CHAPTER 11 ARENES AND AROMATICITY hapter and the next we extend our coverage of conjugated systems to include S. Arenes are hydrocarbons based on the benzene ring as a structural unit. Ben toluene, and naphthalene, for example, are arenes H HH HH HH Naphthalene 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 det velo an ation for the cept of aromaticity-to see what are the properties of benzene and its derivatives that reflect its special stability, plore 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 rolve the ring its 398 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
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. Benzene, toluene, and naphthalene, for example, are arenes. One factor that makes conjugation in arenes special is its cyclic nature. A conjugated 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 concept 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 H H H H H Benzene H H H H H CH3 Toluene H H H H H H H H Naphthalene Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
11.1 Benzene Lets 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 Faraday is better know called"bicarburet of hydrogen. "Nine years later Eilhardt Mitscherlich of the University chemistry for his lawsor 3 of Berlin prepared the same substance by heating benzoic acid with lime and found to be a hydrocarbon having the empirical formula CnHn proposing the relationship C6HsCO,H Cao →C6H6+ strating the principle of Benzoic acid Calcium oxide 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 ghly prized articles of commerce, especially when the trees that produced them coul 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, enjoin, 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 hydrocarbon.s. 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 KEKULE 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 CH6 toluene is C7H) 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 Back Forward Main MenuToc Study Guide ToC Student o 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 CnHn. 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 Mitscherlich’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 equivalent, 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 benzene 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 C6H6, toluene is C7H8) indicate that, like alkenes and alkynes, they are unsaturated and should undergo addition reactions. Under conditions in which bromine, for example, reacts rapidly with alkenes and C6H5CO2H Benzoic acid C6H6 Benzene CaO Calcium oxide CaCO3 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 magnetic fields and for demonstrating the principle of electromagnetic induction. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER ELEVEN Arenes and Aromaticity alkynes, however, benzene proved to be inert. Benzene does react with Br2 in the pres ence of iron(im) bromide as a catalyst, but even then addition isnt observed Substitu- tion occurs instead no observable reaction Bromobenzene Hydrogen bromide 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 he 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 Kekule applied it to the structure of benzene. He based his reasoning on three premises 1. Benzene is Cah 2. All the hydrogens of benzene are equivalent. 3. The structural theory requires that there be four bonds to each carbon Kekule advanced the venturesome notion that the six carbon atoms of benzene were oschmidt, who was later to 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 University of Vienna, pri- ining a structural formula for benzene similar to that H five years later. Loschmidt's ook reached few readers d his ideas were not well known H an you write? An article in a flaw in Kekule s structure for benzene was soon discovered. Kekule's structure the March 1994 issue of the quires that 1, 2-and 1, 6-disubstitution patterns create different compounds (isomers) Journal of Chemical Educa- ion(pp 222-224)claims ed and draws structural formulas for 25 of them 1. 2-Disubstituted 1.6-Disubstituted derivative of benzene derivative of benzene 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 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
alkynes, however, benzene proved to be inert. Benzene does react with Br2 in the presence of iron(III) bromide as a catalyst, but even then addition isn’t observed. Substitution 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 structural 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 benzene 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 recognize 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 C6H6. 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 system 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 C H C C H C C H H 1 2 3 4 6 5 H H C6H6 Benzene Br2 Bromine CCl4 FeBr3 no observable reaction C6H5Br 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, privately published a book containing 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 C6H6 can you write? An article in the March 1994 issue of the Journal of Chemical Education (pp. 222–224) claims that there are several hundred and draws structural formulas for 25 of them. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
11.2 Kekule and the structure of benzene BENZENE DREAMS AND CREATIVE THINKING t ceremonies in Berlin in 1890 celebrating the Let us learn to dream, then perhaps we shall twenty-fifth anniversary of his proposed struc- find the truth. But let us beware of publishing ture of benzene, August Kekule recalled the our dreams before they have been put to th thinking that led him to it. He began by noting that proof by the waking understanding the idea of the structural theory came to him during The imagery of a whirling circle of snakes evokes a a daydream while on a bus in London. Kekule went vivid picture that engages one's attention when first on to describe the origins of his view of the benzene exposed to Kekule's model of the benzene structure. structure Recently, however, the opinion has been expressed There I sat and wrote for my textbook; but that Kekule might have engaged in some hyperbole with other matters. I turned the chair towards Illinois University suggests that discoveries in science atoms danced before my eyes. This time smaller body of experimental observations to progress to a groups modestly remained in the background. higher level of understanding. Wotiz'view that My mental eye, sharpened by repeated appari- Kekule's account is more fanciful than accurate has joined more densely; everything in motion, yond the history of organic chemistry. How does cre- what was that? One of the snakes caught hold more creative? Because these are questions that have of its own tail and mockingly whirled round be. concerned psychologists for decades, the idea of a fore my eyes. I awoke as if by lightning: this sleepy Kekule being more creative than an alert time, too, I spent the rest of the night working Kekule becomes more than simply a charming story out the consequences of this hypothesis he once told about himself Concluding his remarks, Kekule merged his advocacy of creative imagination with the rigorous standards Hafner published in Angew. Chem. Internat. edEngl! of science by reminding his audience 641-651(1979) none could be found, Kekule suggested that two isomeric structures could exist but inter converted too rapidly to be separated X Kekule'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. Kekule was Benzene is lohexatriene. nor is it air of rapidly equilibrating cyclo hexatriene isomers. But there was no way that Kekule could have gotten it right given the state of chemical know ledge at the time. After all, the electron hadnt even been dis- onding to provi 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 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
none could be found, Kekulé suggested that two isomeric structures could exist but interconverted too rapidly to be separated. Kekulé’s ideas about the structure of benzene left an important question unanswered. 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 cyclohexatriene 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 discovered 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, however, 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 At ceremonies in Berlin in 1890 celebrating the twenty-fifth anniversary of his proposed structure 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 apparitions 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 before 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 beyond the history of organic chemistry. How does creative 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
CHAPTER ELEVEN Arenes and Aromaticity 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- hybridization. Interestingly, the 140-pm bond distances in benzene are exactly midway between the typical sp--sp" single-bond distance of 146 pm and the sp-sp- 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 Kekule 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 Kekule structures, benzene is often represented by a hexagon containing an inscribed 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 T electrons of the three double bonds. Robinsons symbol is a convenient time-saving shorthand device, but Kekule-type formulas are better for counting and keeping track of electrons, especially in chemical reactions PROBLEM 11.1 Write structural formulas for toluene(cshs cha) and for benzoic acid (C,Hs Co2 H)(a)as resonance hybrids of two Kekule forms and( b)with the 140pm 108pm FIGURE 11.1 Bond distances and bond angles of benzen Back Forward Main MenuToc Study Guide ToC Student o 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 correspond to perfect sp2 hybridization. Interestingly, the 140-pm bond distances in benzene are exactly midway between the typical sp2 –sp2 single-bond distance of 146 pm and the sp2 –sp2 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 aromaticity. 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 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 (C6H5CH3) and for benzoic acid (C6H5CO2H) (a) as resonance hybrids of two Kekulé forms and (b) with the Robinson symbol. is equivalent to 402 CHAPTER ELEVEN Arenes and Aromaticity 120 120 120 140 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
11.4 The Stability of Benzene 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 le 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 T electrons are delocalized over a cyclic con system. Both Kekule structures of benzene are of equal energy, and one of the 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 stabilize- tion is can be estimated from heats of hydrogenation. 11. 4 THE STABILITY OF BENZENE Hydrogenation of benzene and other difficult than hydrogenation of alkenes and alkynes. Two of the more are rhodium and platinum, and it is possible to hydrogenate arenes in the presence of these catalysts at room temperature nd modest pressure. Benzene consumes three molar equivalents of hydrogen to give cyclohexane. +3H Benzene Hydrogen Cyclohexane (100%) Nickel catalysts, although less expensive than rhodium and platinum, are also less active. Hydrogenation of arenes in the presence of nickel requires high temperatures (100-200oC)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 k/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 Back Forward Main MenuToc Study Guide ToC Student o 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 intermediate 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 electrons are delocalized over a cyclic conjugated system. Both Kekulé structures of benzene are of equal energy, and one of the principles 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 stabilization than is observed in noncyclic conjugated trienes. How much greater that stabilization 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-cyclohexadiene, 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 whenever 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 compare it with benzene. We can approximate the heat of hydrogenation of 1,3,5-cyclohexatriene 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 noninteracting 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 3H2 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
CHAPTER ELEVEN Arenes and Aromaticity FIGURE 11.2 Heats of hydro An imaginary molecule. 3-cyclohexadiene, a hyp cyclohexatriene thetical 1, 3, 5-cyclohexatriene and benzene. All heats of hy drogenation are in kilojoules per mole. 3×120 2H,+ 231 A real 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 H H H +3H2—>CH3CH2)4CH3△H=-337kJ H (-80.5kca) (2)-1, 3, 5-Hexatriene Hydrogen 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 cal 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 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
We reach a similar conclusion when comparing benzene with the open-chain conjugated triene (Z)-1,3,5-hexatriene. Here we compare two real molecules, both conjugated 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-cycloheptatriene are 110 kJ/mol (26.3 kcal/mol) and 305 kJ/mol (73.0 kcal/mol), respectively. 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 benzene? H° 337 kJ (80.5 kcal) H H H H H H H H (Z)-1,3,5-Hexatriene 3H2 Hydrogen CH3(CH2)4CH3 Hexane 404 CHAPTER ELEVEN Arenes and Aromaticity Energy 2H2 H2 120 231 208 152 3H2 A real molecule, benzene An imaginary molecule, cyclohexatriene 3 120 360 3H2 FIGURE 11.2 Heats of hydrogenation of cyclohexene, 1,3-cyclohexadiene, a hypothetical 1,3,5-cyclohexatriene, and benzene. All heats of hydrogenation are in kilojoules per mole. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
11.6 The r Molecular Orbitals of benzene FIGURE 11.3(a) The framework of bonds shown in the tube model of benzene are o bonds. b)Each carbon ybridized and has a 2p orbital perpendicular to the o framework. Overlap of the 2p orbitals generates a system encompa ne entire ring. (c) Electrostatic potential plot of benzene. the red area in the center corresponds to the region above and he plane of the ring where the t electrons are concentrated 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- hybridization for carbon and the frame work of g bonds shown in Fi In addition to its three sp- hybrid orbitals, each carbon has a half-filled 2p orbital that can participate in T bonding. Figure 11.3b shows the continuous TT system that encompasses all of the carbons that result from overlap of these 2p orbitals. The six TT electrons of benzene are delocalized over all six carbons The electrostatic potential map of benzene(Figure 113c) 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 T electrons) to be 11.6 THE TT MOLECULAR ORBITALS OF BENZENE The picture of benzene as a planar framework of o bonds with six electrons in a delo- calized T 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 T molecular orbitals. These six T molecular orbitals include three which are bonding and three which are antibonding. The relative energies of these orbitals and the distribution of the a electrons among them are illustrated in figure 114. Benzene is said to have a closed-shell Tr electron configuration. All the bonding orbitals are filled, and there are no electrons in antibonding orbitals. FIGURE 11.4 molecular orbitals f be Antibonding zene arranged in order of in- orbitals easing energy. The six T electrons of benzene occup bitals, all of which are bond- 两2+ I3 Bonding ing. The nodal properties of x these orbitals may be viewed on Learning By Modeling Back Forward Main MenuToc Study Guide ToC Student o 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 carbon is bonded to three other atoms, suggest sp2 hybridization for carbon and the framework of bonds shown in Figure 11.3a. In addition to its three sp2 hybrid orbitals, each carbon has a half-filled 2p orbital that can participate in bonding. Figure 11.3b shows the continuous system that encompasses all of the carbons that result from overlap of these 2p orbitals. The six 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 electrons) to be. 11.6 THE MOLECULAR ORBITALS OF BENZENE The picture of benzene as a planar framework of bonds with six electrons in a delocalized 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 molecular orbitals. These six molecular orbitals include three which are bonding and three which are antibonding. The relative energies of these orbitals and the distribution of the electrons among them are illustrated in Figure 11.4. Benzene is said to have a closed-shell electron configuration. All the bonding orbitals are filled, and there are no electrons in antibonding orbitals. 11.6 The Molecular Orbitals of Benzene 405 (a) (b) (c) Antibonding orbitals Energy Bonding orbitals π4 π2 π1 π3 π5 π6 FIGURE 11.3 (a) The framework of bonds shown in the tube model of benzene are bonds. (b) Each carbon is sp2 - hybridized and has a 2p orbital perpendicular to the framework. Overlap of the 2p orbitals generates a 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 electrons are concentrated. FIGURE 11.4 The molecular orbitals of benzene arranged in order of increasing energy. The six electrons of benzene occupy the three lowest energy orbitals, all of which are bonding. 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
CHAPTER ELEVEN Arenes and Aromaticity 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 T 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 ar named by attaching the name of the substituent as a prefix to benzene C(CH3) Bromobenzene tert-Butylbenzene Ni 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 3 o-Xylene m-Xylene Xylene (1, 2-dimethylbenzene) (1, 3-dimethylbenzene) (1, 4-dimethylben 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, O O-Dichlorobenzene IN-Nitrotoluene Back Forward Main MenuToc Study Guide ToC Student o 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 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 chapter (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 standing 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-disubstitution, 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 acetophenone) is used. For example, Cl Cl o-Dichlorobenzene (1,2-dichlorobenzene) NO2 CH3 m-Nitrotoluene (3-nitrotoluene) C F O CH3 p-Fluoroacetophenone (4-fluoroacetophenone) CH3 CH3 o-Xylene (1,2-dimethylbenzene) CH3 CH3 m-Xylene (1,3-dimethylbenzene) CH3 CH3 p-Xylene (1,4-dimethylbenzene) Br Bromobenzene C(CH3)3 tert-Butylbenzene NO2 Nitrobenzene 406 CHAPTER ELEVEN Arenes and Aromaticity Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
11.7 Substituted derivatives of benzene and their nomenclature TABLE 11.1 Names of Some Frequently Encountered Derivatives of Benzene Structure Systematic Name Common name Benzenecarbaldehyd Benzaldehyde Benzenecarboxylic acid Benzoic acid CH=CH2 Vinylbenzene Styrene - cch Methyl phenyl ketone Acetophenone Benzenol Phenol Methoxybenzene Anisole Benzenamine Aniline "These common names are acceptable in IUPaC nomenclature and are the names that will be used in this PROBLEM 11.3 Write a structural formula for each of the following compounds (b) m-Chlorostyrene SAMPLE SOLUTION (a) The parent compound in o-ethylanisole is anisole Anisole, as shown in Table 11.1, h ethoxy(CH3O-)substituent on the ben- zene ring. The ethyl group in o-ethylanisole is attached to the carbon adjacent to he one that bears the metho OCH3 CH2 CH Back Forward Main MenuToc Study Guide ToC Student o 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 (CH3O±) substituent on the benzene ring. The ethyl group in o-ethylanisole is attached to the carbon adjacent to the one that bears the methoxy substituent. OCH3 CH2CH3 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 ±CCH3 O X ±CHœCH2 ±OH ±OCH3 ±NH2 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website