《有机化学》课程教学资源（教材文献，英文版）CHAPTER 11 ARENES AND AROMATICITY

CHAPTER ELEVEN Arenes and Aromaticity he prefixes o, m, and p are not used when three or more substituents are present n benzene: numerical locants must be used instead F O,N OCH CH,CH NO 4-Ethyl-2-fluoroanisole 2, 4, 6-Trinitrotoluene 3-Ethyl-2-methylaniline In these examples the base name of the benzene derivative determines the carbon at which numbering begins: anisole has its methoxy group at C-1, toluene its methyl group at C-l, and aniline its amino group at C-1. The direction of numbering is chosen to give the next substituted position the lowest number irrespective of what substituent it bears The order of appearance of substituents in the name is alphabetical. When no simple base name other than benzene is appropriate, positions are numbered so as to give the "first point of differ- lowest locant at the first point of difference. Thus, each of the following examples is ence"rule was introduced in named as a 1, 2, 4-trisubstituted derivative of benzene rather than as a 1, 3, 4-derivative Section 2.11 CH,CH NO, F 1-Chloro-2, 4-dinitrobenzene 4-Ethyl-1-fluoro-2-nitrobenzene When the benzene ring is named as a substituent, the word"phenyl"stands for C6Hs-. Similarly, an arene named as a substituent is called an ary/ group. a benzyl group is C6Hs CH2 CH,CH,OH CHOBI 2-Phenylethanol Benzyl bromide Biphenyl is the accepted IuPAC name for the compound in which two benzene rings ar 11.8 POLY CYCLIC AROMATIC HYDROCARBONS Members of a class of arenes called polycyclic benzenoid aromatic hydrocarbons possess substantial resonance energies because each is a collection of benzene rings fused together. ithalene is a white cry Naphthalene, anthracene, and phenanthrene are the three simplest members of this that sublimes readily. It has a class. They are all present in coal tar, a mixture of organic substances formed when coal characteristic odor and was is converted to coke by heating at high temperatures(about 1000C)in the absence of formerly used as a moth pellent air. Naphthalene is bicyclic(has two rings), and its two benzene rings share a common side. Anthracene and phenanthrene are both tricyclic aromatic hydrocarbons. Anthracene Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

The prefixes o, m, and p are not used when three or more substituents are present on benzene; numerical locants must be used instead. In these examples the base name of the benzene derivative determines the carbon at which numbering begins: anisole has its methoxy group at C-1, toluene its methyl group at C-1, and aniline its amino group at C-1. The direction of numbering is chosen to give the next substituted position the lowest number irrespective of what substituent it bears. The order of appearance of substituents in the name is alphabetical. When no simple base name other than benzene is appropriate, positions are numbered so as to give the lowest locant at the first point of difference. Thus, each of the following examples is named as a 1,2,4-trisubstituted derivative of benzene rather than as a 1,3,4-derivative: When the benzene ring is named as a substituent, the word “phenyl” stands for C6H5±. Similarly, an arene named as a substituent is called an aryl group. A benzyl group is C6H5CH2±. Biphenyl is the accepted IUPAC name for the compound in which two benzene rings are connected by a single bond. 11.8 POLYCYCLIC AROMATIC HYDROCARBONS Members of a class of arenes called polycyclic benzenoid aromatic hydrocarbons possess substantial resonance energies because each is a collection of benzene rings fused together. Naphthalene, anthracene, and phenanthrene are the three simplest members of this class. They are all present in coal tar, a mixture of organic substances formed when coal is converted to coke by heating at high temperatures (about 1000°C) in the absence of air. Naphthalene is bicyclic (has two rings), and its two benzene rings share a common side. Anthracene and phenanthrene are both tricyclic aromatic hydrocarbons. Anthracene Biphenyl Cl p-Chlorobiphenyl CH2CH2OH 2-Phenylethanol CH2Br Benzyl bromide 1 4 2 3 6 5 Cl NO2 NO2 1-Chloro-2,4-dinitrobenzene 4 1 3 2 5 6 CH2CH3 F NO2 4-Ethyl-1-fluoro-2-nitrobenzene 3 6 2 1 4 5 CH3CH2 F OCH3 4-Ethyl-2-fluoroanisole 1 4 2 3 6 5 CH3 NO2 O2N NO2 2,4,6-Trinitrotoluene 1 4 2 3 6 5 NH2 CH3 CH2CH3 3-Ethyl-2-methylaniline 408 CHAPTER ELEVEN Arenes and Aromaticity The “first point of differ￾ence” rule was introduced in Section 2.11. Naphthalene is a white crys￾talline solid melting at 80°C that sublimes readily. It has a characteristic odor and was formerly used as a moth re￾pellent. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

has three rings fused in a"linear'fashion, and"angular"fusion characterizes phenan- threne. The structural formulas of naphthalene, anthracene, and phenanthrene are shown along with the numbering system used to name their substituted derivatives: Naphthalene Anthracene Phenanthrene 255 kJ/mol 347 k/mol 381 kJ/mol (91 kcal/mol) In general, the most stable resonance structure for a c aromatic hydr arbon is the one which has the greatest number of rings that d to Kekule for- lulations of benzene. Naphthalene provides a fairly typical Most stable resonance Only left ring corresponds ngs cor to Kekule benzene to Kekule benzene kule benzene Notice that anthracene cannot be represented by any single Lewis structure in which all three rings correspond to Kekule formulations of benzene, but phenanthrene can PROBLEM 11.4 Chrysene is an aromatic hydrocarbon found in coal ta. the struc ture shown is not the most stable resonance form write the most stable reso- nance form for chrysene have been synthesized in the laboratory, and several of the others are products of coa ly A large number of polycyclic benzenoid aromatic hydroc bustion. Benzo]pyrene, for example, is present in tobacco smoke, contaminates food cooked on barbecue grills, and collects in the soot of chimneys. Benzo[a]pyrene is a car- cinogen(a cancer-causing substance). It is converted in the liver to an epoxy diol that Sir Percivall Pott suggested can induce mutations leading to the uncontrolled growth of certain cells that scrotal cancer in chim- caused by soot. This was the first pro- al that cancer could be caused by chemicals present oxidation in Benzo(apyrene 7,8-Dihydroxy-9, 10-epoy 7, 8,9, 10-tetrahydrobenzolalpyrer Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

has three rings fused in a “linear” fashion, and “angular” fusion characterizes phenan￾threne. The structural formulas of naphthalene, anthracene, and phenanthrene are shown along with the numbering system used to name their substituted derivatives: In general, the most stable resonance structure for a polycyclic aromatic hydro￾carbon is the one which has the greatest number of rings that correspond to Kekulé for￾mulations of benzene. Naphthalene provides a fairly typical example: Notice that anthracene cannot be represented by any single Lewis structure in which all three rings correspond to Kekulé formulations of benzene, but phenanthrene can. PROBLEM 11.4 Chrysene is an aromatic hydrocarbon found in coal tar. The struc￾ture shown is not the most stable resonance form. Write the most stable reso￾nance form for chrysene. A large number of polycyclic benzenoid aromatic hydrocarbons are known. Many have been synthesized in the laboratory, and several of the others are products of com￾bustion. Benzo[a]pyrene, for example, is present in tobacco smoke, contaminates food cooked on barbecue grills, and collects in the soot of chimneys. Benzo[a]pyrene is a car￾cinogen (a cancer-causing substance). It is converted in the liver to an epoxy diol that can induce mutations leading to the uncontrolled growth of certain cells. Benzo[a]pyrene oxidation in the liver O HO OH 7,8-Dihydroxy-9,10-epoxy- 7,8,9,10-tetrahydrobenzo[a]pyrene Only left ring corresponds to Kekulé benzene. Both rings correspond to Kekulé benzene. Most stable resonance form Only right ring corresponds to Kekulé benzene. 11.8 Polycyclic Aromatic Hydrocarbons 409 Arene: Resonance energy: 7 6 8 5 1 4 2 3 Naphthalene 255 kJ/mol (61 kcal/mol) 7 6 2 3 8 5 1 4 9 10 Anthracene 347 kJ/mol (83 kcal/mol) 7 8 6 5 9 10 1 2 4 3 Phenanthrene 381 kJ/mol (91 kcal/mol) In 1775, the British surgeon Sir Percivall Pott suggested that scrotal cancer in chim￾ney sweeps was caused by soot. This was the first pro￾posal that cancer could be caused by chemicals present in the workplace. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER ELEVEN Arenes and Aromaticity CARBON CLUSTERS, FULLERENES, AND NANOTUBES he 1996 Nobel Prize in chemistry was awarded larized by the American architect and inventor R to Professors Harold W. Kroto(University of Sus- Buckminster Fuller ( It is also often referred to as a sex), Robert F Curl, and Richard E. Smalley(both "buckyball. ")Other carbon clusters, some larger than of Rice University) for groundbreaking work involv- C60 and some smaller, were also formed in the exper- ing elemental carbon that opened up a whole new iment, and the general term fullerene refers to such area of chemistry. The work began when Kroto carbon clusters wondered whether polyacetylenes of the type all of the carbon atoms in buckminster- HC=C-(C=O)n-C=CH might be present in inter- fullerene are equivalent and are sp-hybridized; each stellar space and discussed experiments to test this one simultaneously belongs to one five-membered idea while visiting Curl and Smalley at rice in the ring and two benzene-like six-membered rings. The spring of 1984. Smalley had developed a method for strain caused by distortion of the rings from copla the laser- induced evaporation of metals at very low narity is equally distributed among all of the carbons pressure and was able to measure the molecular Confirmation of the structure proposed for cso eights of the various clusters of atoms produced. required isolation of enough material to allow the ar- Kroto, Curl, and Smalley felt that by applying this senal of modern techniques of structure determina- technique to graphite Figure 11.5)the vaporized car- tion to be applied. a quantum leap in fullerene re- bon produced might be similar to that produced by a search came in 1990 when a team led by Wolfgang carbon-rich star Kratschmer of the max planck institute for nuclear When the experiment was carried out in the Physics in Heidelberg and donald Huffman of the fall of 1985, Kroto, Curl, and Smalley found that un- University of Arizona successfully prepared buckman- der certain conditions a species with a molecular for- sterfullerene in amounts sufficient for its isolation mula of Cso was present in amounts much greater purification and detailed study. Not only was the than any other On speculating about what Cso might buckminsterfullerene structure shown to be correct, be, they concluded that its most likely structure is the but academic and industrial scientists around the spherical cluster of carbon atoms shown in Figure world seized the opportunity afforded by the avail- 11.6 and suggested it be called buckminsterfullerene ability of cso in quantity to study its properties because of its similarity to the geodesic domes popu- Speculation about the stability of Coo centered on the extent to which the aromaticity associated with its 20 benzene rings is degraded by their non- FIGURE 11.5 Graphite is a form of elemental carbon FIGURE 11.6 Buckminsterfullerene(Cso). Note that composed of parallel sheets of fused benzene-like rings. all carbons are equivalent and that no five-membered rings acent to one anothe -Cont Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

410 CHAPTER ELEVEN Arenes and Aromaticity CARBON CLUSTERS, FULLERENES, AND NANOTUBES The 1996 Nobel Prize in chemistry was awarded to Professors Harold W. Kroto (University of Sus￾sex), Robert F. Curl, and Richard E. Smalley (both of Rice University) for groundbreaking work involv￾ing elemental carbon that opened up a whole new area of chemistry. The work began when Kroto wondered whether polyacetylenes of the type HCPC±(CPC)n±CPCH might be present in inter￾stellar space and discussed experiments to test this idea while visiting Curl and Smalley at Rice in the spring of 1984. Smalley had developed a method for the laser-induced evaporation of metals at very low pressure and was able to measure the molecular weights of the various clusters of atoms produced. Kroto, Curl, and Smalley felt that by applying this technique to graphite (Figure 11.5) the vaporized car￾bon produced might be similar to that produced by a carbon-rich star. When the experiment was carried out in the fall of 1985, Kroto, Curl, and Smalley found that un￾der certain conditions a species with a molecular for￾mula of C60 was present in amounts much greater than any other. On speculating about what C60 might be, they concluded that its most likely structure is the spherical cluster of carbon atoms shown in Figure 11.6 and suggested it be called buckminsterfullerene because of its similarity to the geodesic domes popu￾larized by the American architect and inventor R. Buckminster Fuller. (It is also often referred to as a “buckyball.”) Other carbon clusters, some larger than C60 and some smaller, were also formed in the exper￾iment, and the general term fullerene refers to such carbon clusters. All of the carbon atoms in buckminster￾fullerene are equivalent and are sp2 -hybridized; each one simultaneously belongs to one five-membered ring and two benzene-like six-membered rings. The strain caused by distortion of the rings from copla￾narity is equally distributed among all of the carbons. Confirmation of the structure proposed for C60 required isolation of enough material to allow the ar￾senal of modern techniques of structure determina￾tion to be applied. A quantum leap in fullerene re￾search came in 1990 when a team led by Wolfgang Krätschmer of the Max Planck Institute for Nuclear Physics in Heidelberg and Donald Huffman of the University of Arizona successfully prepared buckmin￾sterfullerene in amounts sufficient for its isolation, purification and detailed study. Not only was the buckminsterfullerene structure shown to be correct, but academic and industrial scientists around the world seized the opportunity afforded by the avail￾ability of C60 in quantity to study its properties. Speculation about the stability of C60 centered on the extent to which the aromaticity associated with its 20 benzene rings is degraded by their non￾FIGURE 11.5 Graphite is a form of elemental carbon composed of parallel sheets of fused benzene-like rings. FIGURE 11.6 Buckminsterfullerene (C60). Note that all carbons are equivalent and that no five-membered rings are adjacent to one another. —Cont. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

11.10 Reactions of Arenes: a preview planarity and the accompanying angle strain. It is tube of fused six-membered carbon rings between now clear that Cso is a relatively reactive substance, the two halves. reacting with many substances toward which ben Thus far, the importance of carbon cluster zene itself is inert. Many of these reactions are char- chemistry has been in the discovery of new knowl- acterized by the addition of nucleophilic substances edge. Many scientists feel that the earliest industrial to buckminsterfullerene, converting sp--hybridized applications of the fullerenes will be based on their carbons to sp-hybridized ones and reducing the novel electrical properties. Buckminsterfullerene is overall strain in insulator, but has a high electron affinity and is a he field of fullerene chemistry expanded in an superconductor in its reduced form. Nanotubes have unexpected direction in 1991 when Sumio lijima of aroused a great deal of interest for their electrical he NEC Fundamental Research Laboratories in Japan properties and as potential sources of carbon fibers discovered fibrous carbon clusters in one of his of great strength fullerene preparations. this led, within a short time Although the question that began the to substances of the type portrayed in Figure 11.7 fullerene story, the possibility that carbon clusters are called single-walled nanotubes. The best way to think formed in stars, still remains unanswered, the at- about this material is as a"stretched"fullerene Take tempt to answer that question has opened the door a molecule of Cao, cut it in half, and place a cylindrical to novel structures and materials. aononongoooooonanoomaoanm FIGURE 11.7 A portion of a nanotube. The closed end is approximately one half of a buckyball. the main length cannot close as long as all of the rings are hexagons. 11.9 PHYSICAL PROPERTIES OF ARENES n general, arenes resemble other hydrocarbons in their physical properties. They are nonpolar, insoluble in water, and less dense than water. In the absence of polar sub- enes are stituents, intermolecular forces are weak and limited to van der waals attractions of the Appendix 1 luced-dipole type At one time, benzene was widely used as a solvent. This use virtually disappeared when statistical studies revealed an increased incidence of leukemia among workers exposed to atmospheric levels of benzene as low as I ppm. Toluene has replaced ben zene as an inexpensive organic solvent, because it has similar solvent properties but has not been determined to be carcinogenic in the cell systems and at the dose levels that benzene is 11.10 REACTIONS OF ARENES: A PREVIEW We'll examine the chemical properties of aromatic compounds from two different per- 1. One mode of chemical reactivity involves the ring itself as a functional group and includes (a) Reduction (b)Electrophilic aromatic substitution Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

11.10 Reactions of Arenes: A Preview 411 planarity and the accompanying angle strain. It is now clear that C60 is a relatively reactive substance, reacting with many substances toward which ben￾zene itself is inert. Many of these reactions are char￾acterized by the addition of nucleophilic substances to buckminsterfullerene, converting sp2 -hybridized carbons to sp3 -hybridized ones and reducing the overall strain. The field of fullerene chemistry expanded in an unexpected direction in 1991 when Sumio lijima of the NEC Fundamental Research Laboratories in Japan discovered fibrous carbon clusters in one of his fullerene preparations. This led, within a short time, to substances of the type portrayed in Figure 11.7 called single-walled nanotubes. The best way to think about this material is as a “stretched” fullerene. Take a molecule of C60, cut it in half, and place a cylindrical tube of fused six-membered carbon rings between the two halves. Thus far, the importance of carbon cluster chemistry has been in the discovery of new knowl￾edge. Many scientists feel that the earliest industrial applications of the fullerenes will be based on their novel electrical properties. Buckminsterfullerene is an insulator, but has a high electron affinity and is a superconductor in its reduced form. Nanotubes have aroused a great deal of interest for their electrical properties and as potential sources of carbon fibers of great strength. Although the question that began the fullerene story, the possibility that carbon clusters are formed in stars, still remains unanswered, the at￾tempt to answer that question has opened the door to novel structures and materials. FIGURE 11.7 A portion of a nanotube. The closed end is approximately one half of a buckyball. The main length cannot close as long as all of the rings are hexagons. 11.9 PHYSICAL PROPERTIES OF ARENES In general, arenes resemble other hydrocarbons in their physical properties. They are nonpolar, insoluble in water, and less dense than water. In the absence of polar sub￾stituents, intermolecular forces are weak and limited to van der Waals attractions of the induced-dipole/induced-dipole type. At one time, benzene was widely used as a solvent. This use virtually disappeared when statistical studies revealed an increased incidence of leukemia among workers exposed to atmospheric levels of benzene as low as 1 ppm. Toluene has replaced ben￾zene as an inexpensive organic solvent, because it has similar solvent properties but has not been determined to be carcinogenic in the cell systems and at the dose levels that benzene is. 11.10 REACTIONS OF ARENES: A PREVIEW We’ll examine the chemical properties of aromatic compounds from two different per￾spectives: 1. One mode of chemical reactivity involves the ring itself as a functional group and includes (a) Reduction (b) Electrophilic aromatic substitution Selected physical properties for a number of arenes are listed in Appendix 1. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER ELEVEN Arenes and Aromaticity Reduction of arenes by catalytic hydrogenation was described in Section 11. 4. a dif- ferent method using Group I metals as reducing agents, which gives 1, 4-cyclohexadiene derivatives, will be presented in Section 11.11. Electrophilic aromatic substitution is the most important reaction type exhibited by benzene and its derivatives and constitutes stituent and affects the reactivity of a functional unit to which it is attached vn 2. The second family of reactions are those in which the aryl group acts as a sub A carbon atom that is directly attached to a benzene ring is called a benzylic car- analogous to the allylic carbon of C=C-C). A phenyl group(C6Hs-)is an even better conjugating substituent than a vinyl group(CH2-CH-), and benzylic carboca- tions and radicals are more highly stabilized than their allylic counterparts. The double bond of an alkenylbenzene is stabilized to about the same extent as that of a conjugated Benzylic carbocation Benzylic radical Alkenylbenzene Reactions involving benzylic cations, benzylic radicals, and alkenylbenzenes will be dis- cussed in Sections 11 12 through 11.17 11.11 THE BIRCH REDUCTION We saw in Section 9 10 that the combination of a Group I metal and liquid ammonia is a powerful reducing system capable of reducing alkynes to trans alkenes In the pres- ence of an alcohol, this same combination reduces arenes to nonconjugated dienes. Thus treatment of benzene with sodium and methanol or ethanol in liquid ammonia converts it to 1, 4-cyclohexadiene. 1, 4-Cyclohexadiene(80%) Metal-ammonia-alcohol reductions of aromatic rings are known as birch reductions, after the Australian chemist Arthur J. Birch, who demonstrated their usefulness begin ning in the 1940s The mechanism by which the Birch reduction of benzene takes place is analogous to the mechanism for the metal-ammonia reduction of alkynes(Figure 11.8). It involves a sequence of four steps in which steps I and 3 are single-electron transfers from the The Birch reduction not only provides a method to prepare dienes from arenes, be accomplished by catalytic hy gated diene system rather than the more stable conjugated one Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

Reduction of arenes by catalytic hydrogenation was described in Section 11.4. A dif￾ferent method using Group I metals as reducing agents, which gives 1,4-cyclohexadiene derivatives, will be presented in Section 11.11. Electrophilic aromatic substitution is the most important reaction type exhibited by benzene and its derivatives and constitutes the entire subject matter of Chapter 12. 2. The second family of reactions are those in which the aryl group acts as a sub￾stituent and affects the reactivity of a functional unit to which it is attached. A carbon atom that is directly attached to a benzene ring is called a benzylic car￾bon (analogous to the allylic carbon of CœC±C). A phenyl group (C6H5±) is an even better conjugating substituent than a vinyl group (CH2œCH±), and benzylic carboca￾tions and radicals are more highly stabilized than their allylic counterparts. The double bond of an alkenylbenzene is stabilized to about the same extent as that of a conjugated diene. Reactions involving benzylic cations, benzylic radicals, and alkenylbenzenes will be dis￾cussed in Sections 11.12 through 11.17. 11.11 THE BIRCH REDUCTION We saw in Section 9.10 that the combination of a Group I metal and liquid ammonia is a powerful reducing system capable of reducing alkynes to trans alkenes. In the pres￾ence of an alcohol, this same combination reduces arenes to nonconjugated dienes. Thus, treatment of benzene with sodium and methanol or ethanol in liquid ammonia converts it to 1,4-cyclohexadiene. Metal–ammonia–alcohol reductions of aromatic rings are known as Birch reductions, after the Australian chemist Arthur J. Birch, who demonstrated their usefulness begin￾ning in the 1940s. The mechanism by which the Birch reduction of benzene takes place is analogous to the mechanism for the metal–ammonia reduction of alkynes (Figure 11.8). It involves a sequence of four steps in which steps 1 and 3 are single-electron transfers from the metal and steps 2 and 4 are proton transfers from the alcohol. The Birch reduction not only provides a method to prepare dienes from arenes, which cannot be accomplished by catalytic hydrogenation, but also gives a nonconju￾gated diene system rather than the more stable conjugated one. Benzene Na, NH3 CH3OH H H H H 1,4-Cyclohexadiene (80%) C Benzylic carbocation C Benzylic radical C C Alkenylbenzene 412 CHAPTER ELEVEN Arenes and Aromaticity Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website