有机化学（英文原版）：Chapt11

398 CHAPTER 11 ARENES AND AROMATICITY I n this chapter and the next we extend our coverage of conjugated systems to include arenes. Arenes are hydrocarbons based on the benzene ring as a structural unit. Ben- zene, toluene, and naphthalene, for example, are arenes. One factor that makes conjugation in arenes special is its cyclic nature. A conju- gated system that closes upon itself can have properties that are much different from those of open-chain polyenes. Arenes are also referred to as aromatic hydrocarbons. Used in this sense, the word “aromatic” has nothing to do with odor but means instead that arenes are much more stable than we expect them to be based on their formulation as conjugated trienes. Our goal in this chapter is to develop an appreciation for the con- cept of aromaticity—to see what are the properties of benzene and its derivatives that reflect its special stability, and to explore the reasons for it. This chapter develops the idea of the benzene ring as a fundamental structural unit and examines the effect of a benzene ring as a substituent. The chapter following this one describes reactions that involve the ring itself. H HH H H H Benzene H HH H H CH 3 Toluene H H H H H H H H Naphthalene Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website Let’s begin by tracing the history of benzene, its origin, and its structure. Many of the terms we use, including aromaticity itself, are of historical origin. We’ll begin with the discovery of benzene. 11.1 BENZENE In 1825, Michael Faraday isolated a new hydrocarbon from illuminating gas, which he called “bicarburet of hydrogen.” Nine years later Eilhardt Mitscherlich of the University of Berlin prepared the same substance by heating benzoic acid with lime and found it to be a hydrocarbon having the empirical formula C n H n . Eventually, because of its relationship to benzoic acid, this hydrocarbon came to be named benzin, then later benzene, the name by which it is known today. Benzoic acid had been known for several hundred years by the time of Mitscher- lich’s experiment. Many trees exude resinous materials called balsams when cuts are made in their bark. Some of these balsams are very fragrant, which once made them highly prized articles of commerce, especially when the trees that produced them could be found only in exotic, faraway lands. Gum benzoin is a balsam obtained from a tree that grows in Java and Sumatra. “Benzoin” is a word derived from the French equiva- lent, benjoin, which in turn comes from the Arabic luban jawi, meaning “incense from Java.” Benzoic acid is itself odorless but can easily be isolated from gum benzoin. Compounds related to benzene were obtained from similar plant extracts. For example, a pleasant-smelling resin known as tolu balsam was obtained from the South American tolu tree. In the 1840s it was discovered that distillation of tolu balsam gave a methyl derivative of benzene, which, not surprisingly, came to be named toluene. Although benzene and toluene are not particularly fragrant compounds themselves, their origins in aromatic plant extracts led them and compounds related to them to be classified as aromatic hydrocarbons. Alkanes, alkenes, and alkynes belong to another class, the aliphatic hydrocarbons. The word “aliphatic” comes from the Greek aleiphar (meaning “oil” or “unguent”) and was given to hydrocarbons that were obtained by the chemical degradation of fats. Benzene was prepared from coal tar by August W. von Hofmann in 1845. Coal tar remained the primary source for the industrial production of benzene for many years, until petroleum-based technologies became competitive about 1950. Current production is about 6 million tons per year in the United States. A substantial portion of this ben- zene is converted to styrene for use in the preparation of polystyrene plastics and films. Toluene is also an important organic chemical. Like benzene, its early industrial production was from coal tar, but most of it now comes from petroleum. 11.2 KEKULé AND THE STRUCTURE OF BENZENE The classification of hydrocarbons as aliphatic or aromatic took place in the 1860s when it was already apparent that there was something special about benzene, toluene, and their derivatives. Their molecular formulas (benzene is C 6 H 6 , toluene is C 7 H 8 ) indicate that, like alkenes and alkynes, they are unsaturated and should undergo addition reac- tions. Under conditions in which bromine, for example, reacts rapidly with alkenes and H11001C 6 H 5 CO 2 H Benzoic acid C 6 H 6 Benzene CaO Calcium oxide H11001 CaCO 3 Calcium carbonate heat 11.1 Benzene 399 Faraday is better known in chemistry for his laws of electrolysis and in physics for proposing the relationship between electric and mag- netic fields and for demon- strating the principle of electromagnetic induction. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website alkynes, however, benzene proved to be inert. Benzene does react with Br 2 in the pres- ence of iron(III) bromide as a catalyst, but even then addition isn’t observed. Substitu- tion occurs instead! Furthermore, only one monobromination product of benzene was ever obtained, which suggests that all the hydrogen atoms of benzene are equivalent. Substitution of one hydrogen by bromine gives the same product as substitution of any of the other hydrogens. Chemists came to regard the six carbon atoms of benzene as a fundamental struc- tural unit. Reactions could be carried out that altered its substituents, but the integrity of the benzene unit remained undisturbed. There must be something “special” about ben- zene that makes it inert to many of the reagents that add to alkenes and alkynes. In 1866, only a few years after publishing his ideas concerning what we now rec- ognize as the structural theory of organic chemistry, August Kekulé applied it to the structure of benzene. He based his reasoning on three premises: 1. Benzene is C 6 H 6 . 2. All the hydrogens of benzene are equivalent. 3. The structural theory requires that there be four bonds to each carbon. Kekulé advanced the venturesome notion that the six carbon atoms of benzene were joined together in a ring. Four bonds to each carbon could be accommodated by a sys- tem of alternating single and double bonds with one hydrogen on each carbon. A flaw in Kekulé’s structure for benzene was soon discovered. Kekulé’s structure requires that 1,2- and 1,6-disubstitution patterns create different compounds (isomers). The two substituted carbons are connected by a double bond in one but by a single bond in the other. Since no such cases of isomerism in benzene derivatives were known, and X X 1 4 2 3 6 5 1,2-Disubstituted derivative of benzene X X 1 4 2 3 6 5 1,6-Disubstituted derivative of benzene C HC C C H C C H H 1 2 3 4 6 5 H H H11001 C 6 H 6 Benzene Br 2 Bromine H11001 CCl 4 FeBr 3 no observable reaction C 6 H 5 Br Bromobenzene HBr Hydrogen bromide 400 CHAPTER ELEVEN Arenes and Aromaticity In 1861, Johann Josef Loschmidt, who was later to become a professor at the University of Vienna, pri- vately published a book con- taining a structural formula for benzene similar to that which Kekulé would propose five years later. Loschmidt’s book reached few readers, and his ideas were not well known. How many isomers of C 6 H 6 can you write? An article in the March 1994 issue of the Journal of Chemical Educa- tion (pp. 222–224) claims that there are several hun- dred and draws structural formulas for 25 of them. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website none could be found, Kekulé suggested that two isomeric structures could exist but inter- converted too rapidly to be separated. Kekulé’s ideas about the structure of benzene left an important question unan- swered. What is it about benzene that makes it behave so much differently from other unsaturated compounds? We’ll see in this chapter that the answer is a simple one—the low reactivity of benzene and its derivatives reflects their special stability. Kekulé was wrong. Benzene is not cyclohexatriene, nor is it a pair of rapidly equilibrating cyclo- hexatriene isomers. But there was no way that Kekulé could have gotten it right given the state of chemical knowledge at the time. After all, the electron hadn’t even been dis- covered yet. It remained for twentieth-century electronic theories of bonding to provide insight into why benzene is so stable. We’ll outline these theories shortly. First, how- ever, let’s look at the structure of benzene in more detail. X X X X fast 11.2 Kekulé and the Structure of Benzene 401 BENZENE, DREAMS, AND CREATIVE THINKING A t ceremonies in Berlin in 1890 celebrating the twenty-fifth anniversary of his proposed struc- ture of benzene, August Kekulé recalled the thinking that led him to it. He began by noting that the idea of the structural theory came to him during a daydream while on a bus in London. Kekulé went on to describe the origins of his view of the benzene structure. There I sat and wrote for my textbook; but things did not go well; my mind was occupied with other matters. I turned the chair towards the fireplace and began to doze. Once again the atoms danced before my eyes. This time smaller groups modestly remained in the background. My mental eye, sharpened by repeated appari- tions of similar kind, now distinguished larger units of various shapes. Long rows, frequently joined more densely; everything in motion, twisting and turning like snakes. And behold, what was that? One of the snakes caught hold of its own tail and mockingly whirled round be- fore my eyes. I awoke, as if by lightning; this time, too, I spent the rest of the night working out the consequences of this hypothesis. * Concluding his remarks, Kekulé merged his advocacy of creative imagination with the rigorous standards of science by reminding his audience: Let us learn to dream, then perhaps we shall find the truth. But let us beware of publishing our dreams before they have been put to the proof by the waking understanding. The imagery of a whirling circle of snakes evokes a vivid picture that engages one’s attention when first exposed to Kekulé’s model of the benzene structure. Recently, however, the opinion has been expressed that Kekulé might have engaged in some hyperbole during his speech. Professor John Wotiz of Southern Illinois University suggests that discoveries in science are the result of a disciplined analysis of a sufficient body of experimental observations to progress to a higher level of understanding. Wotiz’ view that Kekulé’s account is more fanciful than accurate has sparked a controversy with ramifications that go be- yond the history of organic chemistry. How does cre- ative thought originate? What can we do to become more creative? Because these are questions that have concerned psychologists for decades, the idea of a sleepy Kekulé being more creative than an alert Kekulé becomes more than simply a charming story he once told about himself. * The Kekulé quotes are taken from the biographical article of K. Hafner published in Angew. Chem. Internat. ed. Engl. 18, 641–651 (1979). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website Benzene is planar and its carbon skeleton has the shape of a regular hexagon. There is no evidence that it has alternating single and double bonds. As shown in Figure 11.1, all the carbon–carbon bonds are the same length (140 pm) and the 120° bond angles cor- respond to perfect sp 2 hybridization. Interestingly, the 140-pm bond distances in benzene are exactly midway between the typical sp 2 –sp 2 single-bond distance of 146 pm and the sp 2 –sp 2 double-bond distance of 134 pm. If bond distances are related to bond type, what kind of carbon–carbon bond is it that lies halfway between a single bond and a double bond in length? 11.3 A RESONANCE PICTURE OF BONDING IN BENZENE Twentieth-century theories of bonding in benzene provide a rather clear picture of aro- maticity. We’ll start with a resonance description of benzene. The two Kekulé structures for benzene have the same arrangement of atoms, but differ in the placement of electrons. Thus they are resonance forms, and neither one by itself correctly describes the bonding in the actual molecule. As a hybrid of the two Kekulé structures, benzene is often represented by a hexagon containing an inscribed circle. The circle-in-a-hexagon symbol was first suggested by the British chemist Sir Robert Robinson to represent what he called the “aromatic sextet”—the six delocalized H9266 electrons of the three double bonds. Robinson’s symbol is a convenient time-saving shorthand device, but Kekulé-type formulas are better for counting and keeping track of electrons, especially in chemical reactions. PROBLEM 11.1 Write structural formulas for toluene (C 6 H 5 CH 3 ) and for benzoic acid (C 6 H 5 CO 2 H) (a) as resonance hybrids of two Kekulé forms and (b) with the Robinson symbol. is equivalent to 402 CHAPTER ELEVEN Arenes and Aromaticity 120H11034 120H11034 120H11034 140 pm 108 pm FIGURE 11.1 Bond distances and bond angles of benzene. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website Since the carbons that are singly bonded in one resonance form are doubly bonded in the other, the resonance description is consistent with the observed carbon–carbon bond distances in benzene. These distances not only are all identical but also are inter- mediate between typical single-bond and double-bond lengths. We have come to associate electron delocalization with increased stability. On that basis alone, benzene ought to be stabilized. It differs from other conjugated systems that we have seen, however, in that its H9266 electrons are delocalized over a cyclic conjugated system. Both Kekulé structures of benzene are of equal energy, and one of the princi- ples of resonance theory is that stabilization is greatest when the contributing structures are of similar energy. Cyclic conjugation in benzene, then, leads to a greater stabiliza- tion than is observed in noncyclic conjugated trienes. How much greater that stabiliza- tion is can be estimated from heats of hydrogenation. 11.4 THE STABILITY OF BENZENE Hydrogenation of benzene and other arenes is more difficult than hydrogenation of alkenes and alkynes. Two of the more active catalysts are rhodium and platinum, and it is possible to hydrogenate arenes in the presence of these catalysts at room temperature and modest pressure. Benzene consumes three molar equivalents of hydrogen to give cyclohexane. Nickel catalysts, although less expensive than rhodium and platinum, are also less active. Hydrogenation of arenes in the presence of nickel requires high temperatures (100–200°C) and pressures (100 atm). The measured heat of hydrogenation of benzene to cyclohexane is, of course, the same regardless of the catalyst and is 208 kJ/mol (49.8 kcal/mol). To put this value into perspective, compare it with the heats of hydrogenation of cyclohexene and 1,3-cyclo- hexadiene, as shown in Figure 11.2. The most striking feature of Figure 11.2 is that the heat of hydrogenation of benzene, with three “double bonds,” is less than the heat of hydrogenation of the two double bonds of 1,3-cyclohexadiene. Our experience has been that some 125 kJ/mol (30 kcal/mol) is given off when- ever a double bond is hydrogenated. When benzene combines with three molecules of hydrogen, the reaction is far less exothermic than we would expect it to be on the basis of a 1,3,5-cyclohexatriene structure for benzene. How much less? Since 1,3,5-cyclohexatriene does not exist (if it did, it would instantly relax to benzene), we cannot measure its heat of hydrogenation in order to com- pare it with benzene. We can approximate the heat of hydrogenation of 1,3,5-cyclo- hexatriene as being equal to three times the heat of hydrogenation of cyclohexene, or a total of 360 kJ/mol (85.8 kcal/mol). The heat of hydrogenation of benzene is 152 kJ/mol (36 kcal/mol) less than expected for a hypothetical 1,3,5-cyclohexatriene with noninter- acting double bonds. This is the resonance energy of benzene. It is a measure of how much more stable benzene is than would be predicted on the basis of its formulation as a pair of rapidly interconverting 1,3,5-cyclohexatrienes. Benzene H11001 3H 2 Hydrogen (2–3 atm pressure) Cyclohexane (100%) Pt acetic acid 30°C 11.4 The Stability of Benzene 403 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website We reach a similar conclusion when comparing benzene with the open-chain con- jugated triene (Z)-1,3,5-hexatriene. Here we compare two real molecules, both conju- gated trienes, but one is cyclic and the other is not. The heat of hydrogenation of (Z)- 1,3,5-hexatriene is 337 kJ/mol (80.5 kcal/mol), a value which is 129 kJ/mol (30.7 kcal/mol) greater than that of benzene. The precise value of the resonance energy of benzene depends, as comparisons with 1,3,5-cyclohexatriene and (Z)-1,3,5-hexatriene illustrate, on the compound chosen as the reference. What is important is that the resonance energy of benzene is quite large, six to ten times that of a conjugated triene. It is this very large increment of resonance energy that places benzene and related compounds in a separate category that we call aromatic. PROBLEM 11.2 The heats of hydrogenation of cycloheptene and 1,3,5-cyclo- heptatriene are 110 kJ/mol (26.3 kcal/mol) and 305 kJ/mol (73.0 kcal/mol), respec- tively. In both cases cycloheptane is the product. What is the resonance energy of 1,3,5-cycloheptatriene? How does it compare with the resonance energy of ben- zene? H11001H9004H° H11005 H11002337 kJ (H1100280.5 kcal) H H H H H H H H (Z)-1,3,5-Hexatriene 3H 2 Hydrogen CH 3 (CH 2 ) 4 CH 3 Hexane 404 CHAPTER ELEVEN Arenes and Aromaticity Energy 2H 2 H11001 H 2 H11001 120 231 208 152 3H 2 A real molecule, benzene An imaginary molecule, cyclohexatriene 3 H11003 120 H11005 360 3H 2 H11001 H11001 FIGURE 11.2 Heats of hydro- genation of cyclohexene, 1,3-cyclohexadiene, a hypo- thetical 1,3,5-cyclohexatriene, and benzene. All heats of hy- drogenation are in kilojoules per mole. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 11.5 AN ORBITAL HYBRIDIZATION VIEW OF BONDING IN BENZENE The structural facts that benzene is planar, all of the bond angles are 120°, and each car- bon is bonded to three other atoms, suggest sp 2 hybridization for carbon and the frame- work of H9268 bonds shown in Figure 11.3a. In addition to its three sp 2 hybrid orbitals, each carbon has a half-filled 2p orbital that can participate in H9266 bonding. Figure 11.3b shows the continuous H9266 system that encompasses all of the carbons that result from overlap of these 2p orbitals. The six H9266 electrons of benzene are delocalized over all six carbons. The electrostatic potential map of benzene (Figure 11.3c) shows regions of high electron density above and below the plane of the ring, which is where we expect the most loosely held electrons (the H9266 electrons) to be. 11.6 THE H9266 MOLECULAR ORBITALS OF BENZENE The picture of benzene as a planar framework of H9268 bonds with six electrons in a delo- calized H9266 orbital is a useful, but superficial, one. Six electrons cannot simultaneously occupy any one orbital, be it an atomic orbital or a molecular orbital. A more rigorous molecular orbital analysis recognizes that overlap of the six 2p atomic orbitals of the ring carbons generates six H9266 molecular orbitals. These six H9266 molecular orbitals include three which are bonding and three which are antibonding. The relative energies of these orbitals and the distribution of the H9266 electrons among them are illustrated in Figure 11.4. Benzene is said to have a closed-shell H9266 electron configuration. All the bonding orbitals are filled, and there are no electrons in antibonding orbitals. 11.6 The H9266 Molecular Orbitals of Benzene 405 (a)(b)(c) Antibonding orbitals Energy Bonding orbitals π 4 π 2 π 1 π 3 π 5 π 6 FIGURE 11.3 (a) The framework of bonds shown in the tube model of benzene are H9268 bonds. (b) Each carbon is sp 2 - hybridized and has a 2p orbital perpendicular to the H9268 framework. Overlap of the 2p orbitals generates a H9266 system encompassing the entire ring. (c) Electrostatic potential plot of benzene. The red area in the center corresponds to the region above and below the plane of the ring where the H9266 electrons are concentrated. FIGURE 11.4 The H9266 molecular orbitals of ben- zene arranged in order of in- creasing energy. The six H9266 electrons of benzene occupy the three lowest energy or- bitals, all of which are bond- ing. The nodal properties of these orbitals may be viewed on Learning By Modeling. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website Higher level molecular orbital theory can provide quantitative information about orbital energies and how strongly a molecule holds its electrons. When one compares aromatic and nonaromatic species in this way, it is found that cyclic delocalization causes the H9266 electrons of benzene to be more strongly bound (more stable) than they would be if restricted to a system with alternating single and double bonds. We’ll come back to the molecular orbital description of benzene later in this chap- ter (Section 11.19) to see how other conjugated polyenes compare with benzene. 11.7 SUBSTITUTED DERIVATIVES OF BENZENE AND THEIR NOMENCLATURE All compounds that contain a benzene ring are aromatic, and substituted derivatives of benzene make up the largest class of aromatic compounds. Many such compounds are named by attaching the name of the substituent as a prefix to benzene. Many simple monosubstituted derivatives of benzene have common names of long stand- ing that have been retained in the IUPAC system. Table 11.1 lists some of the most important ones. Dimethyl derivatives of benzene are called xylenes. There are three xylene isomers, the ortho (o)-, meta (m)-, and para ( p)- substituted derivatives. The prefix ortho signifies a 1,2-disubstituted benzene ring, meta signifies 1,3-disubstitu- tion, and para signifies 1,4-disubstitution. The prefixes o, m, and p can be used when a substance is named as a benzene derivative or when a specific base name (such as ace- tophenone) is used. For example, Cl Cl o-Dichlorobenzene (1,2-dichlorobenzene) NO 2 CH 3 m-Nitrotoluene (3-nitrotoluene) C F CH 3 O p-Fluoroacetophenone (4-fluoroacetophenone) CH 3 CH 3 o-Xylene (1,2-dimethylbenzene) CH 3 CH 3 m-Xylene (1,3-dimethylbenzene) CH 3 CH 3 p-Xylene (1,4-dimethylbenzene) Br Bromobenzene C(CH 3 ) 3 tert-Butylbenzene NO 2 Nitrobenzene 406 CHAPTER ELEVEN Arenes and Aromaticity Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website PROBLEM 11.3 Write a structural formula for each of the following compounds: (a) o-Ethylanisole (c) p-Nitroaniline (b) m-Chlorostyrene SAMPLE SOLUTION (a) The parent compound in o-ethylanisole is anisole. Anisole, as shown in Table 11.1, has a methoxy (CH 3 O±) substituent on the ben- zene ring. The ethyl group in o-ethylanisole is attached to the carbon adjacent to the one that bears the methoxy substituent. OCH 3 CH 2 CH 3 o-Ethylanisole 11.7 Substituted Derivatives of Benzene and Their Nomenclature 407 TABLE 11.1 Names of Some Frequently Encountered Derivatives of Benzene *These common names are acceptable in IUPAC nomenclature and are the names that will be used in this text. Benzenecarbaldehyde Systematic Name Benzenecarboxylic acid Vinylbenzene Methyl phenyl ketone Benzenol Methoxybenzene Benzenamine Benzaldehyde Common Name* Benzoic acid Styrene Acetophenone Phenol Anisole Aniline Structure ±CH O X ±COH O X ±CCH 3 O X ±CH?CH 2 ±OH ±OCH 3 ±NH 2 Back Forward Main Menu TOC Study Guide TOC Student OLC 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 C 6 H 5 ±. Similarly, an arene named as a substituent is called an aryl group. A benzyl group is C 6 H 5 CH 2 ±. 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 CH 2 CH 2 OH 2-Phenylethanol CH 2 Br Benzyl bromide 1 4 2 3 6 5 Cl NO 2 NO 2 1-Chloro-2,4-dinitrobenzene 4 1 3 2 5 6 CH 2 CH 3 F NO 2 4-Ethyl-1-fluoro-2-nitrobenzene 3 6 2 1 4 5 CH 3 CH 2 F OCH 3 4-Ethyl-2-fluoroanisole 1 4 2 3 6 5 CH 3 NO 2 O 2 N NO 2 2,4,6-Trinitrotoluene 1 4 2 3 6 5 NH 2 CH 3 CH 2 CH 3 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: 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 65 910 1 2 34 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 410 CHAPTER ELEVEN Arenes and Aromaticity CARBON CLUSTERS, FULLERENES, AND NANOTUBES T he 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 ± C P CH 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 C 60 was present in amounts much greater than any other. On speculating about what C 60 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 C 60 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 sp 2 -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 C 60 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 Kr?tschmer 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 C 60 in quantity to study its properties. Speculation about the stability of C 60 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 (C 60 ). 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 411 planarity and the accompanying angle strain. It is now clear that C 60 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 sp 2 -hybridized carbons to sp 3 -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 C 60 , 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 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 C?C±C). A phenyl group (C 6 H 5 ±) is an even better conjugating substituent than a vinyl group (CH 2 ?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 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, NH 3 CH 3 OH HH HH 1,4-Cyclohexadiene (80%) C H11001 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 11.11 The Birch Reduction 413 H HH H H H H HH H H H H H The overall reaction: Benzene NH 3 Step 1: An electron is transferred from sodium (the reducing agent) to the π system of the aromatic ring. The product is an anion radical. Step 2: The anion radical is a strong base and abstracts a proton from methanol. H11001 2Na Sodium H11001 2CH 3 OH Methanol 1,4-Cyclohexadiene Sodium methoxide H11001 2NaOCH 3 The mechanism: Benzene Sodium Benzene anion radical Sodium ion H11002 H HH H H H Benzene anion radical H11001 Methanol H11002 H±OCH 3 H HH H H H H Cyclohexadienyl radical Methoxide ion H11001 H11002 OCH 3 H11002 Step 3: The cyclohexadienyl radical produced in step 2 is converted to an anion by electron transfer from sodium. Cyclohexadienyl radical H11001 Na Sodium Cyclohexadienyl anion H11001 Na H11001 Sodium ion Step 4: Proton transfer from methanol to the anion gives 1,4-cyclohexadiene. H11002 H HH H H H H Cyclohexadienyl anion H11001 H±OCH 3 H HH H H H H H 1,4-Cyclohexadiene H11001 OCH 3 H11002 Methoxide ion H HH H H H H HH H H H H HH H H H H H HH H H H H H11001H11001 Na Na H11001 FIGURE 11.8 Mechanism of the Birch reduction. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website Alkyl-substituted arenes give 1,4-cyclohexadienes in which the alkyl group is a substituent on the double bond. PROBLEM 11.5 A single organic product was isolated after Birch reduction of p-xylene. Suggest a reasonable structure for this substance. Substituents other than alkyl groups may also be present on the aromatic ring, but their reduction is beyond the scope of the present discussion. 11.12 FREE-RADICAL HALOGENATION OF ALKYLBENZENES The benzylic position in alkylbenzenes is analogous to the allylic position in alkenes. Thus a benzylic C±H bond, like an allylic one, is weaker than a C±H bond of an alkane, as the bond dissociation energies of toluene, propene, and 2-methylpropane attest: We attributed the decreased bond dissociation energy in propene to stabilization of allyl radical by electron delocalization. Similarly, electron delocalization stabilizes benzyl rad- ical and weakens the benzylic C±H bond. The unpaired electron is shared by the ben- zylic carbon and by the ring carbons that are ortho and para to it. In orbital terms, as represented in Figure 11.9, benzyl radical is stabilized by delo- calization of electrons throughout the extended H9266 system formed by overlap of the p orbital of the benzylic carbon with the H9266 system of the ring. The comparative ease with which a benzylic hydrogen is abstracted leads to high selectivity in free-radical halogenations of alkylbenzenes. Thus, chlorination of toluene CH 2 H Toluene CH 2 Benzyl radical H11001 H H9004H° H11005 356 kJ (85 kcal) H Propene CH 2 HCHCH 2 CH 2 CHCH 2 Allyl radical H11001H9004H° H11005 368 kJ (88 kcal) H(CH 3 ) 3 C H 2-Methylpropane (CH 3 ) 3 C tert-Butyl radical H11001H9004H° H11005 397 kJ (95 kcal) rather than Na, NH 3 CH 3 CH 2 OH C(CH 3 ) 3 tert-Butylbenzene C(CH 3 ) 3 1-tert-Butyl-1,4- cyclohexadiene (86%) C(CH 3 ) 3 3-tert-Butyl-1,4- cyclohexadiene 414 CHAPTER ELEVEN Arenes and Aromaticity H HH H H CH 2 Most stable Lewis structure of benzyl radical H HH H H CH 2 H HH H H CH 2 H HH H H CH 2 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website takes place exclusively at the benzylic carbon and is an industrial process for the prepa- ration of the compounds shown. The propagation steps in the formation of benzyl chloride involve benzyl radical as an intermediate. (Dichloromethyl)benzene and (trichloromethyl)benzene arise by further side-chain chlo- rination of benzyl chloride. Benzylic bromination is a more commonly used laboratory procedure than chlori- nation and is typically carried out under conditions of photochemical initiation. As we saw when discussing allylic bromination in Section 10.4, N-bromosuccin- imide (NBS) is a convenient free-radical brominating agent. Benzylic brominations with NBS are normally performed in carbon tetrachloride as the solvent in the presence of peroxides, which are added as initiators. As the example illustrates, free-radical bromi- nation is selective for substitution of benzylic hydrogens. H11001 NO 2 CH 3 p-Nitrotoluene Br 2 Bromine CCl 4 , 80°C light NO 2 CH 2 Br p-Nitrobenzyl bromide (71%) H11001 HBr Hydrogen bromide H11001CH 3 Toluene H11001 Cl Chlorine atom CH 2 Benzyl radical HCl Hydrogen chloride H11001 Cl Chlorine atom CH 2 Benzyl radical Cl 2 Chlorine CH 2 Cl Benzyl chloride H11001 CH 3 Toluene Cl 2 light or heat Cl 2 light or heat Cl 2 light or heat CH 2 Cl Benzyl chloride CHCl 2 (Dichloromethyl)- benzene CCl 3 (Trichloromethyl)- benzene 11.12 Free-Radical Halogenation of Alkylbenzenes 415 FIGURE 11.9 The benzyl rad- ical is stabilized by overlap of its half-filled p orbital with the H9266 system of the aromatic ring. The common names of (di- chloromethyl)benzene and (trichloromethyl)benzene are benzal chloride and benzo- trichloride, respectively. Benzoyl peroxide is a com- monly used free-radical ini- tiator. It has the formula C 6 H 5 COOCC 6 H 5 O X O X Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website PROBLEM 11.6 The reaction of N-bromosuccinimide with the following com- pounds has been reported in the chemical literature. Each compound yields a sin- gle product in 95% yield. Identify the product formed from each starting mate- rial. (a) p-tert-Butyltoluene (b) 4-Methyl-3-nitroanisole SAMPLE SOLUTION (a) The only benzylic hydrogens in p-tert-butyltoluene are those of the methyl group that is attached directly to the ring. Substitution occurs there to give p-tert-butylbenzyl bromide. 11.13 OXIDATION OF ALKYLBENZENES A striking example of the activating effect that a benzene ring has on reactions that take place at benzylic positions may be found in the reactions of alkylbenzenes with oxidiz- ing agents. Chromic acid, for example, prepared by adding sulfuric acid to aqueous sodium dichromate, is a strong oxidizing agent but does not react either with benzene or with alkanes. On the other hand, an alkyl side chain on a benzene ring is oxidized on being heated with chromic acid. The product is benzoic acid or a substituted derivative of benzoic acid. Na 2 Cr 2 O 7 H 2 O, H 2 SO 4 , heat orCH 2 R CHR 2 Alkylbenzene O COH Benzoic acid Na 2 Cr 2 O 7 H 2 O, H 2 SO 4 CH 3 O 2 N p-Nitrotoluene O COHO 2 N p-Nitrobenzoic acid (82–86%) Na 2 Cr 2 O 7 H 2 O, H 2 SO 4 , heat RCH 2 CH 2 RH11032 no reaction Na 2 Cr 2 O 7 H 2 O, H 2 SO 4 , heat no reaction (CH 3 ) 3 C CH 3 p-tert-Butyltoluene NBS CCl 4 , 80°C free-radical initiator CH 2 Br(CH 3 ) 3 C p-tert-Butylbenzyl bromide 416 CHAPTER ELEVEN Arenes and Aromaticity CH 2 CH 3 Ethylbenzene H11001 NBr O O N-Bromosuccinimide (NBS) NH O O Succinimide benzoyl peroxide CCl 4 , 80°C CHCH 3 Br 1-Bromo-1-phenylethane (87%) H11001 An alternative oxidizing agent, similar to chromic acid in its reactions with or- ganic compounds, is potas- sium permanganate (KMnO 4 ). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website When two alkyl groups are present on the ring, both are oxidized. Note that alkyl groups, regardless of their chain length, are converted to carboxyl groups (±CO 2 H) attached directly to the ring. An exception is a tert-alkyl substituent. Because it lacks benzylic hydrogens, a tert-alkyl group is not susceptible to oxidation under these conditions. PROBLEM 11.7 Chromic acid oxidation of 4-tert-butyl-1,2-dimethylbenzene yielded a single compound having the molecular formula C 12 H 14 O 4 . What was this compound? Side-chain oxidation of alkylbenzenes is important in certain metabolic processes. One way in which the body rids itself of foreign substances is by oxidation in the liver to compounds more easily excreted in the urine. Toluene, for example, is oxidized to benzoic acid by this process and is eliminated rather readily. Benzene, with no alkyl side chain, undergoes a different reaction in the presence of these enzymes, which convert it to a substance capable of inducing mutations in DNA. This difference in chemical behavior seems to be responsible for the fact that benzene is car- cinogenic but toluene is not. 11.14 NUCLEOPHILIC SUBSTITUTION IN BENZYLIC HALIDES Primary benzylic halides are ideal substrates for S N 2 reactions, since they are very reac- tive toward good nucleophiles and cannot undergo competing elimination. Benzylic halides that are secondary resemble secondary alkyl halides in that they undergo substitution only when the nucleophile is weakly basic. If the nucleophile is a strong base such as sodium ethoxide, elimination by the E2 mechanism is faster than substitu- tion. PROBLEM 11.8 Give the structure of the principal organic product formed on reaction of benzyl bromide with each of the following reagents: (a) Sodium ethoxide (d) Sodium hydrogen sulfide (b) Potassium tert-butoxide (e) Sodium iodide (in acetone) (c) Sodium azide CH 3 CO 2 H11002 Na H11001 acetic acid CH 2 ClO 2 N p-Nitrobenzyl chloride CH 2 OCCH 3 O O 2 N p-Nitrobenzyl acetate (78–82%) CH 3 Toluene COH O Benzoic acid O 2 cytochrome P-450 (an enzyme in the liver) CH(CH 3 ) 2 CH 3 p-Isopropyltoluene COH O HOC O p-Benzenedicarboxylic acid (45%) Na 2 Cr 2 O 7 H 2 O, H 2 SO 4 , heat 11.14 Nucleophilic Substitution in Benzylic Halides 417 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website SAMPLE SOLUTION (a) Benzyl bromide is a primary bromide and undergoes S N 2 reactions readily. It has no hydrogens H9252 to the leaving group and so cannot undergo elimination. Ethoxide ion acts as a nucleophile, displacing bromide and forming benzyl ethyl ether. Benzylic halides resemble allylic halides in the readiness with which they form carbocations. On comparing the rate of S N 1 hydrolysis in aqueous acetone of the fol- lowing two tertiary chlorides, we find that the benzylic chloride reacts over 600 times faster than does tert-butyl chloride. Just as the odd electron in benzyl radical is shared by the carbons ortho and para to the benzylic carbon, the positive charge in benzyl cation is shared by these same positions. Unlike the case with allylic carbocations, however, dispersal of the positive charge does not result in nucleophilic attack at more than one carbon. There is no “benzylic rearrangement” analogous to allylic rearrangement (Section 10.2), because the aromatic stabilization would be lost if the nucleophile became bonded to one of the ring carbons. Thus, when conditions are chosen that favor S N 1 substitution over E2 elimination (solvolysis, weakly basic nucleophile), benzylic halides give a single substitution prod- uct in high yield. Additional phenyl substituents stabilize carbocations even more. Triphenylmethyl cation is particularly stable. Its perchlorate salt is ionic and stable enough to be isolated and stored indefinitely. 2-Chloro-2-phenylpropane CH 3 CH 3 CCl CH 3 CCl CH 3 CH 3 2-Chloro-2-methylpropane CH 3 CH 2 O H11002 Ethoxide ion CH 2 Br Benzyl bromide CH 2 OCH 2 CH 3 Benzyl ethyl ether 418 CHAPTER ELEVEN Arenes and Aromaticity H HH H H CH 2 Most stable Lewis structure of benzyl cation H HH H H CH 2 H11001 H11001 H11001 H HH H H CH 2 H HH H H CH 2 H11001 2-Chloro-2-phenylpropane CH 3 CH 3 CCl 2-Ethoxy-2-phenylpropane (87%) CH 3 CH 3 COCH 2 CH 3 via CH 3 CH 2 OH C H11001 CH 3 CH 3 See Learning By Model- ing for an electrostatic potential map of benzyl cation. The triphenylmethyl group is often referred to as a trityl group. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 11.15 PREPARATION OF ALKENYLBENZENES Alkenylbenzenes are prepared by the various methods described in Chapter 5 for the preparation of alkenes: dehydrogenation, dehydration, and dehydrohalogenation. Dehydrogenation of alkylbenzenes is not a convenient laboratory method but is used industrially to convert ethylbenzene to styrene. Acid-catalyzed dehydration of benzylic alcohols is a useful route to alkenylben- zenes, as is dehydrohalogenation under E2 conditions. 11.16 ADDITION REACTIONS OF ALKENYLBENZENES Most of the reactions of alkenes that were discussed in Chapter 6 find a parallel in the reactions of alkenylbenzenes. Hydrogenation of the side-chain double bond of an alkenylbenzene is much easier than hydrogenation of the aromatic ring and can be achieved with high selectivity, leav- ing the ring unaffected. 2-(m-Bromophenyl)-2-butene C Br CH 3 CHCH 3 2-(m-Bromophenyl)butane (92%) CHCH 2 CH 3 Br CH 3 H11001 Hydrogen H 2 Pt KHSO 4 heat m-Chlorostyrene (80–82%) Cl CH CH 2 1-(m-Chlorophenyl)ethanol CHCH 3 OH Cl CH 2 CHCH 3 Br H 3 C 2-Bromo-1-(p-methylphenyl)propane CH CHCH 3 H 3 C 1-(p-Methylphenyl)propene (99%) NaOCH 2 CH 3 CH 3 CH 2 OH, 50°C Ethylbenzene CH 2 CH 3 630°C ZnO H11001 H 2 HydrogenStyrene CH CH 2 C H11001 Triphenylmethyl perchlorate [ClO 4 ] H11002 11.16 Addition Reactions of Alkenylbenzenes 419 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website PROBLEM 11.9 Both 1,2-dihydronaphthalene and 1,4-dihydronaphthalene may be selectively hydrogenated to 1,2,3,4-tetrahydronaphthalene. One of these isomers has a heat of hydrogenation of 101 kJ/mol (24.1 kcal/mol), and the heat of hydrogenation of the other is 113 kJ/mol (27.1 kcal/mol). Match the heat of hydrogenation with the appropriate dihydronaphthalene. The double bond in the alkenyl side chain undergoes addition reactions that are typical of alkenes when treated with electrophilic reagents. The regioselectivity of electrophilic addition is governed by the ability of an aro- matic ring to stabilize an adjacent carbocation. This is clearly seen in the addition of hydrogen chloride to indene. Only a single chloride is formed. Only the benzylic chloride is formed, because protonation of the double bond occurs in the direction that gives a carbocation that is both secondary and benzylic. Protonation in the opposite direction also gives a secondary carbocation, but it is not benzylic. This carbocation does not receive the extra increment of stabilization that its benzylic isomer does and so is formed more slowly. The orientation of addition is controlled by H H HCl slower Less stable carbocation H11001 Cl H11002 H HH H H HCl H H11001 H H Cl H11002 Carbocation that leads to observed product Indene H11001 HCl Hydrogen chloride Cl 1-Chloroindane (75–84%) H11001 Br 2 BromineStyrene CH CH 2 CHCH 2 Br Br 1,2-Dibromo-1-phenylethane (82%) H 2 Pt H 2 Pt 1,2-Dihydronaphthalene 1,2,3,4-Tetrahydronaphthalene 1,4-Dihydronaphthalene 420 CHAPTER ELEVEN Arenes and Aromaticity Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website the rate of carbocation formation; the more stable benzylic carbocation is formed faster and is the one that determines the reaction product. PROBLEM 11.10 Each of the following reactions has been reported in the chem- ical literature and gives a single organic product in high yield. Write the structure of the product for each reaction. (a) 2-Phenylpropene H11001 hydrogen chloride (b) 2-Phenylpropene treated with diborane in tetrahydrofuran followed by oxidation with basic hydrogen peroxide (c) Styrene H11001 bromine in aqueous solution (d) Styrene H11001 peroxybenzoic acid (two organic products in this reaction; iden- tify both by writing a balanced equation.) SAMPLE SOLUTION (a) Addition of hydrogen chloride to the double bond takes place by way of a tertiary benzylic carbocation. In the presence of peroxides, hydrogen bromide adds to the double bond of styrene with a regioselectivity opposite to Markovnikov’s rule. The reaction is a free-radical addi- tion, and the regiochemistry is governed by preferential formation of the more stable radical. 11.17 POLYMERIZATION OF STYRENE The annual production of styrene in the United States is on the order of 8 H11003 10 9 lb, with about 65% of this output used to prepare polystyrene plastics and films. Styrofoam cof- fee cups are made from polystyrene. Polystyrene can also be produced in a form that is very strong and impact-resistant and is used widely in luggage, television and radio cab- inets, and furniture. Polymerization of styrene is carried out under free-radical conditions, often with ben- zoyl peroxide as the initiator. Figure 11.10 illustrates a step in the growth of a polystyrene chain by a mechanism analogous to that of the polymerization of ethylene (Section 6.21). C CH 3 CH 3 Cl 2-Chloro-2- phenylpropane C CH 3 CH 3 H11001 Cl H11002 H Cl Hydrogen chloride C CH 2 CH 3 2-Phenylpropene 11.17 Polymerization of Styrene 421 HBr peroxides via Styrene CH CH 2 1-Bromo-2-phenylethane (major product) CH 2 CH 2 Br 2-Bromo-1-phenylethyl radical (secondary; benzylic) CHCH 2 Br As described in the box “Diene Polymers” in Chapter 10, most synthetic rubber is a copolymer of styrene and 1,3-butadiene. Polymer ±CH 2 ±CH H11001 CH 2 ?CHC 6 H 5 ±￡ Polymer C 6 H 5 W ±CH 2 ±CH±CH 2 ±CH C 6 H 5 W C 6 H 5 W C FIGURE 11.10 Chain propagation step in polymerization of styrene. The growing polymer chain has a free-radical site at the benzylic carbon. It adds to a molecule of styrene to extend the chain by one styrene unit. The new polymer chain is also a benzylic radical; it attacks another molecule of styrene, and the process repeats over and over again. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 11.18 CYCLOBUTADIENE AND CYCLOOCTATETRAENE During our discussion of benzene and its derivatives, it may have occurred to you that cyclobutadiene and cyclooctatetraene might be stabilized by H9266 electron delocalization in a manner analogous to that of benzene. The same thought occurred to early chemists. However, the complete absence of natu- rally occurring compounds based on cyclobutadiene and cyclooctatetraene contrasted starkly with the abundance of compounds based on the benzene nucleus. Attempts to synthesize cyclobutadiene and cyclooctatetraene met with failure and reinforced the growing conviction that these compounds would prove to be quite unlike benzene if, in fact, they could be isolated at all. The first breakthrough came in 1911 when Richard Willst?tter prepared cyclooc- tatetraene by a lengthy degradation of pseudopelletierine, a natural product obtained from the bark of the pomegranate tree. Nowadays, cyclooctatetraene is prepared from acety- lene in a reaction catalyzed by nickel cyanide. Thermochemical measurements suggest a value of only about 20 kJ/mol (about 5 kcal/mol) for the resonance energy of cyclooctatetraene, far less than the aromatic sta- bilization of benzene (152 kJ/mol; 36 kcal/mol). PROBLEM 11.11 Both cyclooctatetraene and styrene have the molecular for- mula C 8 H 8 and undergo combustion according to the equation C 8 H 8 H11001 10O 2 ±￡ 8CO 2 H11001 4H 2 O The measured heats of combustion are 4393 and 4543 kJ/mol (1050 and 1086 kcal/mol). Which heat of combustion belongs to which compound? Structural studies confirm the absence of appreciable H9266 electron delocalization in cyclooctatetraene. Its structure is as pictured in Figure 11.11—a nonplanar hydrocarbon with four short carbon–carbon bond distances and four long carbon–carbon bond dis- tances. Cyclooctatetraene is satisfactorily represented by a single Lewis structure having alternating single and double bonds in a tub-shaped eight-membered ring. All the evidence indicates that cyclooctatetraene lacks the “special stability” of benzene, and is more appropriately considered as a conjugated polyene than as an aro- matic hydrocarbon. Cyclobutadiene escaped chemical characterization for more than 100 years. Despite numerous attempts, all synthetic efforts met with failure. It became apparent not only that cyclobutadiene was not aromatic but that it was exceedingly unstable. Beginning in the 1950s, a variety of novel techniques succeeded in generating cyclobutadiene as a transient, reactive intermediate. Cyclooctatetraene (70%) 4HC CH Acetylene Ni(CN) 2 heat, pressure Cyclobutadiene Cyclooctatetraene 422 CHAPTER ELEVEN Arenes and Aromaticity Willst?tter’s most important work, for which he won the 1915 Nobel Prize in chem- istry, was directed toward determining the structure of chlorophyll. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website PROBLEM 11.12 One of the chemical properties that make cyclobutadiene dif- ficult to isolate is that it reacts readily with itself to give a dimer: What reaction of dienes does this resemble? Structural studies of cyclobutadiene and some of its derivatives reveal a pattern of alternating single and double bonds and a rectangular, rather than a square, shape. Bond distances in a stable, highly substituted derivative of cyclobutadiene illustrate this pat- tern of alternating short and long ring bonds. Thus cyclobutadiene, like cyclooctatetraene, is not aromatic. Cyclic conjugation, although necessary for aromaticity, is not sufficient for it. Some other factor or factors must contribute to the special stability of benzene and its derivatives. To understand these factors, let’s return to the molecular orbital description of benzene. 11.19 HüCKEL’S RULE: ANNULENES One of the early successes of molecular orbital theory occurred in 1931 when Erich Hückel discovered an interesting pattern in the H9266 orbital energy levels of benzene, cyclobutadiene, and cyclooctatetraene. By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries, Hückel found that such hydrocarbons are characterized by a set of H9266 molecular orbitals in which one orbital is lowest in energy, another is highest in energy, and the rest are distributed in pairs between them. (CH 3 ) 3 C (CH 3 ) 3 C CO 2 CH 3 C(CH 3 ) 3 138 pm 151 pm Methyl 2,3,4-tri-tert-butylcyclobutadiene-1-carboxylate 11.19 Hückel’s Rule: Annulenes 423 133 pm 146 pm FIGURE 11.11 Molec- ular geometry of cyclooc- tatetraene. The ring is not planar, and the bond dis- tances alternate between short double bonds and long single bonds. Hückel was a German physi- cal chemist. Before his theo- retical studies of aromaticity, Hückel collaborated with Peter Debye in developing what remains the most widely accepted theory of electrolyte solutions. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website The arrangements of H9266 orbitals for cyclobutadiene, benzene, and cyclooctatetraene as determined by Hückel are presented in Figure 11.12. Their interpretation can be sum- marized as follows: Cyclobutadiene According to the molecular orbital picture, square planar cyclobu- tadiene should be a diradical (have two unpaired electrons). The four H9266 electrons are distributed so that two are in the lowest energy orbital and, in accordance with Hund’s rule, each of the two equal- energy nonbonding orbitals is half-filled. (Remember, Hund’s rule tells us that when two orbitals have the same energy, each one is half-filled before either of them reaches its full complement of two electrons.) Benzene As seen earlier in Figure 11.4 (Section 11.6), the six H9266 electrons of benzene are distributed in pairs among its three bonding orbitals. All the bonding orbitals are occupied, and all the electron spins are paired. Cyclooctatetraene Six of the eight H9266 electrons of cyclooctatetraene occupy three bond- ing orbitals. The remaining two H9266 electrons occupy, one each, the two equal-energy nonbonding orbitals. Planar cyclooctatetraene should, like square cyclobutadiene, be a diradical. As it turns out, neither cyclobutadiene nor cyclooctatetraene is a diradical in its most stable electron configuration. The Hückel approach treats them as planar regular polygons. Because the electron configurations associated with these geometries are not particularly stable, cyclobutadiene and cyclooctatetraene adopt structures other than pla- nar regular polygons. Cyclobutadiene, rather than possessing a square shape with two unpaired electron spins, is a spin-paired rectangular molecule. Cyclooctatetraene is non- planar, with all its H9266 electrons paired in alternating single and double bonds. On the basis of his analysis Hückel proposed that only certain numbers of H9266 elec- trons could lead to aromatic stabilization. Only when the number of H9266 electrons is 2, 6, 10, 14, and so on, can a closed-shell electron configuration be realized. These results are summarized in Hückel’s rule: Among planar, monocyclic, fully conjugated polyenes, only those possessing (4n H11545 2) H9266 electrons, where n is an integer, will have special aromatic stability. The general term annulene has been coined to apply to completely conjugated monocyclic hydrocarbons. A numerical prefix specifies the number of carbon atoms. Cyclobutadiene is [4]-annulene, benzene is [6]-annulene, and cyclooctatetraene is [8]- annulene. 424 CHAPTER ELEVEN Arenes and Aromaticity Antibonding Nonbonding Nonbonding Bonding Antibonding Bonding Cyclobutadiene (four π electrons) Benzene (six π electrons) Planar cyclooctatetraene (eight π electrons) FIGURE 11.12 Distribution of H9266 molecular orbitals and H9266 electrons in cyclobutadi- ene, benzene, and planar cyclooctatetraene. Hückel’s rule should not be applied to polycyclic aro- matic hydrocarbons (Section 11.8). Hückel’s analysis is lim- ited to monocyclic systems. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website PROBLEM 11.13 Represent the H9266 electron distribution among the H9266 orbitals in (a) [10]-Annulene (b) [12]-Annulene SAMPLE SOLUTION (a) [10]-Annulene has ten carbons: ten H9266 orbitals and ten H9266 electrons. Like benzene, it should have a closed-shell electron configuration with all its bonding orbitals doubly occupied. The prospect of observing aromatic character in conjugated polyenes having 10, 14, 18, and so on H9266 electrons spurred efforts toward the synthesis of higher annulenes. A problem immediately arises in the case of the all-cis isomer of [10]-annulene, the struc- ture of which is shown in the preceding problem. Geometry requires a ten-sided regular polygon to have 144° bond angles; sp 2 hybridization at carbon requires 120° bond angles. Therefore, aromatic stabilization due to conjugation in all-cis-[10]-annulene is opposed by the destabilizing effect of 24° of angle strain at each of its carbon atoms. All-cis-[10]- annulene has been prepared. It is not very stable and is highly reactive. A second isomer of [10]-annulene (the cis, trans, cis, cis, trans stereoisomer) can have bond angles close to 120° but is destabilized by a close contact between two hydro- gens directed toward the interior of the ring. In order to minimize the van der Waals strain between these hydrogens, the ring adopts a nonplanar geometry, which limits its ability to be stabilized by H9266 electron delocalization. It, too, has been prepared and is not very stable. Similarly, the next higher (4n H11001 2) system, [14]-annulene, is also somewhat destabilized by van der Waals strain and is nonplanar. When the ring contains 18 carbon atoms, it is large enough to be planar while still allowing its interior hydrogens to be far enough apart that they do not interfere with one another. The [18]-annulene shown is planar or nearly so and has all its car- bon–carbon bond distances in the range 137–143 pm—very much like those of ben- zene. Its resonance energy is estimated to be about 418 kJ/mol (100 kcal/mol). Although its structure and resonance energy attest to the validity of Hückel’s rule, which predicts “special stability” for [18]-annulene, its chemical reactivity does not. [18]-Annulene Planar geometry required for aromaticity destabilized by van der Waals repulsions between indicated hydrogens HH cis,trans,cis,cis,trans- [10]-Annulene HH HH [14]-Annulene Antibonding orbitals Bonding orbitals [10]-Annulene 11.19 Hückel’s Rule: Annulenes 425 The size of each angle of a regular polygon is given by the expression 180° H11003 (number of sides) H11002 2 (number of sides) Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website behaves more like a polyene than like benzene in that it is hydrogenated readily, under- goes addition rather than substitution with bromine, and forms a Diels–Alder adduct with maleic anhydride. According to Hückel’s rule, annulenes with 4n H9266 electrons are not aromatic. Cyclobutadiene and cyclooctatetraene are [4n]-annulenes, and their properties are more in accord with their classification as cyclic polyenes than as aromatic hydrocarbons. Among higher [4n]-annulenes, [16]-annulene has been prepared. [16]-Annulene is not planar and shows a pattern of alternating short (average 134 pm) and long (average 146 pm) bonds typical of a nonaromatic cyclic polyene. PROBLEM 11.14 What does a comparison of the heats of combustion of ben- zene (3265 kJ/mol; 781 kcal/mol), cyclooctatetraene (4543 kJ/mol; 1086 kcal/mol), [16]-annulene (9121 kJ/mol; 2182 kcal/mol), and [18]-annulene (9806 kJ/mol; 2346 kcal/mol) reveal? Most of the synthetic work directed toward the higher annulenes was carried out by Franz Sondheimer and his students, first at Israel’s Weizmann Institute and later at the University of London. Sondheimer’s research systematically explored the chemistry of these hydrocarbons and provided experimental verification of Hückel’s rule. 11.20 AROMATIC IONS Hückel realized that his molecular orbital analysis of conjugated systems could be extended beyond the realm of neutral hydrocarbons. He pointed out that cycloheptatrienyl cation contained a H9266 system with a closed-shell electron configuration similar to that of benzene (Figure 11.13). Cycloheptatrienyl cation has a set of seven H9266 molecular orbitals. Three of these are bonding and contain the six H9266 electrons of the cation. These six H9266 electrons are delocalized over seven carbon atoms, each of which contributes one 2p orbital to a planar, monocyclic, completely conjugated H9266 system. Therefore, cyclohepta- trienyl cation should be aromatic. It should be appreciably more stable than expected on the basis of any Lewis structure written for it. [16]-Annulene No serious repulsions among six interior hydrogens; molecule is planar and aromatic. H HH H HH [18]-Annulene 426 CHAPTER ELEVEN Arenes and Aromaticity Molecular models of [10]-, [14]-, [16]-, and [18]-annulene can be inspected on Learning By Modeling. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website It’s important to recognize the difference between the hydrocarbon cycloheptatriene and cycloheptatrienyl (tropylium) cation. The carbocation, as we have just stated, is aro- matic, whereas cycloheptatriene is not. Cycloheptatriene has six H9266 electrons in a conju- gated system, but its H9266 system does not close upon itself. The ends of the triene system are joined by an sp 3 -hybridized carbon, which prevents continuous electron delocaliza- tion. The ends of the triene system in the carbocation are joined by an sp 2 -hybridized carbon, which contributes an empty p orbital, and allows continuous delocalization of the six H9266 electrons. When we say cycloheptatriene is not aromatic but tropylium cation is, we are not comparing the stability of the two to each other. Cycloheptatriene is a sta- ble hydrocarbon but does not possess the special stability required to be called aromatic. Tropylium cation, although aromatic, is still a carbocation and reasonably reactive toward nucleophiles. Its special stability does not imply a rocklike passivity but rather a much greater ease of formation than expected on the basis of the Lewis structure drawn for it. A number of observations indicate that tropylium cation is far more stable than most other carbocations. To emphasize the aromatic nature of tropylium cation, it is some- times written in the Robinson manner, representing the aromatic sextet with a circle in the ring and including a positive charge within the circle. Br H11002 H11001 Tropylium bromide H H Cycloheptatriene H H11001 Cycloheptatrienyl cation (commonly referred to as tropylium cation) 11.20 Aromatic Ions 427 Energy Antibonding orbitals Bonding orbitals (Lowest energy orbital; all bonding) FIGURE 11.13 The H9266 molecular orbitals of cycloheptatrienyl (tropylium) cation. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website Tropylium bromide was first prepared, but not recognized as such, in 1891. The work was repeated in 1954, and the ionic properties of tropylium bromide were demon- strated. The ionic properties of tropylium bromide are apparent in its unusually high melt- ing point (203°C), its solubility in water, and its complete lack of solubility in diethyl ether. PROBLEM 11.15 Write resonance structures for tropylium cation sufficient to show the delocalization of the positive charge over all seven carbons. Cyclopentadienide anion is an aromatic anion. It has six H9266 electrons delocalized over a completely conjugated planar monocyclic array of five sp 2 -hybridized carbon atoms. PROBLEM 11.16 Write resonance structures for cyclopentadienide anion suffi- cient to show the delocalization of the negative charge over all five carbons. Figure 11.14 presents Hückel’s depiction of the molecular orbitals of cyclopenta- dienide anion. Like benzene and tropylium cation, cyclopentadienide anion has a closed- shell configuration of six H9266 electrons. A convincing demonstration of the stability of cyclopentadienide anion can be found in the acidity of cyclopentadiene. K a H11005 10 H1100216 (pK a H11005 16) HH Cyclopentadiene H H11001 H11001 H H11002 Cyclopentadienide anion Cyclopentadienide anion H HH HH H11002 H11002 428 CHAPTER ELEVEN Arenes and Aromaticity Energy Bonding orbitals (Lowest energy orbital; all bonding) Antibonding orbitals FIGURE 11.14 The H9266 molecular orbitals of cyclo- pentadienide anion. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website Cyclopentadiene is only a slightly weaker acid than water. The equilibrium for its deprotonation is more favorable than for other hydrocarbons because cyclopentadienide anion is aromatic. The contrast is striking when we compare this equilibrium with that for loss of a proton from cycloheptatriene. Resonance structures can be written that show delocalization of the negative charge over all of its seven carbons; nevertheless, because cycloheptatrienide anion contains eight H9266 electrons, it is not aromatic. The equilibrium constant for formation from the parent hydrocarbon is more favorable by 10 20 (20 pK a units) for the aromatic cyclopentadienide anion than for the nonaromatic cycloheptatrienide anion. PROBLEM 11.17 A standard method for the preparation of sodium cyclopenta- dienide (C 5 H 5 Na) is by reaction of cyclopentadiene with a solution of sodium amide in liquid ammonia. Write a balanced equation for this reaction. Hückel’s rule is now taken to apply to planar, monocyclic, completely conjugated systems generally, not just to neutral hydrocarbons. A planar, monocyclic, continuous system of p orbitals possesses aromatic stability when it contains (4n H11545 2) H9266 elec- trons. Other aromatic ions include cyclopropenyl cation (two H9266 electrons) and cyclooc- tatetraene dianion (ten H9266 electrons). Here, liberties have been taken with the Robinson symbol. Instead of restricting its use to a sextet of electrons, organic chemists have come to adopt it as an all-purpose sym- bol for cyclic electron delocalization. PROBLEM 11.18 Is either of the following ions aromatic? (a) (b) SAMPLE SOLUTION (a) The crucial point is the number of H9266 electrons in a cyclic conjugated system. If there are (4n H11001 2) H9266 electrons, the ion is aromatic. Electron Cyclononatetraenide anion H11002 Cyclononatetraenyl cation H11001 Cyclopropenyl cation HH H H11001 HH H H11001 2H11002 H H 2H11002 Cyclooctatetraene dianion K a H11005 10 H1100236 (pK a H11005 36) H H11001 H11001 H H Cycloheptatriene Cycloheptatrienide anion H H11002 11.20 Aromatic Ions 429 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website counting is easiest if we write the ion as a single Lewis structure and remember that each double bond contributes two H9266 electrons, a negatively charged carbon contributes two, and a positively charged carbon contributes none. 11.21 HETEROCYCLIC AROMATIC COMPOUNDS Cyclic compounds that contain at least one atom other than carbon within their ring are called heterocyclic compounds, and those that possess aromatic stability are called het- erocyclic aromatic compounds. Some representative heterocyclic aromatic compounds are pyridine, pyrrole, furan, and thiophene. The structures and the IUPAC numbering system used in naming their derivatives are shown. In their stability and chemical behav- ior, all these compounds resemble benzene more than they resemble alkenes. Pyridine, pyrrole, and thiophene, like benzene, are present in coal tar. Furan is pre- pared from a substance called furfural obtained from corncobs. Heterocyclic aromatic compounds can be polycyclic as well. A benzene ring and a pyridine ring, for example, can share a common side in two different ways. One way gives a compound called quinoline; the other gives isoquinoline. Analogous compounds derived by fusion of a benzene ring to a pyrrole, furan, or thio- phene nucleus are called indole, benzofuran, and benzothiophene. PROBLEM 11.19 Unlike quinoline and isoquinoline, which are of comparable stability, the compounds indole and isoindole are quite different from each other. Which one is more stable? Explain the reason for your choice. 1 2 3 4 5 6 7 N H Indole O 1 2 3 4 5 6 7 Benzofuran S 1 2 3 4 5 6 7 Benzothiophene N 1 2 3 45 6 7 8 Quinoline N 1 2 3 45 6 7 8 Isoquinoline 1 2 3 4 5 6 N Pyridine 1 2 3 4 5 N H Pyrrole O 1 2 3 4 5 Furan S 1 2 3 4 5 Thiophene Cyclononatetraenyl cation has eight H9266 electrons; it is not aromatic. H11001 430 CHAPTER ELEVEN Arenes and Aromaticity Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website A large group of heterocyclic aromatic compounds are related to pyrrole by replacement of one of the ring carbons H9252 to nitrogen by a second heteroatom. Com- pounds of this type are called azoles. A widely prescribed drug for the treatment of gastric ulcers has the generic name cimet- idine and is a synthetic imidazole derivative. Firefly luciferin is a thiazole derivative that is the naturally occurring light-emitting substance present in fireflies. Firefly luciferin is an example of an azole that contains a benzene ring fused to the five- membered ring. Such structures are fairly common. Another example is benzimidazole, present as a structural unit in vitamin B 12 . Some compounds related to benzimidazole include purine and its amino-substituted derivative adenine, one of the so-called hetero- cyclic bases found in DNA and RNA (Chapter 27). PROBLEM 11.20 Can you deduce the structural formulas of benzoxazole and benzothiazole? The structural types described in this section are but a tiny fraction of those pos- sible. The chemistry of heterocyclic aromatic compounds is a rich and varied field with numerous applications. N H N Benzimidazole N H N N N Purine NH 2 N H N N N Adenine N H N CH 3 CH 2 SCH 2 CH 2 NHCNHCH 3 NCN Cimetidine S HO N S N CO 2 H Firefly luciferin 1 2 3 4 5 N N H Imidazole 1 2 3 4 5 N O Oxazole 1 2 3 4 5 N S Thiazole N H Indole NH Isoindole 11.21 Heterocyclic Aromatic Compounds 431 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 11.22 HETEROCYCLIC AROMATIC COMPOUNDS AND HüCKEL’S RULE Hückel’s rule can be extended to heterocyclic aromatic compounds. A single heteroatom can contribute either 0 or 2 of its lone-pair electrons as needed to the H9266 system so as to satisfy the (4n H11001 2) H9266 electron requirement. The lone pair in pyridine, for example, is associated entirely with nitrogen and is not delocalized into the aromatic H9266 system. As shown in Figure 11.15a, pyridine is simply a benzene ring in which a nitrogen atom has replaced a CH group. The nitrogen is sp 2 -hybridized, and the three double bonds of the ring contribute the necessary six H9266 electrons to make pyridine a heterocyclic aromatic compound. The unshared electron pair of nitrogen occupies an sp 2 orbital in the plane of the ring, not a p orbital aligned with the H9266 system. In pyrrole, on the other hand, the unshared pair belonging to nitrogen must be added to the four H9266 electrons of the two double bonds in order to meet the six-H9266-elec- tron requirement. As shown in Figure 11.15b, the nitrogen of pyrrole is sp 2 -hybridized and the pair of electrons occupies a p orbital where both electrons can participate in the aromatic H9266 system. Pyridine and pyrrole are both weak bases, but pyridine is much more basic than pyrrole. When pyridine is protonated, its unshared pair is used to bond to a proton and, since the unshared pair is not involved in the H9266 system, the aromatic character of the ring is little affected. When pyrrole acts as a base, the two electrons used to form a bond to hydrogen must come from the H9266 system, and the aromaticity of the molecule is sac- rificed on protonation. 432 CHAPTER ELEVEN Arenes and Aromaticity 2 π electrons 2 π electrons 2 π electrons 2 π electrons 2 π electrons 2 π electrons 2 π electrons 2 π electrons 2 π electrons These electrons are not involved in the π system These electrons are not involved in the π system (a) Pyridine (b) Pyrrole (c) Furan H O N N N N±H O FIGURE 11.15 (a) Pyridine has six H9266 electrons plus an unshared pair in a nitrogen sp 2 orbital. (b) Pyrrole has six H9266 electrons. (c) Furan has six H9266 electrons plus an unshared pair in an oxygen sp 2 orbital, which is perpendicular to the H9266 system and does not inter- act with it. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website PROBLEM 11.21 Imidazole is a much stronger base than pyrrole. Predict which nitrogen is protonated when imidazole reacts with an acid, and write a structural formula for the species formed. The oxygen in furan has two unshared electron pairs (Figure 11.15c). One pair is like the pair in pyrrole, occupying a p orbital and contributing two electrons to complete the six-H9266-electron requirement for aromatic stabilization. The other electron pair in furan is an “extra” pair, not needed to satisfy the 4n H11001 2 rule for aromaticity, and occupies an sp 2 -hybridized orbital like the unshared pair in pyridine. The bonding in thiophene is similar to that of furan. 11.23 SUMMARY Section 11.1 Benzene is the parent of a class of hydrocarbons called arenes, or aro- matic hydrocarbons. Section 11.2 An important property of aromatic hydrocarbons is that they are much more stable and less reactive than other unsaturated compounds. Ben- zene, for example, does not react with many of the reagents that react rapidly with alkenes. When reaction does take place, substitution rather than addition is observed. The Kekulé formulas for benzene seem incon- sistent with its low reactivity and with the fact that all of the C±C bonds in benzene are the same length (140 pm). Section 11.3 One explanation for the structure and stability of benzene and other arenes is based on resonance, according to which benzene is regarded as a hybrid of the two Kekulé structures. Section 11.4 The extent to which benzene is more stable than either of the Kekulé structures is its resonance energy, which is estimated to be 125–150 kJ/mol (30–36 kcal/mol) from heats of hydrogenation data. Section 11.5 According to the orbital hybridization model, benzene has six H9266 elec- trons, which are shared by all six sp 2 -hybridized carbons. Regions of high H9266 electron density are located above and below the plane of the ring. H N N Imidazole 11.23 Summary 433 The article “A History of the Structural Theory of Benzene—The Aromatic Sex- tet and Hückel’s Rule” in the February 1997 issue of the Journal of Chemical Educa- tion (pp. 194–201) is a rich source of additional informa- tion about this topic. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website Section 11.6 A molecular orbital description of benzene has three H9266 orbitals that are bonding and three that are antibonding. Each of the bonding orbitals is fully occupied (two electrons each), and the antibonding orbitals are vacant. Section 11.7 Many aromatic compounds are simply substituted derivatives of benzene and are named accordingly. Many others have names based on some other parent aromatic compound. Section 11.8 Polycyclic aromatic hydrocarbons, of which anthracene is an example, contain two or more benzene rings fused together. Section 11.9 The physical properties of arenes resemble those of other hydrocarbons. Section 11.10 Chemical reactions of arenes can take place on the ring itself, or on a side chain. Reactions that take place on the side chain are strongly influ- enced by the stability of benzylic radicals and benzylic carbocations. Section 11.11 An example of a reaction in which the ring itself reacts is the Birch reduction. The ring of an arene is reduced to a nonconjugated diene by treatment with a Group I metal (usually sodium) in liquid ammonia in the presence of an alcohol. Sections Free-radical halogenation and oxidation involve reactions at the benzylic 11.12–11.13 carbon. See Table 11.2. Section 11.14 Benzylic carbocations are intermediates in S N 1 reactions of benzylic halides and are stabilized by electron delocalization. C H11001 C H11001 and so on CH 3 CH 3 o-Xylene CH 3 CH 3 1,2-Dimethyl-1,4- cyclohexadiene (92%) Na, NH 3 CH 3 OH C Benzylic free radical C H11001 Benzylic carbocation Anthracene OH H 3 C CH 3 2,6-Dimethylphenol C(CH 3 ) 3 tert-Butylbenzene Cl CH 3 m-Chlorotoluene 434 CHAPTER ELEVEN Arenes and Aromaticity Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website Section 11.15 The simplest alkenylbenzene is styrene (C 6 H 5 CH?CH 2 ). An aryl group stabilizes a double bond to which it is attached. Alkenylbenzenes are usu- ally prepared by dehydration of benzylic alcohols or dehydrohalogena- tion of benzylic halides. 11.23 Summary 435 TABLE 11.2 Reactions Involving Alkyl and Alkenyl Side Chains in Arenes and Arene Derivatives Reaction (section) and comments Halogenation (Section 11.12) Free-radical halogenation of alkylbenzenes is highly selective for substitution at the benzylic position. In the example shown, elemental bromine was used. Alternatively, N-bromosuccinimide is a convenient reagent for benzylic bromination. Hydrogenation (Section 11.16) Hydrogena- tion of aromatic rings is somewhat slower than hydrogenation of alkenes, and it is a simple matter to reduce the double bond of an unsaturated side chain in an arene while leaving the ring intact. Electrophilic addition (Section 11.16) An aryl group stabilizes a benzylic carbocation and controls the regioselectivity of addition to a double bond involving the benzylic car- bon. Markovnikov’s rule is obeyed. Oxidation (Section 11.13) Oxidation of alkylbenzenes occurs at the benzylic posi- tion of the alkyl group and gives a benzoic acid derivative. Oxidizing agents include sodium or potassium dichromate in aque- ous sulfuric acid. Potassium permanganate (KMnO 4 ) is also an effective oxidant. General equation and specific example ArCHR 2 Arene 1-Arylalkyl bromide ArCR 2 Br NBS benzoyl peroxide CCl 4 , 80°C p-Ethylnitrobenzene O 2 N CH 2 CH 3 1-(p-Nitrophenyl)ethyl bromide (77%) O 2 N CHCH 3 Br Br 2 CCl 4 light oxidize ArCHR 2 Arene ArCO 2 H Arenecarboxylic acid 2,4,6-Trinitrotoluene O 2 N CH 3 NO 2 NO 2 2,4,6-Trinitrobenzoic acid (57–69%) O 2 N CO 2 H NO 2 NO 2 Na 2 Cr 2 O 7 H 2 SO 4 H 2 O ArCH 2 CHR 2 Alkylarene H 2 Hydrogen H11001 Alkenylarene ArCH CR 2 Pt H 2 Pt 1-(m-Bromophenyl)propene CH Br CHCH 3 m-Bromopropylbenzene (85%) CH 2 CH 2 CH 3 Br Product of electrophilic addition ArCH Y CH 2 E Alkenylarene ArCH CH 2 H9254H11001 E±Y H9254H11002 Styrene CH CH 2 1-Phenylethyl bromide (85%) CHCH 3 Br HBr Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website Section 11.16 Addition reactions to alkenylbenzenes occur at the double bond of the alkenyl substituent, and the regioselectivity of electrophilic addition is governed by carbocation formation at the benzylic carbon. See Table 11.2. Section 11.17 Polystyrene is a widely used vinyl polymer prepared by the free-radical polymerization of styrene. Section 11.18 Although cyclic conjugation is a necessary requirement for aromaticity, this alone is not sufficient. If it were, cyclobutadiene and cyclooctate- traene would be aromatic. They are not. Section 11.19 An additional requirement for aromaticity is that the number of H9266 elec- trons in conjugated, planar, monocyclic species must be equal to 4n H11001 2, where n is an integer. This is called Hückel’s rule. Benzene, with six H9266 electrons, satisfies Hückel’s rule for n H11005 1. Cyclobutadiene (four H9266 electrons) and cyclooctatetraene (eight H9266 electrons) do not. Planar, mono- cyclic, completely conjugated polyenes are called annulenes. Section 11.20 Species with six H9266 electrons that possess “special stability” include cer- tain ions, such as cyclopentadienide anion and cycloheptatrienyl cation. Section 11.21 Heterocyclic aromatic compounds are compounds that contain at least one atom other than carbon within an aromatic ring. Cyclopentadienide anion (six H9266 electrons) H HH HH H11002 H H H HH H H H11001 Cycloheptatrienyl cation (six H9266 electrons) Cyclobutadiene (not aromatic) Cyclooctatetraene (not aromatic) Benzene (aromatic) Polystyrene UU UU H 2 SO 4 heat 1-Phenylcyclohexene (83%)1-Phenylcyclohexanol OH 436 CHAPTER ELEVEN Arenes and Aromaticity Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website Section 11.22 Hückel’s rule can be extended to heterocyclic aromatic compounds. Unshared electron pairs of the heteroatom may be used as H9266 electrons as necessary to satisfy the 4n H11001 2 rule. PROBLEMS 11.22 Write structural formulas and give the IUPAC names for all the isomers of C 6 H 5 C 4 H 9 that contain a monosubstituted benzene ring. 11.23 Write a structural formula corresponding to each of the following: (a) Allylbenzene (g) 2-Nitrobenzenecarboxylic acid (b) (E)-1-Phenyl-1-butene (h) p-Diisopropylbenzene (c) (Z)-2-Phenyl-2-butene (i) 2,4,6-Tribromoaniline (d) (R)-1-Phenylethanol (j) m-Nitroacetophenone (e) o-Chlorobenzyl alcohol (k) 4-Bromo-3-ethylstyrene (f) p-Chlorophenol 11.24 Using numerical locants and the names in Table 11.1 as a guide, give an acceptable IUPAC name for each of the following compounds: (a) Estragole (principal (b) Diosphenol (used in (c) m-Xylidine (used in component of wormwood veterinary medicine synthesis of lidocaine, oil) to control parasites a local anesthetic) in animals) 11.25 Write structural formulas and give acceptable names for all the isomeric (a) Nitrotoluenes (d) Tetrafluorobenzenes (b) Dichlorobenzoic acids (e) Naphthalenecarboxylic acids (c) Tribromophenols (f) Bromoanthracenes 11.26 Mesitylene (1,3,5-trimethylbenzene) is the most stable of the trimethylbenzene isomers. Can you think of a reason why? Which isomer do you think is the least stable? Make a molecular model of each isomer and compare their calculated strain energies with your predictions. Do space- filling models support your explanation? 11.27 Which one of the dichlorobenzene isomers does not have a dipole moment? Which one has the largest dipole moment? Compare your answers with the dipole moments calculated using the molecular-modeling software in Learning By Modeling. NH 2 CH 3 CH 3 OH NO 2 II CH 2 OCH 3 CH 2 CH Nicotine N N CH 3 Problems 437 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 11.28 Identify the longest and the shortest carbon–carbon bonds in styrene. Make reasonable esti- mates of their bond distances and compare them to the distances in a molecular model. 11.29 The resonance form shown is not the most stable one for the compound indicated. Write the most stable resonance form. 11.30 Each of the following may be represented by at least one alternative resonance structure in which all the six-membered rings correspond to Kekulé forms of benzene. Write such a resonance form for each. (a) (c) (b) (d) 11.31 Give the structure of the expected product from the reaction of isopropylbenzene with (a) Hydrogen (3 mol), Pt (b) Sodium and ethanol in liquid ammonia (c) Sodium dichromate, water, sulfuric acid, heat (d) N-Bromosuccinimide in CCl 4 , heat, benzoyl peroxide (e) The product of part (d) treated with sodium ethoxide in ethanol 11.32 Each of the following reactions has been described in the chemical literature and gives a single organic product in good yield. Identify the product of each reaction. (a) (b) (c) (d) (e) H 2 SO 4 acetic acid OH H 3 C (E)-C 6 H 5 CH CHC 6 H 5 CH 3 CO 2 OH acetic acid excess Cl 2 CCl 4 , light CH 3 (C 6 H 5 ) 2 CH C 20 H 14 Cl 4 H11001 H 2 (1 mol) Pt CH 2 CH 3 C 6 H 5 1. B 2 H 6 , diglyme 2. H 2 O 2 , HO H11002 438 CHAPTER ELEVEN Arenes and Aromaticity The common name of iso- propylbenzene is cumene. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website (f) (g) (h) (i) 11.33 A certain compound A, when treated with N-bromosuccinimide and benzoyl peroxide under photochemical conditions in refluxing carbon tetrachloride, gave 3,4,5-tribromobenzyl bromide in excellent yield. Deduce the structure of compound A. 11.34 A compound was obtained from a natural product and had the molecular formula C 14 H 20 O 3 . It contained three methoxy (±OCH 3 ) groups and a ±CH 2 CH?C(CH 3 ) 2 substituent. Oxidation with either chromic acid or potassium permanganate gave 2,3,5-trimethoxybenzoic acid. What is the structure of the compound? 11.35 Hydroboration–oxidation of (E )-2-( p-anisyl)-2-butene yielded an alcohol A, mp 60°C, in 72% yield. When the same reaction was performed on the Z alkene, an isomeric liquid alcohol B was obtained in 77% yield. Suggest reasonable structures for A and B, and describe the relation- ship between them. 11.36 Dehydrohalogenation of the diastereomeric forms of 1-chloro-1,2-diphenylpropane is stereo- specific. One diastereomer yields (E )-1,2-diphenylpropene, and the other yields the Z isomer. Which diastereomer yields which alkene? Why? 11.37 Suggest reagents suitable for carrying out each of the following conversions. In most cases more than one synthetic operation will be necessary. (a) (b) C 6 H 5 CHCH 2 Br Br C 6 H 5 CHCH 3 Br C 6 H 5 CH 2 CH 3 C 6 H 5 CHCH 3 Br C 6 H 5 CHCHC 6 H 5 ClH 3 C 1-Chloro-1,2-diphenylpropane C C 6 H 5 H 3 C CHC 6 H 5 1,2-Diphenylpropene CH 3 O C H 3 C CHCH 3 2-(p-Anisyl)-2-butene K 2 CO 3 water CH 2 ClNC C 8 H 7 NO CH 3 C 11 H 9 Br N-bromosuccinimide CCl 4 , heat ) 2 CHCCl 3 (Cl (DDT) NaOCH 3 CH 3 OH C 14 H 8 Cl 4 C 12 H 14 (CH 3 ) 2 COH (CH 3 ) 2 COH KHSO 4 heat Problems 439 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website (c) C 6 H 5 CH?CH 2 ±￡ C 6 H 5 CPCH (d) C 6 H 5 CPCH ±￡ C 6 H 5 CH 2 CH 2 CH 2 CH 3 (e) C 6 H 5 CH 2 CH 2 OH ±￡ C 6 H 5 CH 2 CH 2 CPCH (f) 11.38 The relative rates of reaction of ethane, toluene, and ethylbenzene with bromine atoms have been measured. The most reactive hydrocarbon undergoes hydrogen atom abstraction a million times faster than does the least reactive one. Arrange these hydrocarbons in order of decreasing reactivity. 11.39 Write the principal resonance structures of o-methylbenzyl cation and m-methylbenzyl cation. Which one has a tertiary carbocation as a contributing resonance form? 11.40 The same anion is formed by loss of the most acidic proton from 1-methyl-1,3-cyclopenta- diene as from 5-methyl-1,3-cyclopentadiene. Explain. 11.41 There are two different tetramethyl derivatives of cyclooctatetraene that have methyl groups on four adjacent carbon atoms. They are both completely conjugated and are not stereoisomers. Write their structures. 11.42 Evaluate each of the following processes applied to cyclooctatetraene, and decide whether the species formed is aromatic or not. (a) Addition of one more H9266 electron, to give C 8 H 8 H11002 (b) Addition of two more H9266 electrons, to give C 8 H 8 2H11002 (c) Removal of one H9266 electron, to give C 8 H 8 H11001 (d) Removal of two H9266 electrons, to give C 8 H 8 2H11001 11.43 Evaluate each of the following processes applied to cyclononatetraene, and decide whether the species formed is aromatic or not: (a) Addition of one more H9266 electron, to give C 9 H 10 H11002 (b) Addition of two more H9266 electrons, to give C 9 H 10 2H11002 (c) Loss of H H11001 from the sp 3 -hybridized carbon (d) Loss of H H11001 from one of the sp 2 -hybridized carbons 11.44 From among the molecules and ions shown, all of which are based on cycloundecapentaene, identify those which satisfy the criteria for aromaticity as prescribed by Hückel’s rule. (a) (c) (b) (d) H11002 Cycloundecapentaenide anionCycloundecapentaenyl radical H11001 Cycloundecapentaenyl cationCycloundecapentaene Cyclononatetraene C 6 H 5 CHCH 2 Br OH C 6 H 5 CH 2 CH 2 Br 440 CHAPTER ELEVEN Arenes and Aromaticity Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 11.45 (a) Figure 11.16 is an electrostatic potential map of calicene, so named because its shape resembles a chalice (calix is the Latin word for “cup”). Both the electrostatic potential map and its calculated dipole moment (H9262 H11005 4.3 D) indicate that calicene is an unusu- ally polar hydrocarbon. Which of the dipolar resonance forms, A or B, better corresponds to the electron distribution in the molecule? Why is this resonance form more important than the other? (b) Which one of the following should be stabilized by resonance to a greater extent? (Hint: Consider the reasonableness of dipolar resonance forms.) 11.46 Classify each of the following heterocyclic molecules as aromatic or not, according to Hückel’s rule: (a) (c) (b) (d) O O NH BH NH N H HN H B HB O C or D Calicene H11002 H11001 A H11001 H11002 B Problems 441 FIGURE 11.16 Elec- trostatic potential map of calicene (problem 11.45). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 11.47 Pellagra is a disease caused by a deficiency of niacin (C 6 H 5 NO 2 ) in the diet. Niacin can be synthesized in the laboratory by the side-chain oxidation of 3-methylpyridine with chromic acid or potassium permanganate. Suggest a reasonable structure for niacin. 11.48 Nitroxoline is the generic name by which 5-nitro-8-hydroxyquinoline is sold as an antibac- terial drug. Write its structural formula. 11.49 Acridine is a heterocyclic aromatic compound obtained from coal tar that is used in the syn- thesis of dyes. The molecular formula of acridine is C 13 H 9 N, and its ring system is analogous to that of anthracene except that one CH group has been replaced by N. The two most stable reso- nance structures of acridine are equivalent to each other, and both contain a pyridine-like struc- tural unit. Write a structural formula for acridine. 11.50 Make molecular models of the two chair conformations of cis-1-tert-butyl-4-phenylcyclo- hexane. What is the strain energy calculated for each conformation by molecular mechanics? Which has a greater preference for the equatorial orientation, phenyl or tert-butyl? 442 CHAPTER ELEVEN Arenes and Aromaticity Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website