15_BRCLoudon_pgs5-0.qxd 12/9/08 12:22 PM Page 716 716 CHAPTER 15 • DIENES, RESONANCE, AND AROMATICITY PROBLEMS 15.27 In each of the following sets, show by the curved-arrow or fishhook notation how each reso- nance structure is derived from the other one, and indicate which structure is more important and why. .. .. (a) .. (b) .. C O.. C O H3CC| A NH H3CC ' NH| L 2 L carbon monoxide (c) (CH3)2CA CH CH| 2 (CH3)2C| CHA CH2 L L (d) N .. N C C CH3 CH3 15.28 Show the 2p orbitals, and indicate the orbital overlap symbolized by the resonance structures for the carbocation in Eq. 15.32 on p. 711. 15.29 Using resonance arguments, state which ion or radical within each set is more stable. Explain. (a) O _ O _ 332 332 H3C"CH CHA CH2 or H3C"C A CH CH3 L L L L (b) CH2 S A A A H2CCH CHCH CH8 2 or H2C8 C CH CH2 LL LL (c) CH3 H2CA"C CH| 2 or H3C CH A CH CH| 2 L L L 15.30 The following isomers do not differ greatly in stability. Predict which one should react more rapidly in an SN1 solvolysis reaction in aqueous acetone. Explain. H CH3 H CH2CH3 $CCA ) $CCA ) H3CCH CH)2 $H H3C CH) $H L L L "Cl "Cl AB 15.7 INTRODUCTION TO AROMATIC COMPOUNDS The term aromatic, as we’ll come to understand it in this section, is a precisely defined struc- tural term that applies to cyclic conjugated molecules that meet certain criteria. Benzene and its derivatives are the best known examples of aromatic compounds. H H "C H C C %S % # % or C "C r H C H % % ( % "H benzene 15_BRCLoudon_pgs5-0.qxd 12/9/08 12:22 PM Page 717 15.7 INTRODUCTION TO AROMATIC COMPOUNDS 717 The origin of the term aromatic is historical: many fragrant compounds known from earliest times, such as the following ones, proved to be derivatives of benzene. O S CHA O C OCH3 CH3 L A " " OH " H2C CH O % % i i i i OCH O % 3 OH methyl salicylate CH saffrole " (oil of wintergreen) " (oil of sassafras) H3C CH vanillin % % 3 (vanilla) p-cymene (cumin and thyme) Although it is known today that benzene derivatives are not distinguished by unique odors, the term aromatic—which has nothing to do with odor—has stuck, and it is now a class name for benzene, its derivatives, and a number of other organic compounds. Development of the theory of aromaticity was a major theoretical advance in organic chem- istry because it solved a number of intriguing problems that centered on the structure and reac- tivity of benzene. Before considering this theory, let’s see what some of these problems were. A. Benzene, a Puzzling “Alkene” The structure used today for benzene was proposed in 1865 by August Kekulé (p. 47), who claimed later that it came to him in a dream. The Kekulé structure portrays benzene as a cyclic, conjugated triene. Yet benzene does not undergo any of the addition reactions that are associ- ated with either conjugated dienes or ordinary alkenes. Benzene itself, as well as benzene rings in other compounds, are inert to the usual conditions of halogen addition, hydroboration, hydra- tion, or ozonolysis. This property of the benzene ring is illustrated by the addition of bromine to styrene, a compound that contains both a benzene ring and one additional double bond: CH A CH2 Br2 CH CH2 (15.38) cL + cL L "Br "Br styrene The noncyclic double bond in styrene rapidly adds bromine, but the benzene ring remains un- affected, even if excess bromine is used. This lack of alkenelike reactivity defined the unique- ness of benzene and its derivatives to early chemists. Does benzene’s lack of reactivity have something to do with its cyclic structure? Cyclo- hexene, however, adds bromine readily. Perhaps, then, it is the cyclic structure and the con- jugated double bonds that together account for the unusual behavior of benzene. However, 1,3,5,7-cyclooctatetraene (abbreviated in this text as COT) adds bromine smoothly even at low temperature. H 55 °C L " Br (15.39) Br2 L -CHCl3 H + Br 1,3,5,7-cyclooctatetraene (100% yield) (COT) 15_BRCLoudon_pgs5-0.qxd 12/9/08 12:22 PM Page 718 718 CHAPTER 15 • DIENES, RESONANCE, AND AROMATICITY Thus, the Kekulé structure clearly had difficulties that could not be easily explained away, but there were some ingenious attempts. In 1869, Albert Ladenburg proposed a structure for ben- zene, called both Ladenburg benzene and prismane, that seemed to overcome these objections. Ladenburg benzene or prismane Although Ladenburg benzene is recognized today as a highly strained molecule (it has been described as a “caged tiger”), an attractive feature of this structure to nineteenth-century chemists was its lack of double bonds. Several facts, however, ultimately led to the adoption of the Kekulé structure. One of the most compelling arguments was that all efforts to prepare the alkene 1,3,5-cyclohexatriene using standard alkene syntheses led to benzene. The argument was, then, that benzene and 1,3,5-cyclohexatriene must be one and the same compound. The reactions used in these routes received additional credibility because they were also used to prepare COT, which, as Eq. 15.39 illustrates, has the reactivity of an ordinary alkene. Although the Ladenburg benzene structure had been discarded for all practical purposes decades earlier, its final refutation came in 1973 with its synthesis by Professor Thomas J. Katz and his colleagues at Columbia University. These chemists found that Ladenburg ben- zene is an explosive liquid with properties that are quite different from those of benzene. How, then, can the Kekulé “cyclic triene” structure for benzene be reconciled with the fact that benzene is inert to the usual reactions of alkenes? The answer to this question will occupy our attention in the next three parts of this section. B. Structure of Benzene The structure of benzene is given in Fig. 15.10a. This structure shows that benzene has one type of carbon–carbon bond with a bond length (1.395 Å) that is the average the lengths of sp2–sp2 single bonds (1.46 Å) and double bonds (1.33 Å, Fig. 15.10c). All atoms in the benzene mole- cule lie in one plane. The Kekulé structure for benzene shows two types of carbon-carbon bond: single bonds and double bonds. This inadequacy of the Kekulé structure can be remedied, how- ever, by depicting benzene as the hybrid of two equally contributing resonance structures: 1.33 Å 1.395 Å 1.395 Å (15.40) 1.395ir Å 1.395 Å 1.46 Å Kekulé structure resonance hybrid Benzene is an average of these two structures; it is one compound with one type of carbon–carbon bond that is neither a single bond nor a double bond, but something in be- tween. A benzene ring is often represented with either of the following hybrid structures, which show the “smearing out” of double-bond character: ` ` hybrid structures of benzene 15_BRCLoudon_pgs5-0.qxd 12/9/08 12:22 PM Page 719 15.7 INTRODUCTION TO AROMATIC COMPOUNDS 719 H H H H 1.08 Å H 1.467 Å H C C 1.09 Å 120° 1.344 Å C C H 1.395 Å 122.9° 119.5° H H H H H 1,3-butadiene benzene (b) (a) HH CC1.09 Å H H C C 118.3° C 126.5° C H H 1.462 Å CC 1.334 Å H H 1,3,5,7-cyclooctatetraene (COT) ball-and-stick model of COT (c) (d) Figure 15.10 Comparison of the structures of benzene, 1,3-butadiene, and COT. (a) The structure of benzene. (“Double bonds” are not shown.) (b) The structure of 1,3-butadiene, a conjugated diene. (c) Structure of 1,3,5,7- cyclooctatetraene (COT). (d) A ball-and-stick model of COT. The carbon skeleton of benzene is a planar hexagon and all of the carbon–carbon bonds are equivalent with a bond length that is the average of the lengths of car- bon–carbon single and double bonds in COT. In contrast, COT has distinct single and double bonds with lengths that are almost the same as those in 1,3-butadiene, and COT is tub-shaped rather than planar. As with other resonance-stabilized molecules, we’ll continue to represent benzene as one of its resonance contributors because the curved-arrow notation and electronic bookkeeping de- vices are easier to apply to structures with fixed bonds. It is interesting to compare the structures of benzene and 1,3,5,7-cyclooctatetraene (COT) in view of their greatly different chemical reactivities (Eqs. 15.38 and 15.39). Their structures are remarkably different (Fig. 15.10). First, although benzene has a single type of carbon–car- bon bond, COT has alternating single and double bonds, which have almost the same lengths as the single and double bonds in 1,3-butadiene. Second, COT is not planar like benzene, but instead is tub-shaped. The p bonds of benzene and COT are also different (Fig. 15.11, p. 720). The Kekulé struc- tures for benzene suggest that each carbon atom should be trigonal, and therefore sp2-hy- bridized. This means each carbon atom has a 2p orbital (Fig. 15.11a). Because the benzene molecule is planar, and the axes of all six 2p orbitals of benzene are parallel, these 2p orbitals can overlap to form six p molecular orbitals.