北京化工大学：《有机化学》课程教学资源（习题与答案）Chapter 15 Benzene and Aromaticity：Electrophilic Aromatic Substitution.pdf_大学文库

15 Benzene and Aromaticity: Electrophilic Aromatic Substitution The most obvious feature of benzene is its superficial similarity to a molecule containing ordinary double bonds. Benzene and its derivatives are special, however, as a result of the new concept of aromatic stabilization covered in the early sections of this chapter. The electronic structure of benzene, which gives this compound unusual stability relative to that of an ordinary triene, profoundly affects benzene’s chemistry. Each time you encounter a new property or chemical reaction of a benzene derivative, ask yourself, “How would the properties or reactions of a simple alkene (or diene or triene) compare with this?” See if the differences in behavior of benzene and alkenes make sense to you on thermodynamic grounds. After you’ve done that, you will be in a better position to learn the material in this and the next chapter more thoroughly, with a more balanced overview of the entire topic. Benzene and its derivatives make up one of the most important classes of organic compounds (after carbonyl compounds and alcohols). There is, accordingly, a lot of relatively significant material presented in these chapters concerning them. Other kinds of aromatic compounds exist besides simple benzene derivatives. Two common types will be covered in this chapter: polycyclic fused benzenoid hydrocarbons and other cyclic conjugated polyenes with either more or less than six carbons in the ring. In Chapter 25, a third common class, heterocyclic aromatic compounds, will be presented. Outline of the Chapter 15-1 Nomenclature A few new twists. 15-2, 15-3 Structure of Benzene: A First Look at Aromaticity What makes it special. 15-4 Spectral Properties 15-5 Polycyclic Aromatic Hydrocarbons 15-6, 15-7 Other Cyclic Polyenes: Hückel’s Rule Getting to the heart of the matter: Whence aromaticity? 15-8, 15-9, 15-10 Electrophilic Aromatic Substitution 15-11, 15-12, 15-13 Friedel-Crafts Reaction The basic “core” reactions in benzene chemistry. 276 1559T_ch15_276-290 11/03/05 20:24 Page 276

Keys to the Chapter 15-1. Nomenclature The systematic (IUPAC) naming system for benzenes coexists with an assortment of names that have been in common use for over a century and show no signs of going away. So, whether anyone likes it or not, terms like aniline (for benzenamine) and styrene (for ethenylbenzene) are, and will probably always be, the ones people use, both in speaking as well as writing about these compounds. In addition to these common names for simple substituted benzenes, a special system exists exclusively for benzenes with exactly two substituents on the benzene ring: the “ortho/meta/para” system. Note, carefully, that this method is never to be used for benzenes with more than two ring substituents: For those, a name with proper numbering is required. (It is okay, however, to use numbering instead of ortho/meta/para for a benzene with two substituents.) Finally, when numbering around the benzene ring, be careful where you start: If you use one of the special names for a monosubstituted benzene as your parent name (e.g., phenol, aniline, benzaldehyde, toluene), C1 is always the carbon containing the substituent implied by the parent (even if starting somewhere else would result in smaller numbers). For example, this compound is 3,4-dibromotoluene (1,2-dibromotoluene would be nonsense, although 1,2-dibromo-4-methylbenzene is the correct IUPAC name). 15-2 and 15-3. Structure of Benzene: A First Look at Aromaticity; Molecular Orbitals Aromaticity as a special property of molecules like benzene is covered in these sections. Structural, thermodynamic, and electronic considerations are presented and serve as an introduction to the more general discussion that is presented in Section 15-6. For now, note simply that the aromaticity of benzene is reflected in (1) its symmetrical structure (as a resonance hybrid), (2) its unexpectedly enhanced thermodynamic stability, and (3) its unusual electronic structure with a completely filled set of strongly stabilized bonding molecular orbitals. 15-4. Spectral Properties The spectroscopy of benzene is a logical consequence of its structural and electronic properties. Special features in the spectra make the identification of benzenes relatively easy. As has been the case with other compounds, NMR is most useful, followed by IR and UV spectroscopy. A special feature in the infrared spectrum of a benzene derivative is the COH out-of-plane bending vibration pattern, which provides information concerning the arrangement of substituents around the ring. This information is not always as easily derived from the NMR spectrum. Ultraviolet spectroscopy can be suggestive but not definitive regarding the presence of a benzene derivative. If the presence of the benzene ring is already known from, e.g., NMR, the UV spectrum can be useful in deciding whether it is conjugated with one or more bond-containing groups. 15-5. Polycyclic Aromatic Hydrocarbons The fusion of benzene rings leads to a large class of polycyclic hydrocarbons, beginning with naphthalene as the simplest (and only bicyclic) example. 15-6 and 15-7. Other Cyclic Polyenes: Hückel’s Rule Really very simple. For a molecule to be aromatic, it must have (4n 2) electrons contained in a complete, unbroken circle of p orbitals. CH3 Br Br 1 2 3 4 Keys to the Chapter • 277 1559T_ch15_276-290 11/03/05 20:24 Page 277

15-8, 15-9, and 15-10. Electrophilic Aromatic Substitution: Halogenation, Nitration, and Sulfonation of Benzene These sections introduce the main type of chemistry displayed by benzene rings. The keys to this material are in Section 15-8. You are given a general mechanism (which is repeated later in specific form for each type of reaction that is presented). More important for understanding the concepts, you are given information concerning the energetics of this reaction. Pay close attention to Figure 15-20, Exercise 15-21, and the data in Section 15-8. After you understand this basic material, you will find the specific reactions a little easier to learn. Remember that benzene is stabilized by its special aromatic form of resonance. It is less reactive toward electrophiles than are ordinary alkenes; and therefore only strong electrophiles, sometimes generated in strange ways, will attack the electrons in benzene rings. These electrophiles, and the ways they are generated, have to be memorized so that you can use these reactions later on in synthesis problems. 15-11, 15-12, and 15-13. Friedel-Crafts Reaction Because carbon–carbon bond formation is such an important part of organic synthesis, the attachment of carbon electrophiles to benzene rings is of special significance among the reactions in this chapter. Ordinary carbocations and carbocationlike species are the simplest types of carbon electrophiles, but their use in FriedelCrafts alkylation has limitations. Rearrangements of the carbon electrophile often occur, and it is hard to prevent multiple alkylation from happening to a single benzene ring. Friedel-Crafts alkanoylation (acylation), via the acylium ion [ROC GPhSO mn ROCqO GS], is not subject to these drawbacks and, whenever possible, is the preferred method for attachment of a carbon unit to a benzene ring. Solutions to Problems 33. (a) 3-Chlorobenzenecarboxylic acid, m-chlorobenzoic acid (b) 1-Methoxy-4-nitrobenzene, p-nitroanisole (c) 2-Hydroxybenzenecarbaldehyde, o-hydroxybenzaldehyde (d) 3-Aminobenzenecarboxylic acid, m-aminobenzoic acid (e) 4-Ethyl-2-methylbenzenanime, 4-ethyl-2-methylaniline (f) 1-Bromo-2,4-dimethylbenzene (g) 4-Bromo-3,5-dimethoxybenzenol, 4-bromo-3,5-dimethoxyphenol (h) 2-Phenylethanol (i) 3-Ethanoylphenanthrene, 3-acetylphenanthrene 34. (a) 1,2,4,5-Tetramethylbenzene (b) 4-Hexyl-1,3-benzenediol (c) 2-Methoxy-4-(2-propenyl)benzenol 35. (a) Name is acceptable. (IUPAC: 2-chlorobenzenecarbaldehyde) (b) Name is numbered incorrectly; call it 1,3,5-benzenetriol. HO OH OH C O H Cl 278 • Chapter 15 BENZENE AND AROMATICITY: ELECTROPHILIC AROMATIC SUBSTITUTION 1559T_ch15_276-290 11/03/05 20:24 Page 278

45. (c) (f) is shown in the answer to Problem 44. 46. The problem is twofold: What should we make C6D6 from, and how should we do it? The chapter does not give us any practical way to construct benzene rings from nonbenzenoid starting materials, but it does present methods to replace groups on benzene rings—electrophilic aromatic substitution. We consider what we want to accomplish: the attachment of deuterium, D, to every carbon. A plausible electrophile to consider is the deuterium ion, D. In Section 15-10, we see that hydrogen ions, H, are sufficiently electrophilic to attack benzene rings (in that case, benzenesulfonic acid, leading to replacement of the OSO3H group by OH). We could therefore expect that D would behave similarly, attacking benzenesulfonic acid and replacing the OSO3H group by OD. Indeed, it does, but is that really the best way to solve our problem? That reaction in Section 15-10 was presented to show how to get rid of the OSO3H group. But do we need that group in the first place for what we’re trying to accomplish here—replace H by D? The chemistry in Section 15-10 tells us that D can attack a benzene ring as an electrophile. Therefore, we can assume that it can attack benzene itself: Run in the forward direction, we’ve replaced an H with a D! But the reaction is an equilibrium: We need to drive it from left to right. The solution is to treat benzene with a large excess of an acidic solution containing D instead of H, dilute D2SO4 in D2O, for example. The reaction goes to equilibrium, replacing most of the benzene COH bonds with COD bonds, according to the equation above. We repeat the process several times, each time treating the partially deuterated benzene with fresh D2SO4 in D2O, until the amount of residual H in the benzene has been reduced to an acceptably low level (typically well below 1%). 47. Identify a likely electrophilic atom and follow a reasonable mechanistic pattern. H2O then reacts with excess ClSO3H to make H2SO4 and HCl. Note: This is just one of several possible parallel mechanisms. H2O S O O Cl O S S Cl O H O OH Cl HO O H HO You need to make an OH into a leaving group. O S O Cl O OH2 S O Cl D D H D H OH H (CH3)3C (CH3)3C OH2 (CH3)3C H2O C(CH3)3 (CH3)3C H H Solutions to Problems • 283 1559T_ch15_276-290 11/03/05 20:24 Page 283