and nucleus–electron attractions, the same set of fundamental forces we met when talking about chemical bonding (see Section 1.12) and van der Waals forces between molecules (see Section 2.14). When the energy associated with these interactions is cal- culated for all of the nuclei and electrons within a molecule, it is found that the attrac- tive forces increase more than the repulsive forces as the structure becomes more com- pact. Sometimes, though, two atoms in a molecule are held too closely together. We’ll explore the consequences of that in Chapter 3. PROBLEM 2.14 Without consulting Table 2.5, arrange the following compounds in order of decreasing heat of combustion: pentane, isopentane, neopentane, hexane. 5471 kJ/mol 1307.5 kcal/mol 5466 kJ/mol 1306.3 kcal/mol 5458 kJ/mol 1304.6 kcal/mol 5452 kJ/mol 1303.0 kcal/mol 8CO 2 + 9H 2 O Heat of combustion 25 H11001 O 2 2 25 H11001 O 2 2 25 H11001 O 2 2 25 H11001 O 2 2 FIGURE 2.5 Energy diagram comparing heats of combustion of isomeric C 8 H 18 alkanes. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 2.15 Chemical Properties. Combustion of Alkanes 77 THERMOCHEMISTRY T hermochemistry is the study of the heat changes that accompany chemical processes. It has a long history dating back to the work of the French chemist Antoine Laurent Lavoisier in the late eighteenth century. Thermochemistry provides quantitative information that complements the qual- itative description of a chemical reaction and can help us understand why some reactions occur and others do not. It is of obvious importance when as- sessing the relative value of various materials as fuels, when comparing the stability of isomers, or when de- termining the practicality of a particular reaction. In the field of bioenergetics, thermochemical informa- tion is applied to the task of sorting out how living systems use chemical reactions to store and use the energy that originates in the sun. By allowing compounds to react in a calorime- ter, it is possible to measure the heat evolved in an exothermic reaction or the heat absorbed in an en- dothermic reaction. Thousands of reactions have been studied to produce a rich library of thermo- chemical data. These data take the form of heats of reaction and correspond to the value of the enthalpy change H9004H° for a particular reaction of a particular substance. In this section you have seen how heats of com- bustion can be used to determine relative stabilities of isomeric alkanes. In later sections we shall expand our scope to include the experimentally determined heats of certain other reactions, such as bond dissoci- ation energies (Section 4.17) and heats of hydrogena- tion (Section 6.2), to see how H9004H° values from various sources can aid our understanding of structure and reactivity. Heat of formation (H9004H° f ), the enthalpy change for formation of a compound directly from the ele- ments, is one type of heat of reaction. In cases such as the formation of CO 2 or H 2 O from the combustion of carbon or hydrogen, respectively, the heat of forma- tion of a substance can be measured directly. In most other cases, heats of formation are not measured ex- perimentally but are calculated from the measured heats of other reactions. Consider, for example, the heat of formation of methane. The reaction that de- fines the formation of methane from the elements, can be expressed as the sum of three reactions: Equations (1) and (2) are the heats of formation of carbon dioxide and water, respectively. Equation (3) is the reverse of the combustion of methane, and so the heat of reaction is equal to the heat of combustion but opposite in sign. The molar heat of formation of a substance is the enthalpy change for formation of one mole of the substance from the elements. For methane H9004H° f H11005 H1100275 kJ/mol. The heats of formation of most organic com- pounds are derived from heats of reaction by arith- metic manipulations similar to that shown. Chemists find a table of H9004H° f values to be convenient because it replaces many separate tables of H9004H° values for indi- vidual reaction types and permits H9004H° to be calcu- lated for any reaction, real or imaginary, for which the heats of formation of reactants and products are available. It is more appropriate for our purposes, however, to connect thermochemical data to chemi- cal processes as directly as possible, and therefore we will cite heats of particular reactions, such as heats of combustion and heats of hydrogenation, rather than heats of formation. (1) C (graphite) H11001 O 2 (g) C (graphite) H11001 2H 2 CO 2 (g) H9004H° H11005 H11002393 kJ H9004H° H11005 H1100275 kJ (2) 2H 2 (g) H11001 O 2 (g)2H 2 O(l) H9004H° H11005 H11002572 kJ (3) CO 2 (g) H11001 2H 2 O(l)C 4 (g) H11001 2O 2 (g) H9004H° H11005 H11001890 kJ CH 4 H11001C (graphite) Carbon 2H 2 (g) Hydrogen CH 4 (g) Methane Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 78 CHAPTER TWO Alkanes 2.16 OXIDATION–REDUCTION IN ORGANIC CHEMISTRY As we have just seen, the reaction of alkanes with oxygen to give carbon dioxide and water is called combustion. A more fundamental classification of reaction types places it in the oxidation–reduction category. To understand why, let’s review some principles of oxida- tion–reduction, beginning with the oxidation number (also known as oxidation state). There are a variety of methods for calculating oxidation numbers. In compounds that contain a single carbon, such as methane (CH 4 ) and carbon dioxide (CO 2 ), the oxidation number of carbon can be calculated from the molecular formula. Both molecules are neu- tral, and so the algebraic sum of all the oxidation numbers must equal zero. Assuming, as is customary, that the oxidation state of hydrogen is H110011, the oxidation state of carbon in CH 4 is calculated to be H110024. Similarly, assuming an oxidation state of H110022 for oxygen, car- bon is H110014 in CO 2 . This kind of calculation provides an easy way to develop a list of one- carbon compounds in order of increasing oxidation state, as shown in Table 2.6. The carbon in methane has the lowest oxidation number (H110024) of any of the com- pounds in Table 2.6. Methane contains carbon in its most reduced form. Carbon dioxide and carbonic acid have the highest oxidation numbers (H110014) for carbon, corresponding to its most oxidized state. When methane or any alkane undergoes combustion to form carbon dioxide, carbon is oxidized and oxygen is reduced. A useful generalization from Table 2.6 is the following: Oxidation of carbon corresponds to an increase in the number of bonds between carbon and oxygen or to a decrease in the number of carbon–hydrogen bonds. Conversely, reduction corresponds to an increase in the number of carbon–hydro- gen bonds or to a decrease in the number of carbon–oxygen bonds. From Table 2.6 it can be seen that each successive increase in oxidation state increases the number of bonds between carbon and oxygen and decreases the number of car- bon–hydrogen bonds. Methane has four C±H bonds and no C±O bonds; carbon dioxide has four C±O bonds and no C±H bonds. Among the various classes of hydrocarbons, alkanes contain carbon in its most reduced state, and alkynes contain carbon in its most oxidized state. TABLE 2.6 Oxidation Number of Carbon in One-Carbon Compounds Compound Methane Methanol Formaldehyde Formic acid Carbonic acid Carbon dioxide H110024 H110022 0 H110012 H110014 H110014 Oxidation number CH 4 CH 3 OH H 2 C?O O?C?O Structural formula HCOH O X HOCOH O X CH 4 CH 4 O CH 2 O H 2 CO 3 CO 2 Molecular formula CH 2 O 2 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website We can extend the generalization by recognizing that the pattern is not limited to increasing hydrogen or oxygen content. Any element more electronegative than carbon will have the same effect on oxidation number as oxygen. Thus, the oxidation numbers of carbon in CH 3 Cl and in CH 3 OH are the same (H110022), and the reaction of methane with chlorine (to be discussed in Section 4.16) involves oxidation of carbon. Any element less electronegative than carbon will have the same effect on oxida- tion number as hydrogen. Thus, the oxidation numbers of carbon in CH 3 Li and in CH 4 are the same (H110024), and the reaction of CH 3 Cl with lithium (to be discussed in Section 14.3) involves reduction of carbon. The oxidation number of carbon decreases from H110022 in CH 3 Cl to H110024 in CH 3 Li. The generalization can be expressed in terms broad enough to cover both the pre- ceding reactions and many others as well, as follows: Oxidation of carbon occurs when a bond between carbon and an atom which is less electronegative than carbon is replaced by a bond to an atom that is more electronegative than carbon. The reverse process is reduction. Organic chemists are much more concerned with whether a particular reaction is an oxidation or a reduction of carbon than with determining the precise change in oxi- dation number. The generalizations described permit reactions to be examined in this way and eliminate the need for calculating oxidation numbers themselves. PROBLEM 2.15 The reactions shown will all be encountered in Chapter 6. Clas- sify each according to whether it proceeds by oxidation of carbon, by reduction of carbon, or by a process other than oxidation–reduction. (a) CH 2 ?CH 2 H11001 H 2 O ±￡ CH 3 CH 2 OH (b) CH 2 ?CH 2 H11001 Br 2 ±￡ BrCH 2 CH 2 Br (c) 6CH 2 ?CH 2 H11001 B 2 H 6 ±￡ 2(CH 3 CH 2 ) 3 B X is less electronegative than carbon C X C Y Y is more electronegative than carbon oxidation reduction H11001H11001CH 3 Cl Chloromethane 2Li Lithium CH 3 Li Methyllithium LiCl Lithium chloride H11001H11001CH 4 Methane Cl 2 Chlorine CH 3 Cl Chloromethane HCl Hydrogen chloride Increasing oxidation state of carbon (decreasing hydrogen content) CH 3 CH 3 Ethane (6 C±H bonds) CH 2 ?CH 2 Ethylene (4 C±H bonds) HCPCH Acetylene (2 C±H bonds) 2.16 Oxidation–Reduction in Organic Chemistry 79 Methods for calculating oxi- dation numbers in complex molecules are available. They are time-consuming to apply, however, and are rarely used in organic chemistry. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website SAMPLE SOLUTION (a) In this reaction one new C±H bond and one new C±O bond are formed. One carbon is reduced, the other is oxidized. Overall, there is no net change in oxidation state, and the reaction is not classified as an oxida- tion–reduction. The ability to recognize when oxidation or reduction occurs is of value when decid- ing on the kind of reactant with which an organic molecule must be treated in order to convert it into some desired product. Many of the reactions to be discussed in subse- quent chapters involve oxidation–reduction. 2.17 SUMMARY Section 2.1 The classes of hydrocarbons are alkanes, alkenes, alkynes, and arenes. Alkanes are hydrocarbons in which all of the bonds are single bonds and are characterized by the molecular formula C n H 2nH110012 . Section 2.2 Functional groups are the structural units responsible for the character- istic reactions of a molecule. The functional groups in an alkane are its hydrogen atoms. Section 2.3 The families of organic compounds listed on the inside front cover and in Tables 2.1 and 2.2 bear functional groups that are more reactive than H, and the hydrocarbon chain to which they are attached can often be considered as simply a supporting framework. For example, ethanolamine (H 2 NCH 2 CH 2 OH) contains both amine (RNH 2 ) and alcohol (ROH) func- tional groups. Section 2.4 The first three alkanes are methane (CH 4 ), ethane (CH 3 CH 3 ), and propane (CH 3 CH 2 CH 3 ). All can be described according to the orbital hybridization model of bonding based on sp 3 hybridization of carbon. Section 2.5 Two constitutionally isomeric alkanes have the molecular formula C 4 H 10 . One has an unbranched chain (CH 3 CH 2 CH 2 CH 3 ) and is called n-butane; the other has a branched chain [(CH 3 ) 3 CH] and is called isobutane. Both n-butane and isobutane are common names. Section 2.6 Unbranched alkanes of the type CH 3 (CH 2 ) n CH 3 are often referred to as n-alkanes. Section 2.7 There are three constitutional isomers of C 5 H 12 : n-pentane (CH 3 CH 2 CH 2 CH 2 CH 3 ), isopentane [(CH 3 ) 2 CHCH 2 CH 3 ], and neopen- tane [(CH 3 ) 4 C]. Sections A single alkane may have several different names; a name may be a 2.8–2.12 common name, or it may be a systematic name developed by a well- defined set of rules. The most widely used system is IUPAC nomencla- ture. Table 2.7 summarizes the rules for alkanes and cycloalkanes. Table 2.8 gives the rules for naming alkyl groups. Section 2.13 Natural gas is an abundant source of methane, ethane, and propane. Petro- leum is a liquid mixture of many hydrocarbons, including alkanes. Al- kanes also occur naturally in the waxy coating of leaves and fruits. Section 2.14 Alkanes and cycloalkanes are nonpolar and insoluble in water. The forces of attraction between alkane molecules are induced-dipole/induced- dipole attractive forces. The boiling points of alkanes increase as the 80 CHAPTER TWO Alkanes Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 2.17 Summary 81 TABLE 2.7 Summary of IUPAC Nomenclature of Alkanes and Cycloalkanes Rule 1. Find the longest continuous chain of carbon atoms, and assign a basis name to the compound corresponding to the IUPAC name of the unbranched alkane having the same number of carbons. 2. List the substituents attached to the longest con- tinuous chain in alphabetical order. Use the pre- fixes di-, tri-, tetra-, and so on, when the same substituent appears more than once. Ignore these prefixes when alphabetizing. 4. When two different numbering schemes give equivalent sets of locants, choose the direction that gives the lower locant to the group that appears first in the name. Example The longest continuous chain in the alkane shown is six carbons. This alkane is named as a derivative of hexane. The correct name is 4-ethyl-3,3-dimethylhexane. The alkane bears two methyl groups and an ethyl group. It is an ethyldimethylhexane. In the following example, the substituents are locat- ed at carbons 3 and 4 regardless of the direction in which the chain is numbered. Ethyl precedes methyl in the name; therefore 3-ethyl- 4-methylhexane is correct. When numbering from left to right, the substituents appear at carbons 3, 3, and 4. When numbering from right to left the locants are 3, 4, and 4; therefore, number from left to right. (Continued) 3. Number the chain in the direction that gives the lower locant to a substituent at the first point of difference. Ethyl Methyl Methyl 1 2 3 4 5 6 Correct 6 5 4 3 2 1 Incorrect 1 2 3 4 5 6 Correct 6 5 4 3 2 1 Incorrect A. Alkanes Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 82 CHAPTER TWO Alkanes TABLE 2.7 Summary of IUPAC Nomenclature of Alkanes and Cycloalkanes (Continued) Rule 5. When two chains are of equal length, choose the one with the greater number of substituents as the parent. (Although this requires naming more substituents, the substituents have simpler names.) 1. Count the number of carbons in the ring, and assign a basis name to the cycloalkane corre- sponding to the IUPAC name of the unbranched alkane having the same number of carbons. 3. When two or more different substituents are pres- ent, list them in alphabetical order, and number the ring in the direction that gives the lower num- ber at the first point of difference. 4. Name the compound as a cycloalkyl-substituted alkane if the substituent has more carbons than the ring. Example Two different chains contain five carbons in the alkane: The correct name is 3-ethyl-2-methylpentane (disub- stituted chain), rather than 3-isopropylpentane (monosubstituted chain). The compound shown contains five carbons in its ring. The compound shown is 1,1-diethyl-4-hexylcyclooc- tane. The previous compound is isopropylcyclopentane. Alternatively, the alkyl group can be named accord- ing to the rules summarized in Table 2.8, whereupon the name becomes (1-methylethyl)cyclopentane. Parentheses are used to set off the name of the alkyl group as needed to avoid ambiguity. 2. Name the alkyl group, and append it as a prefix to the cycloalkane. No locant is needed if the com- pound is a monosubstituted cycloalkane. It is understood that the alkyl group is attached to C-1. It is named as a derivative of cyclopentane. B. Cycloalkanes CH(CH 3 ) 2 CH 2 CH 2 CH 2 CH 2 CH 3 is pentylcyclopentane CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 is 1-cyclopentylhexane but 32 4 5 67 8 1 CH 3 CH 2 CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 Correct Incorrect 1 2 3 4 55 4 3 1 2 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website number of carbon atoms increases. Branched alkanes have lower boiling points than their unbranched isomers. There is a limit to how closely two molecules can approach each other, which is given by the sum of their van der Waals radii. Section 2.15 Alkanes and cycloalkanes burn in air to give carbon dioxide, water, and heat. This process is called combustion. The heat evolved on burning an alkane increases with the number of car- bon atoms. The relative stability of isomers may be determined by com- paring their respective heats of combustion. The more stable of two iso- mers has the lower heat of combustion. Section 2.16 Combustion of alkanes is an example of oxidation–reduction. Although it is possible to calculate oxidation numbers of carbon in organic mole- cules, it is more convenient to regard oxidation of an organic substance as an increase in its oxygen content or a decrease in its hydrogen con- tent. PROBLEMS 2.16 Write structural formulas, and give the IUPAC names for the nine alkanes that have the molecular formula C 7 H 16 . H9004H° H11005 H110023529 kJ (H11002843.4 kcal) H11001H11001(CH 3 ) 2 CHCH 2 CH 3 2-Methylbutane 8O 2 Oxygen 5CO 2 Carbon dioxide 6H 2 O Water 2.17 Summary 83 TABLE 2.8 Summary of IUPAC Nomenclature of Alkyl Groups Rule 1. Number the carbon atoms beginning at the point of attachment, proceeding in the direction that follows the longest continuous chain. 3. List the substituents on the basis group in alpha- betical order using replicating prefixes when nec- essary. 4. Locate the substituents according to the number- ing of the main chain described in step 1. Example The longest continuous chain that begins at the point of attachment in the group shown contains six carbons. The alkyl group in step 1 is a dimethylpropylhexyl group. The alkyl group is a 1,3-dimethyl-1-propylhexyl group. The alkyl group shown in step 1 is named as a sub- stituent hexyl group. 2. Assign a basis name according to the number of carbons in the corresponding unbranched alkane. Drop the ending -ane and replace it by -yl. CH 3 CH 2 CH 2 CCH 2 CHCH 2 CH 2 CH 3 234561 CH 3 CH 3 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 84 CHAPTER TWO Alkanes 2.17 From among the 18 constitutional isomers of C 8 H 18 , write structural formulas, and give the IUPAC names for those that are named as derivatives of (a) Heptane (c) Pentane (b) Hexane (d) Butane 2.18 Write a structural formula for each of the following compounds: (a) 6-Isopropyl-2,3-dimethylnonane (e) Cyclobutylcyclopentane (b) 4-tert-Butyl-3-methylheptane (f) (2,2-Dimethylpropyl)cyclohexane (c) 4-Isobutyl-1,1-dimethylcyclohexane (g) Pentacosane (d) sec-Butylcycloheptane (h) 10-(1-methylpentyl)pentacosane 2.19 Give the IUPAC name for each of the following compounds: (a) CH 3 (CH 2 ) 25 CH 3 (e) (b) (CH 3 ) 2 CHCH 2 (CH 2 ) 14 CH 3 (f) (c) (CH 3 CH 2 ) 3 CCH(CH 2 CH 3 ) 2 (g) (d) 2.20 All the parts of this problem refer to the alkane having the carbon skeleton shown. (a) What is the molecular formula of this alkane? (b) What is its IUPAC name? (c) How many methyl groups are present in this alkane? Methylene groups? Methine groups? (d) How many carbon atoms are primary? Secondary? Tertiary? Quaternary? 2.21 Give the IUPAC name for each of the following alkyl groups, and classify each one as pri- mary, secondary, or tertiary: (a) CH 3 (CH 2 ) 10 CH 2 ± (b) ± CH 2 CH 2 CHCH 2 CH 2 CH 3 W CH 2 CH 3 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website (c) ±C(CH 2 CH 3 ) 3 (e) (d) (f) 2.22 Pristane is an alkane that is present to the extent of about 14% in shark liver oil. Its IUPAC name is 2,6,10,14-tetramethylpentadecane. Write its structural formula. 2.23 Hectane is the IUPAC name for the unbranched alkane that contains 100 carbon atoms. (a) How many H9268 bonds are there in hectane? (b) How many alkanes have names of the type x-methylhectane? (c) How many alkanes have names of the type 2,x-dimethylhectane? 2.24 Which of the compounds in each of the following groups are isomers? (a) Butane, cyclobutane, isobutane, 2-methylbutane (b) Cyclopentane, neopentane, 2,2-dimethylpentane, 2,2,3-trimethylbutane (c) Cyclohexane, hexane, methylcyclopentane, 1,1,2-trimethylcyclopropane (d) Ethylcyclopropane, 1,1-dimethylcyclopropane, 1-cyclopropylpropane, cyclopentane (e) 4-Methyltetradecane, 2,3,4,5-tetramethyldecane, pentadecane, 4-cyclobutyldecane 2.25 Epichlorohydrin is the common name of an industrial chemical used as a component in epoxy cement. The molecular formula of epichlorohydrin is C 3 H 5 ClO. Epichlorohydrin has an epoxide functional group; it does not have a methyl group. Write a structural formula for epichloro- hydrin. 2.26 (a) Complete the structure of the pain-relieving drug ibuprofen on the basis of the fact that ibuprofen is a carboxylic acid that has the molecular formula C 13 H 18 O 2 , X is an isobutyl group, and Y is a methyl group. (b) Mandelonitrile may be obtained from peach flowers. Derive its structure from the template in part (a) given that X is hydrogen, Y is the functional group that character- izes alcohols, and Z characterizes nitriles. 2.27 Isoamyl acetate is the common name of the substance most responsible for the characteris- tic odor of bananas. Write a structural formula for isoamyl acetate, given the information that it is an ester in which the carbonyl group bears a methyl substituent and there is a 3-methylbutyl group attached to one of the oxygens. 2.28 n-Butyl mercaptan is the common name of a foul-smelling substance obtained from skunk fluid. It is a thiol of the type RX, where R is an n-butyl group and X is the functional group that characterizes a thiol. Write a structural formula for this substance. 2.29 Some of the most important organic compounds in biochemistry are the H9251-amino acids, rep- resented by the general formula shown. RCHCO H11002 X O W H11001 NH 3 ±CH±ZX± W Y ±CH± W CH 3 ±CHCH 2 CH 2 CH 3 ±CH 2 CH 2 ± Problems 85 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website Write structural formulas for the following H9251-amino acids. (a) Alanine (R H11005 methyl) (b) Valine (R H11005 isopropyl) (c) Leucine (R H11005 isobutyl) (d) Isoleucine (R H11005 sec-butyl) (e) Serine (R H11005 XCH 2 , where X is the functional group that characterizes alcohols) (f) Cysteine (R H11005 XCH 2 , where X is the functional group that characterizes thiols) (g) Aspartic acid (R H11005 XCH 2 , where X is the functional group that characterizes car- boxylic acids) 2.30 Uscharidin is the common name of a poisonous natural product having the structure shown. Locate all of the following in uscharidin: (a) Alcohol, aldehyde, ketone, and ester functional groups (b) Methylene groups (c) Primary carbons 2.31 Write the structural formula of a compound of molecular formula C 4 H 8 Cl 2 in which (a) All the carbons belong to methylene groups (b) None of the carbons belong to methylene groups 2.32 Female tiger moths signify their presence to male moths by giving off a sex attractant. The sex attractant has been isolated and found to be a 2-methyl-branched alkane having a molecular weight of 254. What is this material? 2.33 Write a balanced chemical equation for the combustion of each of the following compounds: (a) Decane (c) Methylcyclononane (b) Cyclodecane (d) Cyclopentylcyclopentane 2.34 The heats of combustion of methane and butane are 890 kJ/mol (212.8 kcal/mol) and 2876 kJ/mol (687.4 kcal/mol), respectively. When used as a fuel, would methane or butane gen- erate more heat for the same mass of gas? Which would generate more heat for the same volume of gas? 2.35 In each of the following groups of compounds, identify the one with the largest heat of combustion and the one with the smallest. (Try to do this problem without consulting Table 2.5.) (a) Hexane, heptane, octane (b) Isobutane, pentane, isopentane (c) Isopentane, 2-methylpentane, neopentane O O O O O O O OH H 3 C X X ± X OH H CH CH 3 H H H HH 86 CHAPTER TWO Alkanes Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website (d) Pentane, 3-methylpentane, 3,3-dimethylpentane (e) Ethylcyclopentane, ethylcyclohexane, ethylcycloheptane 2.36 (a) Given H9004H° for the reaction H 2 (g) H11001 1 2 O 2 (g) ±￡ H 2 O(l) H9004H° H11005H11002286 kJ along with the information that the heat of combustion of ethane is 1560 kJ/mol and that of ethylene is 1410 kJ/mol, calculate H9004H° for the hydrogenation of ethylene: CH 2 ?CH 2 (g) H11001 H 2 (g) ±￡ CH 3 CH 3 (g) (b) If the heat of combustion of acetylene is 1300 kJ/mol, what is the value of H9004H° for its hydrogenation to ethylene? To ethane? (c) What is the value of H9004H° for the hypothetical reaction 2CH 2 ?CH 2 (g) ±￡ CH 3 CH 3 (g) H11001 HCPCH(g) 2.37 Each of the following reactions will be encountered at some point in this text. Classify each one according to whether the organic substrate is oxidized or reduced in the process. (a) CH 3 CPCH H11001 2Na H11001 2NH 3 ±￡ CH 3 CH?CH 2 H11001 2NaNH 2 (b) (c) HOCH 2 CH 2 OH H11001 HIO 4 ±￡ 2CH 2 ?O H11001 HIO 3 H11001 H 2 O (d) 2.38 The reaction shown is important in the industrial preparation of dichlorodimethylsilane for eventual conversion to silicone polymers. 2CH 3 Cl H11001 Si ±￡ (CH 3 ) 2 SiCl 2 Is carbon oxidized, or is it reduced in this reaction? 2.39 Compound A undergoes the following reactions: (a) To what class of compounds does compound A belong? (b) Which of the reactions shown require(s) an oxidizing agent? (c) Which of the reactions shown require(s) a reducing agent? (d) Identify the class to which each of the reaction products belongs. 2.40 Each of the following equations describes a reaction of a compound called methyl formate. To what class of compounds does methyl formate belong? Which reactions require a reducing agent? Which require an oxidizing agent? Which reactions are not oxidation–reduction? CH 3 CC(CH 3 ) 3 X O CH 3 CH 2 C(CH 3 ) 3 CH 3 COC(CH 3 ) 3 X O CH 3 CHC(CH 3 ) 3 W OH Compound A ±NO 2 H11001 2Fe H11001 7H H11001 ±NH 3 H11001 2Fe 3H11001 H11001 2H 2 O H11001 3 H20898 OH H20899 H11001 Cr 2 O 7 2H11002 H11001 8H H11001 3 H20899H20898 O H11001 2Cr 3H11001 H11001 7H 2 O Problems 87 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website (a) (b) (c) (d) (e) 2.41 Which atoms in the following reaction undergo changes in their oxidation state? Which atom is oxidized? Which one is reduced? 2CH 3 CH 2 OH H11001 2Na ±￡ 2CH 3 CH 2 ONa H11001 H 2 2.42 We have not talked about heats of combustion of compounds other than hydrocarbons. Nev- ertheless, from among the compounds shown here, you should be able to deduce which one gives off the most heat on combustion (to give CO 2 and H 2 O) and which one the least. 2.43 Make a molecular model of each of the compounds given as a representative example of the various functional group classes in Table 2.1. 2.44 The compound identified as “ethanoic acid” in Table 2.2 is better known as acetic acid. Make a molecular model of acetic acid, and compare the two C±O bond distances. Compare these with the C±O bond distance in ethanol (Problem 2.43). 2.45 You have seen that a continuous chain of sp 3 -hybridized carbons, as in an alkane, is not “straight,” but rather adopts a zigzag geometry. What would the hybridization state of carbon have to be in order for the chain to be truly straight? HOC±COH X O X O CH 3 CH 2 OH HOCH 2 CH 2 OH HCOCH 3 X O CO 2 H 2 OH11001 CH 3 OHH11001 HCOCH 3 X O 2CO 2 H 2 OH11001 HCOCH 3 X O 2CH 3 OH HCOCH 3 X O HCONa H11001 CH 3 OH X O HCOCH 3 X O HCOH H11001 CH 3 OH X O 88 CHAPTER TWO Alkanes Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website