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