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Unformatted text preview: Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website CHAPTER 24 Organic Chemistry INTRODUCTION ORGANIC ING CHEMISTRY IS THE STUDY OF CARBON COMPOUNDS, EXCLUD- CO, CO2, CS2, 24.1 CLASSES OF ORGANIC COMPOUNDS AND VARIOUS BICARBONATES, CARBONATES, AND 24.2 ALIPHATIC HYDROCARBONS CYANIDES, WHICH ARE TRADITIONALLY CONSIDERED TO BE INORGANIC 24.3 AROMATIC HYDROCARBONS COMPOUNDS. 24.4 CHEMISTRY OF THE FUNCTIONAL GROUPS THE WORD “ORGANIC” WAS ORIGINALLY USED BY EIGH- TEENTH-CENTURY CHEMISTS TO DESCRIBE SUBSTANCES OBTAINED FROM LIVING SOURCES — PLANTS AND ANIMALS. THESE CHEMISTS BELIEVED THAT NATURE POSSESSED A CERTAIN VITAL FORCE AND THAT ONLY LIVING THINGS COULD PRODUCE ORGANIC COMPOUNDS. TION WAS DISPROVED IN 1828 BY THIS ROMANTIC NO- FRIEDRICH WOHLER, A GERMAN CHEMIST WHO PREPARED UREA, AN ORGANIC COMPOUND, FROM THE REACTION BETWEEN THE INORGANIC COMPOUNDS LEAD CYANATE AND AQUEOUS AMMONIA: Pb(OCN)2 2NH3 2H2O 88n 2(NH2)2CO Pb(OH)2 UREA TODAY, WELL OVER 13 MILLION SYNTHETIC AND NATURAL ORGANIC COM- POUNDS ARE KNOWN. THIS NUMBER IS SIGNIFICANTLY GREATER THAN THE 100,000 OR SO KNOWN INORGANIC COMPOUNDS. 939 Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 940 ORGANIC CHEMISTRY 24.1 Recall that the linking of like atoms is called catenation. The ability of carbon to catenate is discussed in Section 21.3. 24.2 CLASSES OF ORGANIC COMPOUNDS Carbon can form more compounds than any other element because carbon atoms are able not only to form single, double, and triple carbon-carbon bonds, but also to link up with each other in chains and ring structures. The branch of chemistry that deals with carbon compounds is organic chemistry. Classes of organic compounds can be distinguished according to functional groups they contain. A functional group is a group of atoms that is largely responsible for the chemical behavior of the parent molecule. Different molecules containing the same kind of functional group or groups undergo similar reactions. Thus, by learning the characteristic properties of a few functional groups, we can study and understand the properties of many organic compounds. In the second half of this chapter we will discuss the functional groups known as alcohols, ethers, aldehydes and ketones, carboxylic acids, and amines. All organic compounds are derived from a group of compounds known as hydrocarbons because they are made up of only hydrogen and carbon. On the basis of structure, hydrocarbons are divided into two main classes — aliphatic and aromatic. Aliphatic hydrocarbons do not contain the benzene group, or the benzene ring, whereas aromatic hydrocarbons contain one or more benzene rings. ALIPHATIC HYDROCARBONS Aliphatic hydrocarbons are divided into alkanes, alkenes, and alkynes, discussed below (Figure 24.1). ALKANES For a given number of carbon atoms, the saturated hydrocarbon contains the largest number of hydrogen atoms. FIGURE 24.1 hydrocarbons. Alkanes have the general formula CnH2n 2, where n 1, 2, . . . . The essential characteristic of alkane hydrocarbon molecules is that only single covalent bonds are present. The alkanes are known as saturated hydrocarbons because they contain the maximum number of hydrogen atoms that can bond with the number of carbon atoms present. The simplest alkane (that is, with n 1) is methane CH4, which is a natural product of the anaerobic bacterial decomposition of vegetable matter under water. Because Classification of Hydrocarbons Aromatic Aliphatic Alkanes Back Forward Main Menu TOC Cycloalkanes Study Guide TOC Alkenes Alkynes Textbook Website MHHE Website 24.2 FIGURE 24.2 Structures of the first four alkanes. Note that butane can exist in two structurally different forms, called structural isomers. ALIPHATIC HYDROCARBONS H A H O CO H A H HH AA HOC OC OH AA HH HHH AAA HOC OC OC OH AAA HHH Methane Ethane Propane HHHH AAAA HOC OCO COC OH AAAA HHHH H A HOC OH A HAH AAA H O C O C O CO H AAA HHH n-Butane Termites are a natural source of methane. 941 Isobutane it was first collected in marshes, methane became known as “marsh gas.” A rather improbable but proven source of methane is termites. When these voracious insects consume wood, the microorganisms that inhabit their digestive system break down cellulose (the major component of wood) into methane, carbon dioxide, and other compounds. An estimated 170 million tons of methane are produced annually by termites! It is also produced in some sewage treatment processes. Commercially, methane is obtained from natural gas. The Chemistry in Action essay on p. 952 describes an interesting compound formed by methane and water molecules. Figure 24.2 shows the structures of the first four alkanes (n 1 to n 4). Natural gas is a mixture of methane, ethane, and a small amount of propane. We discussed the bonding scheme of methane in Chapter 10. Indeed the carbon atoms in all the alkanes can be assumed to be sp3-hybridized. The structures of ethane and propane are straightforward, for there is only one way to join the carbon atoms in these molecules. Butane, however, has two possible bonding schemes resulting in the structural isomers n-butane (n stands for normal) and isobutane, molecules that have the same molecular formula, but different structures. Alkanes such as the structural isomers of butane are described as having the straight chain or branched chain structures. n-Butane is a straight-chain alkane because the carbon atoms are joined along one line. In a branchedchain alkane like isobutane, one or more carbon atoms are bonded to at least three other carbon atoms. In the alkane series, as the number of carbon atoms increases, the number of structural isomers increases rapidly. For example, butane, C4H10, has two isomers; decane, C10H22, has 75 isomers; and the alkane C30H62 has over 400 million, or 4 108, possible isomers! Obviously, most of these isomers do not exist in nature nor have they been synthesized. Nevertheless, the numbers help to explain why carbon is found in so many more compounds than any other element. The following example deals with the number of structural isomers of an alkane. EXAMPLE 24.1 How many structural isomers can be identified for pentane, C5H12? Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 942 ORGANIC CHEMISTRY For small hydrocarbon molecules (eight or fewer carbon atoms), it is relatively easy to determine the number of structural isomers by trial and error. The first step is to write down the straight-chain structure: Answer n-pentane HHHHH AAAAA HOCO COC OCO COH AAAAA HHHHH n -pentane (b.p. 36.1 C) The second structure, by necessity, must be a branched chain: 2-methylbutane H CH3 AA HOCO C AA HH HH AA COC OH AA HH 2-methylbutane (b.p. 27.9 C) Yet another branched-chain structure is possible: 2,2-dimethylpropane H CH3 AA HO COC AA H CH3 H A C OH A H 2,2-dimethylpropane (b.p. 9.5 C) Similar problem: 24.11. We can draw no other structure for an alkane having the molecular formula C5H12. Thus pentane has three structural isomers, in which the numbers of carbon and hydrogen atoms remain unchanged despite the differences in structure. PRACTICE EXERCISE How many structural isomers are there in the alkane C6H14? Table 24.1 shows the melting and boiling points of the straight-chain isomers of the first 10 alkanes. The first four are gases at room temperature; and pentane through decane are liquids. As molecular size increases, so does the boiling point, because of the increasing dispersion forces (see Section 11.2). Alkane Nomenclature The nomenclature of alkanes and all other organic compounds is based on the recommendations of the International Union of Pure and Applied Chemistry (IUPAC). The first four alkanes (methane, ethane, propane, and butane) have nonsystematic names. As Table 24.1 shows, the number of carbon atoms is reflected in the Greek prefixes for the alkanes containing five to ten carbons. We now apply the IUPAC rules to the following examples: 1. Back Forward Main Menu The parent name of the hydrocarbon is that given to the longest continuous chain of carbon atoms in the molecule. Thus the parent name of the following compound is heptane because there are seven carbon atoms in the longest chain TOC Study Guide TOC Textbook Website MHHE Website 24.2 TABLE 24.1 ALIPHATIC HYDROCARBONS 943 The First Ten Straight-Chain Alkanes NAME OF NUMBER OF HYDROCARBON Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane MELTING BOILING CARBON ATOMS MOLECULAR FORMULA POINT (°C) POINT (°C) CH4 CH3OCH3 CH3OCH2OCH3 CH3O(CH2)2OCH3 CH3O(CH2)3OCH3 CH3O(CH2)4OCH3 CH3O(CH2)5OCH3 CH3O(CH2)6OCH3 CH3O(CH2)7OCH3 CH3O(CH2)8OCH3 1 2 3 4 5 6 7 8 9 10 182.5 183.3 189.7 138.3 129.8 95.3 90.6 56.8 53.5 29.7 161.6 88.6 42.1 0.5 36.1 68.7 98.4 125.7 150.8 174.0 CH3 1 2 3 4A 5 6 7 CH3 OCH2 OCH2 OCH2 OCH2 OCH2 OCH3 An alkane less one hydrogen atom is an alkyl group. For example, when a hydrogen atom is removed from methane, we are left with the CH3 fragment, which is called a methyl group. Similarly, removing a hydrogen atom from the ethane molecule gives an ethyl group, or C2H5. Table 24.2 lists the names of several common alkyl groups. Any chain branching off the longest chain is named as an alkyl group. 3. When one or more hydrogen atoms are replaced by other groups, the name of the compound must indicate the locations of carbon atoms where replacements are made. The procedure is to number each carbon atom on the longest chain in the direction that gives the smaller numbers for the locations of all branches. Consider 2. TABLE 24.2 Common Alkyl Groups NAME FORMULA Methyl OCH3 Ethyl OCH2 OCH3 n-Propyl OCH2 OCH2 O CH3 n-Butyl OCH2 OCH2 O CH2 O CH3 Isopropyl CH3 A O C OH A CH3 t-Butyl* CH3 A O CO CH3 A CH3 *The letter t stands for tertiary. Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 944 ORGANIC CHEMISTRY the two different systems for the same compound shown below: CH3 1 2A CH3 3 4 5 1 CH3 OCHO CH2 O CH2 O CH3 2 4A 3 5 CH3 O CH2 O CH2 O CHOCH3 2-methylpentane 4-methylpentane The compound on the left is numbered correctly since the methyl group is located at carbon 2 of the pentane chain; in the compound on the right, the methyl group is located at carbon 4. Thus the name of the compound is 2-methylpentane, and not 4-methylpentane. Note that the branch name and the parent name are written as a single word, and a hyphen follows the number. 4. When there is more than one alkyl branch of the same kind present, we use a prefix such as di-, tri-, or tetra- with the name of the alkyl group. Consider the following examples: CH3 CH3 3A 4 5 6 CH3 OCHO CHOCH2 OCH2 OCH3 1 CH3 2A 1 3A 2 4 5 6 CH3 O CH2 O COCH2 OCH2 OCH3 A CH3 2,3-dimethylhexane 3,3-dimethylhexane When there are two or more different alkyl branches, the name of each branch, with its position number, precedes the base name. For example CH3 C2H5 4A 5 6 7 CH3 O CH2 OCHOCHO CH2 O CH2 O CH3 1 3A 2 3-methyl-4-ethylheptane TABLE 24.3 Names of Common Substituent Groups FUNCTIONAL GROUP Amino Fluoro Chloro Bromo Iodo Nitro Vinyl Of course, alkanes can have many different types of substituents. Table 24.3 lists the names of some substituents, including nitro and bromo. Thus the compound NAME ONH2 OF OCl OBr OI ONO2 OCHPCH2 5. Br 2A 1 NO2 3A 4 CH3 OCHO CHOCH3 is called 2-bromo-3-nitrobutane. If the Br is attached to the first carbon atom NO2 1 2 3A 4 Br OCH2 OCH2 OCHO CH3 the compound becomes 1-bromo-3-nitrobutane. EXAMPLE 24.2 Give the IUPAC name of the following compound: CH3 CH3 A A CH3 O COCH2 OCHOCH2 O CH3 A CH3 The parent name of this compound is hexane. Note that there are two methyl groups attached to carbon number 2 and one methyl group attached to carbon number 4. Answer Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 24.2 CH3 2A 1 ALIPHATIC HYDROCARBONS 945 CH3 3 4A 5 6 CH3 OCO CH2 O CHOCH2 OCH3 A CH3 Similar problem: 24.26. Therefore we call the compound 2,2,4-trimethylhexane. PRACTICE EXERCISE Give the IUPAC name of the following compound: C2H5 C2H5 CH3 A A A CH3 O CHO CH2 O CHOCH2 OCHO CH2 O CH3 EXAMPLE 24.3 Write the structural formula of 2,2-dimethyl-3-ethylpentane. Answer The parent compound is a pentane. There are two methyl groups attached to carbon number 2 and one ethyl group attached to carbon number 3. Therefore, the structural formula of the compound is CH3 C2H5 3A 4 5 CH3 O COO CHOCH2 OCH3 A CH3 1 Similar problem: 24.27. 2A PRACTICE EXERCISE Write the structural formula of 2-propyl-4-methylhexane. REACTIONS OF ALKANES Alkanes are generally not considered to be very reactive substances. However, under suitable conditions they do react. For example, natural gas, gasoline, and fuel oil are alkanes that undergo highly exothermic combustion reactions: CH4(g) 2C2H6(g) The systematic names of methyl chloride, methylene chloride, and chloroform are monochloromethane, dichloromethane, and trichloromethane, respectively. 2O2(g) 88n CO2(g) 2H2O(l ) 7O2(g) 88n 4CO2(g) H° 890.4 kJ H° 6H2O(l ) 3119 kJ These, and similar combustion reactions, have long been utilized in industrial processes and in domestic heating and cooking. Halogenation of alkanes — that is, the replacement of one or more hydrogen atoms by halogen atoms — is another type of reaction that alkanes undergo. When a mixture of methane and chlorine is heated above 100°C or irradiated with light of a suitable wavelength, methyl chloride is produced: CH4(g) Cl2(g) 88n CH3Cl(g) HCl(g) methyl chloride If an excess of chlorine gas is present, the reaction can proceed further: CH3Cl(g) Cl2(g) 88n CH2Cl2(l ) HCl(g) methylene chloride Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 946 ORGANIC CHEMISTRY CH2Cl2(g) Cl2(g) 88n CHCl3(l ) HCl(g) chloroform CHCl3(l ) Cl2(g) 88n CCl4(l ) HCl(g) carbon tetrachloride A great deal of experimental evidence suggests that the initial step of the first halogenation reaction occurs as follows: Cl2 energy 88n Cl Cl Thus the covalent bond in Cl2 breaks and two chlorine atoms form. We know it is the ClOCl bond that breaks when the mixture is heated or irradiated because the bond energy of Cl2 is 242.7 kJ/mol, whereas about 414 kJ/mol are needed to break COH bonds in CH4. A chlorine atom is a free radical, which contains an unpaired electron (shown by a single dot). Chlorine atoms are highly reactive and attack methane molecules according to the equation CH4 Cl 88n CH3 HCl This reaction produces hydrogen chloride and the methyl radical CH3. The methyl radical is another reactive species; it combines with molecular chlorine to give methyl chloride and a chlorine atom: CH3 Cl2 88n CH3Cl Cl The production of methylene chloride from methyl chloride and any further reactions can be explained in the same way. The actual mechanism is more complex than the scheme we have shown because “side reactions” that do not lead to the desired products often take place, such as Cl CH3 Cl 88n Cl2 CH3 88n C2H6 Alkanes in which one or more hydrogen atoms have been replaced by a halogen atom are called alkyl halides. Among the large number of alkyl halides, the best known are chloroform (CHCl3), carbon tetrachloride (CCl 4), methylene chloride (CH2Cl2), and the chlorofluorohydrocarbons. Chloroform is a volatile, sweet-tasting liquid that was used for many years as an anesthetic. However, because of its toxicity (it can severely damage the liver, kidneys, and heart) it has been replaced by other compounds. Carbon tetrachloride, also a toxic substance, serves as a cleaning liquid, for it removes grease stains from clothing. Methylene chloride is used as a solvent to decaffeinate coffee and as a paint remover. The preparation of chlorofluorocarbons and the effect of these compounds on ozone in the stratosphere were discussed in Chapter 17. OPTICAL ISOMERISM OF SUBSTITUTED ALKANES Optical isomerism was first discussed in Section 22.4. Back Forward Optical isomers are compounds that are nonsuperimposable mirror images of each other. Figure 24.3 shows perspective drawings of the substituted methanes CH2ClBr and CHFClBr and their mirror images. The mirror images of CH2ClBr are superimposable but those of CHFClBr are not, no matter how we rotate the molecules. Thus the CHFClBr molecule is chiral. Most simple chiral molecules contain at least one asymmetric carbon atom — that is, a carbon atom bonded to four different atoms or groups of atoms. Main Menu TOC Study Guide TOC Textbook Website MHHE Website 24.2 FIGURE 24.3 (a) The CH2ClBr molecule and its mirror image. Since the molecule and its mirror image are superimposable, the molecule is said to be achiral. (b) The CHFClBr molecule and its mirror image. Since the molecule and its mirror image are not superimposable, no matter how we rotate one with respect to the other, the molecule is said to be chiral. Mirror Mirror Br H Br Br H Cl H Cl H F Br H Cl Br H F Br H Cl F Cl H Br H Cl H Br H Cl 947 ALIPHATIC HYDROCARBONS H (a) Cl F (b) EXAMPLE 24.4 Is the following molecule chiral? Cl A HOC OCH2 OCH3 A CH3 We note that the central carbon atom is bonded to a hydrogen atom, a chlorine atom, a OCH3 group, and a OCH2OCH3 group. Therefore the central carbon atom is asymmetric and the molecule is chiral. Answer Similar problems: 24.25. PRACTICE EXERCISE Is the following molecule chiral? Br A I OC OCH2 OCH3 A Br CYCLOALKANES Alkanes whose carbon atoms are joined in rings are known as cycloalkanes. They have the general formula CnH2n, where n 3, 4, . . . . The simplest cycloalkane is cyclopropane, C3H6 (Figure 24.4). Many biologically significant substances such as cholesterol, testosterone, and progesterone contain one or more such ring systems. Theoretical analysis shows that cyclohexane can assume two different geometries that Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 948 ORGANIC CHEMISTRY FIGURE 24.4 Structures of the first four cycloalkanes and their simplified forms. H H HH H C C C H H H H C C H C C H H H H C C C H H H H C C HH Cyclobutane Cyclopentane H C H C C C C H HH H Cyclopropane H H H H H C H H H H H Cyclohexane are relatively free of strain (Figure 24.5). By “strain” we mean that bonds are compressed, stretched, or twisted out of normal geometric shapes predicted by sp3 hybridization. The most stable geometry is the chair form. ALKENES The alkenes (also called olefins) contain at least one carbon-carbon double bond. Alkenes have the general formula CnH2n, where n 2, 3, . . . . The simplest alkene is C2H4, ethylene, in which both carbon atoms are sp2-hybridized and the double bond is made up of a sigma bond and a pi bond (see Section 10.5). Alkene Nomenclature In naming alkenes we indicate the positions of the carbon-carbon double bonds. The names of compounds containing CPC bonds end with -ene. As with the alkanes, the name of the parent compound is determined by the number of carbon atoms in the longest chain (see Table 24.1), as shown here: CH2PCHOCH2OCH3 H3COCHPCHOCH3 1-butene 2-butene The numbers in the names of alkenes refer to the lowest numbered carbon atom in the chain that is part of the CPC bond of the alkene. The name “butene” means that there are four carbon atoms in the longest chain. Alkene nomenclature must specify whether a given molecule is cis or trans if it is a geometric isomer, such as FIGURE 24.5 The cyclohexane molecule can exist in various shapes. The most stable shape is the chair form; the boat form is less stable. Two types of H atoms are labeled axial and equatorial, respectively. Axial Equatorial Chair form Back Forward Main Menu TOC Study Guide TOC Boat form Textbook Website MHHE Website 24.2 4A 1 H3C H G2 3D CP C D G 949 1 CH3 In the cis isomer, the two H atoms are on the same side of the CPC bond; in the trans isomer, the two H atoms are across from each other. Geometric isomerism was introduced in Section 22.4. ALIPHATIC HYDROCARBONS 5 6 CHOCH2 OCH3 H 4-methyl-cis-2-hexene H G2 3D C PC 5 6 D G4 CHOCH2 OCH3 H A CH3 H3C 4-methyl-trans-2-hexene Properties and Reactions of Alkenes Ethylene is an extremely important substance because it is used in large quantities for the manufacture of organic polymers (to be discussed in the next chapter) and in the preparation of many other organic chemicals. Ethylene is prepared industrially by the cracking process, that is, the thermal decomposition of a large hydrocarbon into smaller molecules. When ethane is heated to about 800°C, it undergoes the following reaction: Pt catalyst C2H6(g) 888888n CH2PCH2(g) H2(g) Other alkenes can be prepared by cracking the higher members of the alkane family. Alkenes are classified as unsaturated hydrocarbons, compounds with double or triple carbon-carbon bonds that enable them to add hydrogen atoms. Unsaturated hydrocarbons commonly undergo addition reactions, in which one molecule adds to another to form a single product. Hydrogenation is an example of addition reaction. Other addition reactions to the CPC bond include C2H4(g) C2H4(g) HX(g) 88n CH3OCH2X(g) X2(g) 88n CH2XOCH2X(g) where X represents a halogen (Cl, Br, or I). Figure 24.6 shows the reaction between ethylene and aqueous bromine to form 1,2-dibromoethane (CH2BrCH2Br). The addition of a hydrogen halide to an unsymmetrical alkene such as propylene is more complicated because two products are possible: FIGURE 24.6 When ethylene gas is bubbled through an aqueous bromine solution, the reddishbrown color gradually disappears due to the formation of 1,2-dibromoethane, which is colorless. Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 950 ORGANIC CHEMISTRY H3C H G D C PC D G HH AA H3C OCO COH AA H Br H HBr H propylene and/or HH AA H3CO COCOH AA Br H 1-bromopropane 2-bromopropane In reality, however, only 2-bromopropane is formed. This phenomenon was observed in all reactions between unsymmetrical reagents and alkenes. In 1871, Vladimir Markovnikov† postulated a generalization that enables us to predict the outcome of such an addition reaction. This generalization, now known as Markovnikov’s rule, states that in the addition of unsymmetrical (that is, polar) reagents to alkenes, the positive portion of the reagent (usually hydrogen) adds to the carbon atom that already has the most hydrogen atoms. Geometric Isomers of Alkenes In a compound such as ethane, C2H6, the rotation of the two methyl groups about the carbon-carbon single bond (which is a sigma bond) is quite free. The situation is different for molecules that contain carbon-carbon double bonds, such as ethylene, C2H4. In addition to the sigma bond, there is a pi bond between the two carbon atoms. Rotation about the carbon-carbon linkage does not affect the sigma bond, but it does move the two 2pz orbitals out of alignment for overlap and, hence, partially or totally destroys the pi bond (see Figure 10.15). This process requires an input of energy on the order of 270 kJ/mol. For this reason, the rotation of a carbon-carbon double bond is considerably restricted, but not impossible. Consequently, molecules containing carboncarbon double bonds (that is, the alkenes) may have geometric isomers, which cannot be interconverted without breaking a chemical bond (see Section 22.4). The molecule dichloroethylene, ClHCPCHCl, can exist as one of the two geometric isomers called cis-dichloroethylene and trans-dichloroethylene: resultant dipole moment Cl cis-dichloroethylene H G D CP C D G Cl H cis-dichloroethylene 1.89 D b.p. 60.3 C trans-dichloroethylene H Cl G D CP C D G Cl H trans-dichloroethylene 0 b.p. 47.5 C where the term cis means that two particular atoms (or groups of atoms) are adjacent to each other, and trans means that the two atoms (or groups of atoms) are across from each other. Generally, cis and trans isomers have distinctly different physical and chemical properties. Heat or irradiation with light is commonly used to bring about the conversion of one geometric isomer to another, a process called cis-trans isomerization, or geometric isomerization. As the above data show, dipole moment measurements can be used to distinguish between geometric isomers. In general, cis isomers possess a dipole moment, whereas trans isomers do not. † Vladimir W. Markovnikov (1838–1904). Russian chemist. Markovnikov’s observations of the addition reactions to alkenes were published a year after his death. Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 24.2 ALIPHATIC HYDROCARBONS 951 ALKYNES Alkynes contain at least one carbon-carbon triple bond. They have the general formula CnH2n 2, where n 2, 3, . . . . Alkyne Nomenclature Names of compounds containing CqC bonds end with -yne. Again the name of the parent compound is determined by the number of carbon atoms in the longest chain (see Table 24.1 for names of alkane counterparts). As in the case of alkenes, the names of alkynes indicate the position of the carbon-carbon triple bond, as, for example, in HCqCOCH2OCH3 H3COCqCOCH3 1-butyne 2-butyne Properties and Reactions of Alkynes The simplest alkyne is ethyne, better known as acetylene (C2H2). The structure and bonding of C2H2 were discussed in Section 10.5. Acetylene is a colorless gas (b.p. 84°C) prepared by the reaction between calcium carbide and water: CaC2(s) 2H2O(l ) 88n C2H2(g) Ca(OH)2(aq) Acetylene has many important uses in industry. Because of its high heat of combustion 2C2H2(g) 5O2(g) 88n 4CO2(g) 2H2O(l ) H° 2599.2 kJ acetylene burned in an “oxyacetylene torch” gives an extremely hot flame (about 3000°C). Thus, oxyacetylene torches are used to weld metals (see p. 219). The standard free energy of formation of acetylene is positive ( G° 209.2 kJ/mol), unlike that of the alkanes. This means that the molecule is unstable (relative to its elements) and has a tendency to decompose: C2H2(g) 88n 2C(s) The reaction of calcium carbide with water produces acetylene, a flammable gas. H2(g) In the presence of a suitable catalyst or when the gas is kept under pressure, this reaction can occur with explosive violence. To be transported safely, the gas must be dissolved in an inert organic solvent such as acetone at moderate pressure. In the liquid state, acetylene is very sensitive to shock and is highly explosive. Acetylene, an unsaturated hydrocarbon, can be hydrogenated to yield ethylene: C2H2(g) H2(g) 88n C2H4(g) It undergoes the following addition reactions with hydrogen halides and halogens: C2H2(g) HX(g) 88n CH2PCHX(g) C2H2(g) C2H2(g) X2(g) 88n CHXPCHX(g) 2X2(g) 88n CHX2OCHX2(g) Methylacetylene (propyne), CH3OCqCOH, is the next member in the alkyne family. It undergoes reactions similar to those of acetylene. The addition reactions of propyne also obey Markovnikov’s rule: H3C CH3 OC q COH HBr Br propyne Back Forward Main Menu TOC Study Guide TOC G D C PC D G H H 2-bromopropene Textbook Website MHHE Website 952 ORGANIC CHEMISTRY Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry Back Ice That Burns Ice that burns? Yes, there is such a thing. It is called methane hydrate, and there is enough of it to meet America’s energy needs for years. But scientists have yet to figure out how to mine it without causing an environmental disaster. Bacteria in the sediments on the ocean floor consume organic material and generate methane gas. Under high pressure and low temperature conditions, methane forms methane hydrate, which consists of single molecules of the natural gas trapped within crystalline cages formed by frozen water molecules. A lump of methane hydrate looks like a gray ice cube, but if one puts a lighted match to it, it will burn. Oil companies have known about methane hydrate since the 1930s, when they began using highpressure pipelines to transport natural gas in cold climates. Unless water is carefully removed before the gas enters the pipeline, chunks of methane hydrate will impede the flow of gas. The total reserve of the methane hydrate in the world’s oceans is estimated to be 1013 tons of carbon content, about twice the amount of carbon in all the coal, oil, and natural gas on land. However, harvesting the energy stored in methane hydrate presents a tremendous engineering challenge. It is believed that methane hydrate acts as a kind of cement to keep the ocean floor sediments together. Tampering with the hydrate deposits could cause underwater landslides, leading to the discharge of methane into the atmosphere. This event could have serious consequences for the environment, because methane is a potent greenhouse gas (see Section 17.5). In fact, scientists have speculated that the abrupt release of methane hydrates may have hastened the end of the last ice age about 10,000 years ago. As the great blanket of continental ice melted, global sea levels swelled by more than 90 meters, submerging Arctic regions rich in hydrate deposits. The relatively warm ocean water would have melted the hydrates, unleashing tremendous amounts of methane, which led to global warming. Methane hydrate. The methane molecule is trapped in a cage of frozen water molecules (blue spheres) held together by hydrogen bonds. Methane hydrate burning in air. Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 24.3 24.3 AROMATIC HYDROCARBONS 953 AROMATIC HYDROCARBONS Benzene, the parent compound of this large family of organic substances, was discovered by Michael Faraday in 1826. Over the next 40 years, chemists were preoccupied with determining its molecular structure. Despite the small number of atoms in the molecule, there are quite a few ways to represent the structure of benzene without violating the tetravalency of carbon. However, most proposed structures were rejected because they did not explain the known properties of benzene. Finally, in 1865, August Kekulé† deduced that the benzene molecule could be best represented by a ring structure — a cyclic compound consisting of six carbon atoms: H H H C H C C H C C C C H C or H C C H H H An electron micrograph of benzene molecule, which shows clearly the ring structure. C C H H As we saw in Section 9.8, the properties of benzene are best represented by both of the above resonance structures. Alternatively, the properties of benzene can be explained in terms of delocalized molecular orbitals (see p. 405): NOMENCLATURE OF AROMATIC COMPOUNDS The naming of monosubstituted benzenes, that is, benzenes in which one H atom has been replaced by another atom or a group of atoms, is quite straightforward, as shown below: CH2CH3 ethylbenzene Cl NH2 aminobenzene (aniline) chlorobenzene NO2 nitrobenzene If more than one substituent is present, we must indicate the location of the second group relative to the first. The systematic way to accomplish this is to number the carbon atoms as follows: 1 6 2 5 3 4 † August Kekulé (1829–1896). German chemist. Kekulé was a student of architecture before he became interested in chemistry. He supposedly solved the riddle of the structure of the benzene molecule after having a dream in which dancing snakes bit their own tails. Kekulé’s work is regarded by many as the crowning achievement of theoretical organic chemistry of the nineteenth century. Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 954 ORGANIC CHEMISTRY Three different dibromobenzenes are possible: Br Br Br Br Br Br 1,2-dibromobenzene (o-dibromobenzene) 1,3-dibromobenzene (m-dibromobenzene) 1,4-dibromobenzene ( p-dibromobenzene) The prefixes o- (ortho-), m- (meta-), and p- ( para-) are also used to denote the relative positions of the two substituted groups, as shown above for the dibromobenzenes. Compounds in which the two substituted groups are different are named accordingly. Thus, NO2 Br is named 3-bromonitrobenzene, or m-bromonitrobenzene. Finally, we note that the group containing benzene minus a hydrogen atom (C6H5) is called the phenyl group. Thus the following molecule is called 2-phenylpropane: This compound is also called isopropyl benzene (see Table 24.2). CH3 O CHOCH3 PROPERTIES AND REACTIONS OF AROMATIC COMPOUNDS Benzene is a colorless, flammable liquid obtained chiefly from petroleum and coal tar. Perhaps the most remarkable chemical property of benzene is its relative inertness. Although it has the same empirical formula as acetylene (CH) and a high degree of unsaturation, it is much less reactive than either ethylene or acetylene. The stability of benzene is the result of electron delocalization. In fact, benzene can be hydrogenated, but only with difficulty. The following reaction is carried out at significantly higher temperatures and pressures than are similar reactions for the alkenes: H H H 3H2 H H H Pt catalyst H H H H HH H H H HH H cyclohexane We saw earlier that alkenes react readily with halogens to form addition products, because the pi bond in CPC can be broken easily. The most common reaction of halogens with benzene is the substitution reaction, in which an atom or group of atoms replaces an atom or groups of atoms in another molecule. For example, Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 24.3 AROMATIC HYDROCARBONS H H Br H H H FeBr3 Br2 HBr catalyst H H 955 H H H H bromobenzene Note that if the reaction were an addition reaction, electron delocalization would be destroyed in the product H H Br H H H Br H and the molecule would not have the aromatic characteristic of chemical unreactivity. Alkyl groups can be introduced into the ring system by allowing benzene to react with an alkyl halide using AlCl3 as the catalyst: CH2CH3 CH3CH2Cl AlCl3 HCl catalyst ethyl chloride ethylbenzene An enormously large number of compounds can be generated from substances in which benzene rings are fused together. Some of these polycyclic aromatic hydrocarbons are shown in Figure 24.7. The best known of these compounds is naphthalene, FIGURE 24.7 Some polycyclic aromatic hydrocarbons. Compounds denoted by * are potent carcinogens. An enormous number of such compounds exist in nature. Naphthalene Anthracene Benz(a)anthracene* Phenanthrene Dibenz(a,h)anthracen* Naphthacene Benzo(a)pyrene Coronene Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 956 ORGANIC CHEMISTRY which is used in mothballs. These and many other similar compounds are present in coal tar. Some of the compounds with several rings are powerful carcinogens — they can cause cancer in humans and other animals. 24.4 CHEMISTRY OF THE FUNCTIONAL GROUPS We now examine in greater depth some organic functional groups, groups that are responsible for most of the reactions of the parent compounds. In particular, we focus on oxygen-containing and nitrogen-containing compounds. ALCOHOLS All alcohols contain the hydroxyl functional group, OOH. Some common alcohols are shown in Figure 24.8. Ethyl alcohol, or ethanol, is by far the best known. It is produced biologically by the fermentation of sugar or starch. In the absence of oxygen the enzymes present in bacterial cultures or yeast catalyze the reaction enzymes C6H12O6(aq) 88888n 2CH3CH2OH(aq) 2CO2(g) ethanol This process gives off energy, which microorganisms, in turn, use for growth and other functions. Commercially, ethanol is prepared by an addition reaction in which water is combined with ethylene at about 280°C and 300 atm: CH2PCH2(g) H SO 2 4 H2O(g) 8888n CH3CH2OH(g) Ethanol has countless applications as a solvent for organic chemicals and as a starting compound for the manufacture of dyes, synthetic drugs, cosmetics, and explosives. It is also a constituent of alcoholic beverages. Ethanol is the only nontoxic (more properly, the least toxic) of the straight-chain alcohols; our bodies produce an enzyme, called alcohol dehydrogenase, which helps metabolize ethanol by oxidizing it to acetaldehyde: alcohol dehydrogenase CH3CH2OH 888888888888888n CH3CHO H2 acetaldehyde FIGURE 24.8 Common alcohols. Note that all the compounds contain the OH group. The properties of phenol are quite different from those of the aliphatic alcohols. H A HOCOOH A H HH AA HOCOC OOH AA HH HHH AAA HOC OCOC OH AAA H OH H Methanol (methyl alcohol) Ethanol (ethyl alcohol) 2-Propanol (isopropyl alcohol) OH Phenol Back Forward Main Menu TOC Study Guide TOC HH AA H O CO CO H AA OH OH Ethylene glycol Textbook Website MHHE Website 24.4 CHEMISTRY OF THE FUNCTIONAL GROUPS 957 This equation is a simplified version of what actually takes place; the H atoms are taken up by other molecules, so that no H2 gas is evolved. Ethanol can also be oxidized by inorganic oxidizing agents, such as acidified dichromate, to acetaldehyde and acetic acid: Cr O2 H Cr O2 H 27 27 CH3CH2OH 88888n CH3CHO 88888n CH3COOH Aliphatic alcohols are derived from alkanes, which are all aliphatic hydrocarbons. Ethanol is called an aliphatic alcohol because it is derived from an alkane (ethane). The simplest aliphatic alcohol is methanol, CH3OH. Called wood alcohol, it was prepared at one time by the dry distillation of wood. It is now synthesized industrially by the reaction of carbon monoxide and molecular hydrogen at high temperatures and pressures: Fe O catalyst 23 2H2(g) 88888n CH3OH(l) CO(g) methanol Methanol is highly toxic. Ingestion of only a few milliliters can cause nausea and blindness. Ethanol intended for industrial use is often mixed with methanol to prevent people from drinking it. Ethanol containing methanol or other toxic substances is called denatured alcohol. The alcohols are very weakly acidic; they do not react with strong bases, such as NaOH. The alkali metals react with alcohols to produce hydrogen: 2CH3OH 2Na 88n 2NaOCH3 H2 sodium methoxide However, the reaction is much less violent than that between Na and water: 2H2O Alcohols react more slowly with sodium metal than water does. 2Na 88n 2NaOH H2 Two other familiar aliphatic alcohols are 2-propanol (or isopropanol), commonly known as rubbing alcohol, and ethylene glycol, which is used as an antifreeze. Note that ethylene glycol has two OOH groups and so can form hydrogen bonds with water molecules more effectively than compounds that have only one OOH group (see Figure 24.8). Most alcohols — especially those with low molar masses — are highly flammable. ETHERS Ethers contain the ROOOR linkage, where R and R are a hydrocarbon (aliphatic or aromatic) group. They are formed by the reaction between two alcohols CH3OH H SO catalyst 2 4 HOCH3 88888n CH3OCH3 H2O dimethyl ether This reaction is an example of a condensation reaction, which is characterized by the joining of two molecules and the elimination of a small molecule, usually water. Like alcohols, ethers are extremely flammable. When left standing in air, they have a tendency to slowly form explosive peroxides: C2H5OC2H5 O2 diethyl ether Back Forward Main Menu TOC Study Guide TOC CH3 A C2H5OO COOOOOH A H 1-ethyoxyethyl hydroperoxide Textbook Website MHHE Website 958 ORGANIC CHEMISTRY Peroxides contain the OOOOO linkage; the simplest peroxide is hydrogen peroxide, H2O2. Diethyl ether, commonly known as “ether,” was used as an anesthetic for many years. It produces unconsciousness by depressing the activity of the central nervous system. The major disadvantages of diethyl ether are its irritating effects on the respiratory system and the occurrence of postanesthetic nausea and vomiting. “Neothyl,” or methyl propyl ether, CH3OCH2CH2CH3, is currently favored as an anesthetic because it is relatively free of side effects. ALDEHYDES AND KETONES Under mild oxidation conditions, it is possible to convert alcohols to aldehydes and ketones: 1 2 CH3OH O2 H2CPO H2O formaldehyde H3C 1 2 C2H5OH O2 G CPO D H H2O acetaldehyde H A CH3 O COCH3 A OH H3C 1 2 O2 H3C G CPO D H2O acetone The functional group in these compounds is the carbonyl group, G CPO. In an aldehyde at least one hydrogen atom is bonded to the carbon in the carbonyl group. In a ketone, the carbon atom in the carbonyl group is bonded to two hydrocarbon groups. The simplest aldehyde, formaldehyde (H2CPO) has a tendency to polymerize; that is, the individual molecules join together to form a compound of high molar mass. This action gives off much heat and is often explosive, so formaldehyde is usually prepared and stored in aqueous solution (to reduce the concentration). This rather disagreeable-smelling liquid is used as a starting material in the polymer industry (see Chapter 25) and in the laboratory as a preservative for animal specimens. Interestingly, the higher molar mass aldehydes, such as cinnamic aldehyde G H D CHP CHOC M O Cinnamic aldehyde gives cinnamon its characteristic aroma. have a pleasant odor and are used in the manufacture of perfumes. Ketones generally are less reactive than aldehydes. The simplest ketone is acetone, a pleasant-smelling liquid that is used mainly as a solvent for organic compounds and nail polish remover. CARBOXYLIC ACIDS Under appropriate conditions both alcohols and aldehydes can be oxidized to carboxylic acids, acids that contain the carboxyl group, OCOOH: CH3CH2OH CH3CHO Back Forward Main Menu TOC O2 88n CH3COOH 1 2 H2O O2 88n CH3COOH Study Guide TOC Textbook Website MHHE Website 24.4 FIGURE 24.9 Some common carboxylic acids. Note that they all contain the COOH group. (Glycine is one of the amino acids found in proteins.) CHEMISTRY OF THE FUNCTIONAL GROUPS O B HOCOOH HO AB HOC OCOOH A H HHHO AAAB HO COC OC OC OOH AAA HHH Formic acid Acetic acid Butyric acid O B COOH Benzoic acid HHO AAB NO COC OOH AA HH O B C OOH A C OOH B O O H OH H O BAAAB HOO COC OC OC OCOOH AAA HCH JG O OH Glycine The oxidization of ethanol to acetic acid in wine is catalyzed by enzymes. 959 Oxalic acid Citric acid These reactions occur so readily, in fact, that wine must be protected from atmospheric oxygen while in storage. Otherwise, it would soon turn to vinegar due to the formation of acetic acid. Figure 24.9 shows the structure of some of the common carboxylic acids. Carboxylic acids are widely distributed in nature; they are found in both the plant and animal kingdoms. All protein molecules are made of amino acids, a special kind of carboxylic acid containing an amino group (ONH2) and a carboxyl group (OCOOH). Unlike the inorganic acids HCl, HNO3, and H2SO4, carboxylic acids are usually weak. They react with alcohols to form pleasant-smelling esters: CH3COOH HOCH2CH3 acetic acid This is a condensation reaction. O B CH3 O COOO CH2CH3 ethanol ethyl acetate H2O Other common reactions of carboxylic acids are neutralization CH3COOH NaOH 88n CH3COONa H2O and formation of acid halides, such as acetyl chloride CH3COOH PCl5 88n CH3COCl HCl acetyl chloride POCl3 phosphoryl chloride Acid halides are reactive compounds used as intermediates in the preparation of many other organic compounds. They hydrolyze in much the same way as many nonmetallic halides, such as SiCl4: CH3COCl(l ) SiCl4(l ) H2O(l ) 88n CH3COOH(aq) 3H2O(l ) 88n H2SiO3(s) HCl(g) 4HCl(g) silicic acid ESTERS Esters have the general formula R COOR, where R can be H or a hydrocarbon group and R is a hydrocarbon group. Esters are used in the manufacture of perfumes and as flavoring agents in the confectionery and soft-drink industries. Many fruits owe their characteristic smell and flavor to the presence of small quantities of esters. For exam- Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 960 ORGANIC CHEMISTRY ple, bananas contain 3-methylbutyl acetate [CH3COOCH2CH2CH(CH3)2], oranges contain octyl acetate (CH3COOCHCH3C6H13), and apples contain methyl butyrate (CH3CH2CH2COOCH3). The functional group in esters is the OCOOR group. In the presence of an acid catalyst, such as HCl, esters undergo hydrolysis to yield a carboxylic acid and an alcohol. For example, in acid solution, ethyl acetate hydrolyzes as follows: CH3COOC2H5 H2O 34 CH3COOH ethyl acetate The odor of fruits is mainly due to the ester compounds in them. ethanol However, this reaction does not go to completion because the reverse reaction, that is, the formation of an ester from an alcohol and an acid, also occurs to an appreciable extent. On the other hand, when NaOH solution is used in hydrolysis the sodium acetate does not react with ethanol, so this reaction does go to completion from left to right: CH3COOC2H5 NaOH 88n CH3COO Na ethyl acetate The action of soap is discussed on p. 495. C2H5OH acetic acid C2H5OH sodium acetate ethanol For this reason, ester hydrolysis is usually carried out in basic solutions. Note that NaOH does not act as a catalyst; rather, it is consumed by the reaction. The term saponification (meaning soapmaking) was originally used to describe the alkaline hydrolysis of fatty acid esters to yield soap molecules (sodium stearate): C17H35COOC2H5 NaOH 88n C17H35COO Na ethyl stearate C2H5OH sodium stearate Saponification has now become a general term for alkaline hydrolysis of any type of ester. AMINES Amines are organic bases with the general formula R3N, where R may be H or a hydrocarbon group. As with ammonia, the reaction of amines with water is RNH2 H2O 88n RNH3 OH where R represents a hydrocarbon group. Like all bases, the amines form salts when allowed to react with acids: CH3CH2NH2 HCl 88n CH3CH2NH3 Cl ethylamine ethylammonium chloride These salts are usually colorless, odorless solids. Aromatic amines are used mainly in the manufacture of dyes. Aniline, the simplest aromatic amine, itself is a toxic compound; a number of other aromatic amines such as 2-naphthylamine and benzidine are potent carcinogens: NH2 NH2 H2N aniline Back Forward Main Menu TOC 2-naphthylamine Study Guide TOC NH2 benzidine Textbook Website MHHE Website 24.4 TABLE 24.4 961 CHEMISTRY OF THE FUNCTIONAL GROUPS Important Functional Groups and Their Reactions FUNCTIONAL GROUP NAME TYPICAL REACTIONS G D CP C D G Carbon-carbon double bond Addition reactions with halogens, hydrogen halides, and water; hydrogenation to yield alkanes OC q C O Carbon-carbon triple bond Addition reactions with halogens, hydrogen halides; hydrogenation to yield alkenes and alkanes O OXS Q (X F, Cl, Br, I) Halogen Exchange reactions: CH3CH2Br KI 88n CH3CH2I O O OQOH Hydroxyl Esterification (formation of an ester) with carboxylic acids; oxidation to aldehydes, ketones, and carboxylic acids G CPO O Q D Carbonyl Reduction to yield alcohols; oxidation of aldehydes to yield carboxylic acids Carboxyl Esterification with alcohols; reaction with phosphorus pentachloride to yield acid chlorides Ester Hydrolysis to yield acids and alcohols Amine Formation of ammonium salts with acids SOS B OCOOOH O Q SOS B O OCOOOR Q (R hydrocarbon) D N OO G (R KBr R R H or hydrocarbon) SUMMARY OF FUNCTIONAL GROUPS Table 24.4 summarizes the common functional groups, including the CPC and CqC groups. Organic compounds commonly contain more than one functional group. Generally, the reactivity of a compound is determined by the number and types of functional groups in its makeup. Example 24.3 shows how we can use the functional groups to predict reactions. EXAMPLE 24.5 An artery becoming blocked by cholesterol. Back Forward Main Menu Cholesterol is a major component of gallstones, and it is believed that the cholesterol level in the blood is a contributing factor in certain types of heart disease. From the following structure of the compound, predict its reaction with (a) Br2, (b) H2 (in the presence of a Pt catalyst), (c) CH3COOH: TOC Study Guide TOC Textbook Website MHHE Website 962 ORGANIC CHEMISTRY C8H17 CH3 CH3 HO The first step is to identify the functional groups in cholesterol. There are two: the hydroxyl group and the carbon-carbon double bond. (a) The reaction with bromine results in the addition of bromine to the doublebonded carbons, which become single-bonded. (b) This is a hydrogenation reaction. Again, the carbon-carbon double bond is converted to a carbon-carbon single bond. (c) The acid reacts with the hydroxyl group to form an ester and water. Figure 24.10 shows the products of these reactions. Answer FIGURE 24.10 The products formed by the reaction of cholesterol with (a) molecular bromine, (b) molecular hydrogen, and (c) acetic acid. CH3 C8H17 CH3 i HO f Br CH3 CH3 CH3 i C8H17 CH3 i O B H3CO COO HO Br (a) Similar problem: 24.41. C8H17 (b) (c) PRACTICE EXERCISE Predict the products of the following reaction: CH3OH CH3CH2COOH 88n ? Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry Back The Petroleum Industry In 1995 an estimated 40 percent of the energy needs of the United States were supplied by oil or petroleum. The rest was provided by natural gas (approximately 25 percent), coal (23 percent), hydroelectric power (4 percent), nuclear power (8 percent), and other sources (0.5 percent). In addition to energy, petroleum is the source of numerous organic chemicals used to manufacture drugs, clothing, and many other products. Unrefined petroleum, a viscous, dark-brown liquid, is often called crude oil. A complex mixture of alkanes, alkenes, cycloalkanes, and aromatic compounds, petroleum was formed in Earth’s crust over the course of millions of years by the anaerobic decomposition of animal and vegetable matter by bacteria. Petroleum deposits are widely distributed through- Forward Main Menu TOC out the world, but they are found mainly in North America, Mexico, Russia, China, Venezuela, and, of course, the Middle East. The actual composition of petroleum varies with location. In the United States, for example, Pennsylvania crude oils are mostly aliphatic hydrocarbons, whereas the major components of western crude oils are aromatic in nature. Although petroleum contains literally thousands of hydrocarbon compounds, we can classify its compo- Study Guide TOC Crude oil. Textbook Website MHHE Website 24.4 Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Back CHEMISTRY OF THE FUNCTIONAL GROUPS 963 Major Fractions of Petroleum FRACTION CARBON ATOMS* BOILING POINT RANGE (°C) Natural gas Petroleum ether Ligroin Gasoline Kerosene C1 – C4 C5 – C6 C7 C6 – C12 C11 – C16 161 to 20 30 – 60 20 – 135 30 – 180 170 – 290 Heating fuel oil C14 – C18 260 – 350 Lubricating oil C15 – C24 300 – 370 USES Fuel and cooking gas Solvent for organic compounds Solvent for organic compounds Automobile fuels Rocket and jet engine fuels, domestic heating Domestic heating and fuel for electricity production Lubricants for automobiles and machines *The entries in this column indicate the numbers of carbon atoms in the compounds involved. For example, C1 – C4 tells us that in natural gas the compounds contain 1 to 4 carbon atoms, and so on. nents according to the range of their boiling points. These hydrocarbons can be separated on the basis of molar mass by fractional distillation. Heating crude oil Gas Gasoline 30°C–180°C Naphtha 110°C–195°C Kerosene 170°C–290°C Heating oil 260°C–350°C Lubricating oil 300°C–370°C Heated crude oil at 370° C Residue A fractional distillation column for separating the components of petroleum crude oil. As the hot vapor moves upward, it condenses and the various components of the crude oil are separated according to their boiling points and are drawn off as shown. Forward Main Menu TOC to about 400°C converts the viscous oil into hot vapor and fluid. In this form it enters the fractionating tower. The vapor rises and condenses on various collecting trays according to the temperatures at which the various components of the vapor liquefy. Some gases are drawn off at the top of the column, and the unvaporized residual oil is collected at the bottom. Gasoline is probably the best-known petroleum product. A mixture of volatile hydrocarbons, gasoline contains mostly alkanes, cycloalkanes, and a few aromatic hydrocarbons. Some of these compounds are far more suitable for fueling an automobile engine than others, and herein lies the problem of the further treatment and refinement of gasoline. Most automobiles employ the four-stroke operation of the Otto cycle engine. A major engineering concern is to control the burning of the gasoline-air mixture inside each cylinder to obtain a smooth expansion of the gas mixture. If the mixture burns too rapidly, the piston receives a hard jerk rather than a smooth, strong push. This action produces a “knocking” or “pinging” sound, as well as a decrease in efficiency in the conversion of combustion energy to mechanical energy. It turns out that straight-chain hydrocarbons have the greatest tendency to produce knocking, whereas the branched-chain and aromatic hydrocarbons give the desired smooth push. Gasolines are usually rated according to the octane number, a measure of their tendency to cause knocking. On this scale, a branched C8 compound (2,2,4-trimethylpentane, or isooctane) has been arbitrarily assigned an octane number of 100, and that of n-heptane, a straight-chain compound, is zero. The higher the octane number of the hydrocarbon, the bet- Study Guide TOC Textbook Website MHHE Website 964 Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Back ORGANIC CHEMISTRY Intake valve open Spark plug fires Exhaust valve open (a) ( b) The four stages of operation of an internal combustion engine. This is the type of engine used in practically all automobiles and is described technically as a four-stroke Otto cycle engine. (a) The intake valve opens to let in a gaso- (c) (d) line-air mixture. (b) During the compression stage the two valves are closed. (c) The spark plug fires and the piston is pushed outward. (d) Finally, as the piston is pushed downward, the exhaust valve opens to let out the exhaust gas. ter its performance in the internal combustion engine. Aromatic hydrocarbons such as benzene and toluene have high octane numbers (106 and 120, respectively), as do aliphatic hydrocarbons with branched chains. The octane rating of hydrocarbons can be improved by the addition of small quantities of compounds called antiknocking agents. Among the most widely used antiknocking agents are the following: The addition of 2 to 4 g of either of these compounds to a gallon of gasoline increases the octane rating by 10 or more. However, lead is a highly toxic metal, and the constant discharge of automobile exhaust into the atmosphere has become a serious environmental problem. Federal regulations require that all automobiles made after 1974 use “unleaded” gasolines. The catalytic converters with which late-model automobiles are equipped can be “poisoned” by lead, another reason for its exclusion from gasoline. To minimize knocking, unleaded gasolines contain a higher proportion of branched-chain hydrocarbons such as 2,2,3trimethylbutane and methyl tert-butyl ether, which also acts as an antiknocking agent. CH3 A CH3 O Pb O CH3 A CH3 CH3 A CH2 A CH3 O CH2 O Pb OCH2 O CH3 A CH2 A CH3 tetramethyllead tetraethyllead Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website QUESTIONS AND PROBLEMS 965 1. Because carbon atoms can link up with other carbon atoms in straight and branched chains, carbon can form more compounds than any other element. 2. Organic compounds are derived from two types of hydrocarbons: aliphatic hydrocarbons and aromatic hydrocarbons. 3. Methane, CH4, is the simplest of the alkanes, a family of hydrocarbons with the general formula CnH2n 2. Cyclopropane, C3H6, is the simplest of the cycloalkanes, a family of alkanes whose carbon atoms are joined in a ring. Alkanes and cycloalkanes are saturated hydrocarbons. 4. Ethylene, CH2PCH2, is the simplest of the olefins, or alkenes, a class of hydrocarbons containing carbon-carbon double bonds and having the general formula CnH2n. 5. Acetylene, CHqCH, is the simplest of the alkynes, which are compounds that have the general formula CnH2n 2 and contain carbon-carbon triple bonds. 6. Compounds that contain one or more benzene rings are called aromatic hydrocarbons. These compounds undergo substitution by halogens and alkyl groups. 7. Functional groups impart specific types of chemical reactivity to molecules. Classes of compounds characterized by their functional groups include alcohols, ethers, aldehydes and ketones, carboxylic acids and esters, and amines. SUMMARY OF FACTS AND CONCEPTS KEY WORDS Addition reactions, p. 949 Alcohol, p. 956 Aldehyde, p. 958 Aliphatic hydrocarbon, p. 940 Alkane, p. 940 Alkene, p. 948 Alkyne, p. 951 Amine, p. 960 Aromatic hydrocarbon, p. 940 Carboxylic acid, p. 958 Condensation reaction, p. 957 Cycloalkane, p. 947 Ester, p. 959 Ether, p. 957 Functional group, p. 940 Hydrocarbon, p. 940 Ketone, p. 958 Organic chemistry, p. 940 Saponification, p. 960 Saturated hydrocarbon, p. 940 Structural isomer, p. 941 Unsaturated hydrocarbon, p. 949 QUESTIONS AND PROBLEMS CLASSES OF ORGANIC COMPOUNDS Review Questions 24.1 Explain why carbon is able to form so many more compounds than any other element. 24.2 What is the difference between aliphatic and aromatic hydrocarbons? ALIPHATIC HYDROCARBONS 24.8 Describe reactions that are characteristic of alkanes, alkenes, and alkynes. 24.9 What factor determines whether a carbon atom in a compound is chiral? 24.10 Give examples of a chiral substituted alkane and an achiral substituted alkane. Review Questions 24.3 What do “saturated” and “unsaturated” mean when applied to hydrocarbons? Give examples of a saturated hydrocarbon and an unsaturated hydrocarbon. 24.4 Give three sources of methane. 24.5 Alkenes exhibit geometric isomerism because rotation about the CPC bond is restricted. Explain. 24.6 Why is it that alkanes and alkynes, unlike alkenes, have no geometric isomers? 24.7 What is Markovnikov’s rule? Back Forward Main Menu TOC Problems 24.11 Draw all possible structural isomers for the following alkane: C7H16. 24.12 How many distinct chloropentanes, C5H11Cl, could be produced in the direct chlorination of n-pentane, CH3(CH2)3CH3? Draw the structure of each molecule. 24.13 Draw all possible isomers for the molecule C4H8. 24.14 Draw all possible isomers for the molecule C3H5Br. Study Guide TOC Textbook Website MHHE Website 966 ORGANIC CHEMISTRY 24.15 The structural isomers of pentane, C5H12, have quite different boiling points (see Example 24.1). Explain the observed variation in boiling point, in terms of structure. 24.16 Discuss how you can determine which of the following compounds might be alkanes, cycloalkanes, alkenes, or alkynes, without drawing their formulas: (a) C6H12, (b) C4H6, (c) C5H12, (d) C7H14, (e) C3H4. 24.17 Draw the structures of cis-2-butene and trans-2butene. Which of the two compounds would have the higher heat of hydrogenation? Explain. 24.18 Would you expect cyclobutadiene to be a stable molecule? Explain. H H H E CO C BB EC O C H H H 24.19 How many different isomers can be derived from ethylene if two hydrogen atoms are replaced by a fluorine atom and a chlorine atom? Draw their structures and name them. Indicate which are structural isomers and which are geometric isomers. 24.20 Suggest two chemical tests that would help you distinguish between these two compounds: (a) CH3CH2CH2CH2CH3 (b) CH3CH2CH2CHPCH2 24.21 Sulfuric acid (H2SO4) adds to the double bond of alkenes as H and OSO3H. Predict the products when sulfuric acid reacts with (a) ethylene and (b) propylene. 24.22 Acetylene is an unstable compound. It has a tendency to form benzene as follows: 3C2H2(g) 88n C6H6(l ) Calculate the standard enthalpy change in kilojoules for this reaction at 25°C. 24.23 Predict products when HBr is added to (a) 1-butene and (b) 2-butene. 24.24 Geometric isomers are not restricted to compounds containing the CPC bond. For example, certain disubstituted cycloalkanes can exist in the cis and the trans forms. Label the following molecules as the cis and trans isomer of the same compound: H (a) Cl H 24.25 Which of the following amino acids are chiral: (a) CH3CH(NH2)COOH, (b) CH2(NH2)COOH, (c) CH2(OH)CH(NH2)COOH? 24.26 Name the following compounds: CH3 A (a) CH3 OCHO CH2 OCH2 OCH3 C2H5 CH3 CH3 A A A (b) CH3 OCH OO CHOCHOCH3 (c) CH3 OCH2 O CHOCH2 O CH3 A CH2 O CH2 OCH3 CH3 A (d) CH2 P CHOCHOCHP CH2 (e) CH3 OC q CO CH2 OCH3 (f ) CH3 OCH2 OCHOCHP CH2 24.27 Write structural formulas for the following organic compounds: (a) 3-methylhexane, (b) 1,3,5-trichlorocyclohexane, (c) 2,3-dimethylpentane, (d) 2-phenyl4-bromopentane, (e) 3,4,5-trimethyloctane. 24.28 Write structural formulas for the following compounds: (a) trans-2-pentene, (b) 2-ethyl-1-butene, (c) 4-ethyl-trans-2-heptene, (d) 3-phenylbutyne. AROMATIC HYDROCARBONS Review Questions 24.29 Comment on the extra stability of benzene compared to ethylene. Why does ethylene undergo addition reactions while benzene usually undergoes substitution reactions? 24.30 Benzene and cyclohexane molecules both contain six-membered rings. Benzene is a planar molecule, and cyclohexane is nonplanar. Explain. Problems 24.31 Write structures for the following compounds: (a) 1-bromo-3-methylbenzene, (b) 1-chloro-2-propylbenzene, (c) 1,2,4,5-tetramethylbenzene. 24.32 Name the following compounds: H Cl (b) Cl H NO2 Cl (b) (a) H CH2CH3 Cl H Back Forward H Main Menu H Cl TOC CH3 Study Guide TOC NO2 Textbook Website MHHE Website QUESTIONS AND PROBLEMS structure for the original compound that is consistent with this information. 24.41 Predict the product or products of each of the following reactions: (a) CH3CH2OH HCOOH 88n (b) HOC q CO CH3 H2 (c) C2H5 H CH3 CH3 (c) H3C CH3 CHEMISTRY OF THE FUNCTIONAL GROUPS Review Questions H 24.33 What are functional groups? Why is it logical and useful to classify organic compounds according to their functional groups? 24.34 Draw the Lewis structure for each of the following functional groups: alcohol, ether, aldehyde, ketone, carboxylic acid, ester, amine. 24.35 Draw structures for molecules with the following formulas: (a) CH4O, (b) C2H6O, (c) C3H6O2, (d) C3H8O. 24.36 Classify each of the following molecules as alcohol, aldehyde, ketone, carboxylic acid, amine, or ether: (a) CH3 OOO CH2 O CH3 (b) H3COCH2 ONH2 (c) CH3 OCH2 O C O J G H (d) CH3 O COCH2 O CH3 H C2H4(g) 3O2(g) 88n 2CO2(g) 2H2O(l ) H° 1411 kJ 2C2H2(g) 5O2(g) 88n 4CO2(g) 2H2O(l ) H° 2599 kJ 1 2 O2(g) 88n H2O(l ) H° (f ) H3COCH2CH2 OOH NH2 O B A CH2 O CO COOH O A H HCOOH 24.43 Draw all the possible structural isomers for the molecule having the formula C7H7Cl. The molecule contains one benzene ring. 24.44 Given these data H2(g) CH3OH 88n 24.39 A compound has the empirical formula C5H12O. Upon controlled oxidation, it is converted into a compound of empirical formula C5H10O, which behaves as a ketone. Draw possible structures for the original compound and the final compound. 24.40 A compound having the molecular formula C4H10O does not react with sodium metal. In the presence of light, the compound reacts with Cl2 to form three compounds having the formula C4H9OCl. Draw a Main Menu TOC 285.8 kJ calculate the heat of hydrogenation for acetylene: C2H2(g) 24.37 Generally aldehydes are more susceptible to oxidation in air than are ketones. Use acetaldehyde and acetone as examples and show why ketones such as acetone are more stable than aldehydes in this respect. 24.38 Complete the following equation and identify the products: Forward HBr 24.42 Identify the functional groups in each of the following molecules: (a) CH3CH2COCH2CH2CH3 (b) CH3COOC2H5 (c) CH3CH2OCH2CH2CH2CH3 B O O B (e) HOCOOH Back G D C PC D G ADDITIONAL PROBLEMS Problems (g) 967 H2(g) 88n C2H4(g) 24.45 State which member of each of the following pairs of compounds is the more reactive and explain why: (a) propane and cyclopropane, (b) ethylene and methane, (c) acetaldehyde and acetone. 24.46 State which of the following types of compounds can form hydrogen bonds with water molecules: (a) carboxylic acids, (b) alkenes, (c) ethers, (d) aldehydes, (e) alkanes, (f ) amines. 24.47 An organic compound is found to contain 37.5 percent carbon, 3.2 percent hydrogen, and 59.3 percent fluorine by mass. The following pressure and volume data were obtained for 1.00 g of this substance at 90°C: P (atm) V (L) 2.00 1.50 1.00 0.50 0.332 0.409 0.564 1.028 Study Guide TOC Textbook Website MHHE Website 968 ORGANIC CHEMISTRY 24.48 24.49 24.50 24.51 24.52 24.53 24.54 24.55 The molecule is known to have no dipole moment. (a) What is the empirical formula of this substance? (b) Does this substance behave as an ideal gas? (c) What is its molecular formula? (d) Draw the Lewis structure of this molecule and describe its geometry. (e) What is the systematic name of this compound? State at least one commercial use for each of the following compounds: (a) 2-propanol (isopropanol), (b) acetic acid, (c) naphthalene, (d) methanol, (e) ethanol, (f ) ethylene glycol, (g) methane, (h) ethylene. How many liters of air (78 percent N2, 22 percent O2 by volume) at 20°C and 1.00 atm are needed for the complete combustion of 1.0 L of octane, C8H18, a typical gasoline component that has a density of 0.70 g/mL? How many carbon-carbon sigma bonds are present in each of the following molecules? (a) 2-butyne, (b) anthracene (see Figure 24.5), (c) 2,3-dimethylpentane How many carbon-carbon sigma bonds are present in each of the following molecules? (a) benzene, (b) cyclobutane, (c) 2-methyl-3-ethylpentane The combustion of 20.63 mg of compound Y, which contains only C, H, and O, with excess oxygen gave 57.94 mg of CO2 and 11.85 mg of H2O. (a) Calculate how many milligrams of C, H, and O were present in the original sample of Y. (b) Derive the empirical formula of Y. (c) Suggest a plausible structure for Y if the empirical formula is the same as the molecular formula. Draw all the structural isomers of compounds with the formula C4H8Cl2. Indicate which isomers are chiral and give them systematic names. The combustion of 3.795 mg of liquid B, which contains only C, H, and O, with excess oxygen gave 9.708 mg of CO2 and 3.969 mg of H2O. In a molar mass determination, 0.205 g of B vaporized at 1.00 atm and 200.0°C and occupied a volume of 89.8 mL. Derive the empirical formula, molar mass, and molecular formula of B and draw three plausible structures. Beginning with 3-methyl-1-butyne, show how you would prepare the following compounds: Br CH3 AA (a) CH2 PC OCHOCH3 CH3 A (b) CH2Br OCBr2 O CHO CH3 Br CH3 A A (c) CH3 O CHO CHOCH3 Back Forward Main Menu TOC 24.56 Indicate the asymmetric carbon atoms in the following compounds: O CH3 A B (a) CH3 O CH2 O CHO CHOCONH2 A NH2 H (b) H H Br H Br 24.57 Suppose benzene contained three distinct single bonds and three distinct double bonds. How many different isomers would there be for dichlorobenzene (C6H4Cl2)? Draw all your proposed structures. 24.58 Write the structural formula of an aldehyde that is a structural isomer of acetone. 24.59 Draw structures for the following compounds: (a) cyclopentane, (b) cis-2-butene, (c) 2-hexanol, (d) 1,4-dibromobenzene, (e) 2-butyne. 24.60 Name the classes to which the following compounds belong: (a) C4H9OH (b) CH3OC2H5 (c) C2H5CHO (d) C6H5COOH (e) CH3NH2 24.61 Ethanol, C2H5OH, and dimethyl ether, CH3OCH3, are structural isomers. Compare their melting points, boiling points, and solubilities in water. 24.62 Amines are Brønsted bases. The unpleasant smell of fish is due to the presence of certain amines. Explain why cooks often add lemon juice to suppress the odor of fish (in addition to enhancing the flavor). 24.63 You are given two bottles, each containing a colorless liquid. You are told that one liquid is cyclohexane and the other is benzene. Suggest one chemical test that would allow you to distinguish between these two liquids. 24.64 Give the chemical names of the following organic compounds and write their formulas: marsh gas, grain alcohol, wood alcohol, rubbing alcohol, antifreeze, mothballs, chief ingredient of vinegar. 24.65 The compound CH3OCqCOCH3 is hydrogenated to an alkene using platinum as the catalyst. Predict whether the product is the pure trans isomer, the pure cis isomer, or a mixture of cis and trans isomers. Based on your prediction, comment on the mechanism of the heterogeneous catalysis. Study Guide TOC Textbook Website MHHE Website QUESTIONS AND PROBLEMS 24.66 How many asymmetric carbon atoms are present in each of the following compounds? HHH AAA (a) HOCO CO COCl AAA H Cl H OH A (b) H3CO C A H H A (c) C A HO CH3 A COCH2OH A H CH2OH A C O A H OH H A A C C A A H OH OH A C A H 24.67 Isopropanol is prepared by reacting propylene (CH3CHCH2) with sulfuric acid, followed by treatment with water. (a) Show the sequence of steps leading to the product. What is the role of sulfuric acid? (b) Draw the structure of an alcohol that is an isomer of isopropanol. (c) Is isopropanol a chiral molecule? (d) What property of isopropanol makes it useful as a rubbing alcohol? 24.68 When a mixture of methane and bromine is exposed to light, the following reaction occurs slowly: CH4(g) Br2(g) 88n CH3Br(g) HBr(g) Suggest a mechanism for this reaction. (Hint: Bromine vapor is deep red; methane is colorless.) 24.69 Fat and oil are names for the same class of compounds, called triglycerides, which contain three ester groups O B CH2 OOO COR 969 erol and carboxylic acids (see p. 425 for structure of glycerol). (b) In the old days, soaps were made by hydrolyzing animal fat with lye (a sodium hydroxide solution). Write an equation for this reaction. (c) The difference between fats and oils is that at room temperature, the former are solids and the latter are liquids. Fats are usually produced by animals, whereas oils are commonly found in plants. The melting points of these substances are determined by the number of CPC bonds (or the extent of unsaturation) present — the larger the number of CPC bonds, the lower the melting point and the more likely that the substance is a liquid. Explain. (d) One way to convert liquid oil to solid fat is to hydrogenate the oil, a process by which some or all of the CPC bonds are converted to COC bonds. This procedure prolongs shelf life of the oil by removing the more reactive CPC group and facilitates packaging. How would you carry out such a process (that is, what reagents and catalyst would you employ)? (e) The degree of unsaturation of oil can be determined by reacting the oil with iodine, which reacts with the CPC bond as follows: AAAA OC O C PC O C O A A I2 II AAAA OC O C OC O C O AAAA The procedure is to add a known amount of iodine to the oil and allow the reaction to go to completion. The amount of excess (unreacted) iodine is determined by titrating the remaining iodine with a standard sodium thiosulfate (Na2S2O3) solution: I2 2Na2S2O3 88n Na2S4O6 2I The number of grams of iodine that react with 100 grams of oil is called the iodine number. In one case, 43.8 g of I2 was treated with 35.3 g of corn oil. The excess iodine required 20.6 mL of a 0.142 M Na2S2O3 for neutralization. Calculate the iodine number of the corn oil. O B CHOOOCO R O B CH2 OOO COR Answers to Practice Exercises: A fat or oil where R, R , and R represent long hydrocarbon chains. (a) Suggest a reaction that leads to the formation of a triglyceride molecule, starting with glyc- Back Forward Main Menu TOC 24.1 5. 24.2 2-methyl-4,6-diethyloctane. C3 H7 CH3 24.3 A A CH3 O CHOCH2 OCHOCH2 OCH3 24.4 No. 24.5 CH3CH2COOCH3 and H2O. Study Guide TOC Textbook Website MHHE Website ...
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