15597ch15276-29011/03/0520:24Page276 风 EQA 15 Benzene and Aromaticity:Electrophilic Aromatic Substitution eredn the arly sections of this chapter.stucture of benzene.which gives this compoundun or reactions ofa simple or diene or tricne)compar with this?sce if the differences in behavior of nced overview Benzene and its derivatives make up one of the most important classes of organic compounds(after car bonyl compounds and cohols).There is,accordingly.a lot of relatively significant material presented in thes Other kinds of aromatic compounds exist besides simple benzene derivatives.Two common types will be Outline of the Chapter 15-1 Nomenclature A few new twists 12.F Aromily 15-4 Spectral Properties 15-5 Polycyclic Aromatic Hydrocarbons 15-6,15-7 Hng【o 15-8,15-9,15-10 Electrophilic Aromatic Substitution 11,112,1513 sin benzene chemistry 276
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
1559T_ch15_276-29011/03/0520:24Pa9e277 ⊕ EQA Keys to the Chapter·277 Keys to the Chapter 15-1.Nomenclature The systematic (TUPAC)naming system for benzenes coexists with an assortment of names that have been in it or not,term ple use.both in speaking as well as writing about these compounds.In addition to imple substituted benzer a special system exists exclusiv y for ber zenes wit h exactly two substit ents on instead of orho/meta/para for abenzene ith two substituents)Finally.whenum ene ring.be care )Cl is a bon contining the substituent implied by the parent (even if starting would result in smaller ermo sion that is presented in Section 15-6.For now.note simply that the of benzene is in() 15-4. Spectral Prope e of its str ies.Special fea tures in the with other com nds.NMR s most u by IR and UV copy.A special fe ature in the infrared spectrun ing the arrangem t of substituents around the ring.This infor mation is not aways aseasily derived from the NMR spectrum. of can be usefulin decidin whether it is conjugated with one or morebond-contann spectru 15-5 Pol the simplest (and only bicyclic)example. 15-6and15-7. Other Cyclic Poly nes:Huckel's Rule to be aromatic.it must have (n)etron contained in a complcte
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
1559Tch15276-29011/03/0520:24Page278 EQA 278.Chapter 15 BENZENE AND AROMATICITY:ELECTROPHILIC AROMATIC SUBSTITUTION 15-8,15-9,and15-10. Elect atic Substitution:Halogenation, E.I on These sections introduce the main ty gs.Thene ys to this material ar in Section 15-8.You are given a general mechanism (which is repeated later in specific form for each type of More imp or und ing the c tion 15-8.Aftery ou und d this bas Rem er that benz e is s atic form of res nce.It is less reactive to ward el 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-ca n bond formation is such an important part of organic synthesis,the attachment of car- bon electrophiles to benze rings is of special significance among the reaction are the sim of the multiple alkyla iofrom happening toa single benzene ring.Friedel-Crafts (acylation),via the for attach of a carbon Solutions to Problems 33.(a)3-Chlorobenzenecarboxylic acid,mchorobenzoic acid ⊕ (b)1-Methoxy-4-nitrobenzene.p-nitroanisole (c)2-Hydroxybenzenecarbaldehyde.-ydroxybenzaldehyde (d)3-Aminobenzenecarboxylic acid,m-aminobenzoic acid (e)4-Ethyl-2-methylbenzenanime.4-ethyl-2-methylanilin (f)1-Bromo-2,4-dimethylbenzene (g)4-Bromo-3.5-dimethoxybenzenol.4-bromo-3.5-dimethoxypheno (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 H 35.(a) Name is acceptable.(IUPAC:2-chlorobenzenecarbaldehyde) OH Name is numbered incorrectly;call it 1.3.5-benzenetriol OH
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
1559T_ch15_276-29011/03/0520:24Pa9e279 EQA Soufionso Problems7 CH: Name is acceptable.(IUPAC:-(1-methylethyl)-benzenecarboxylic acid CH(CH2)2 ong numbers:3.4-dibromoaniline or 3.4-dibromobenzenamine. CH,O CH:Use only numbers:4-methoxy-3-nitroacetophenone or 1-(4-methoxy-3-nitrophenyl)ethanone. 36.Benzene would be higher in energy by about 30 kcal mol-1.soH would be-819 kcal mol- ons 1 4 5 and g are deshielded because thev ays:the ring curren es ca oo eh e nne e
(c) Name is incorrect. Never mix o, m, p with numbers; call it 1,2-dimethyl-4-nitrobenzene. (d) Name is acceptable. (IUPAC: 3-(1-methylethyl)-benzenecarboxylic acid) (e) Wrong numbers; 3,4-dibromoaniline or 3,4-dibromobenzenamine. (f) Use only numbers: 4-methoxy-3-nitroacetophenone or 1-(4-methoxy-3-nitrophenyl)ethanone. 36. Benzene would be higher in energy by about 30 kcal mol1 , so Hcomb would be 819 kcal mol1 . 37. The hydrogens at carbons 1, 4, 5, and 8 are deshielded because they are closer to the other benzene ring in the molecule. They feel the deshielding effects of -electron ring currents three ways: the ring current around the whole molecule (i), the ring current of their own benzene ring (ii), and the ring current of the adjacent benzene ring (iii): The hydrogens at carbons 2, 3, 6, and 7 are too far away to feel a significant amount of the ring current of the other benzene ring. H H (i) (ii) (iii) H H8 H1 H5 H4 H 7.77 7.40 NO2 CH3O CCH3 O NH2 Br Br CH(CH3)2 COOH NO2 CH3 CH3 Solutions to Problems • 279 1559T_ch15_276-290 11/03/05 20:24 Page 279
15597_eh15.276-29011/03/0520:24Page280 280.chapter 15 BENZENE AND AROMATICITY:ELECTROPHILIC AROMATIC SUBSTITUTION 38.Yes .Cyclo ctat traene is described as 两caionoe the6 cnergyh女gnn山> e Aromaticity ir )coa complete,unbroken cirle ofp (a)No.3 electrons.(b)Yes.Benzene is intact:extra double bond is an irrelevant substituent.not being part of the c (c)No.Ih ed ca (d)y here is does not interrupt the p orbital circle.(e)No.:wron number rated ning fusion ca ave B aeonds orimds The IR(single band at 745 cm)agrees with the conclusion (b)'H NMR:four benzene H's (one quite different from the other three).CHaO-(=3.7).IR:meta disubstituted benzene.The answer is symmetry to it The answer (usetrial and emror)is
38. Yes. Cyclooctatetraene is described as lacking any special stabilization such as aromaticity. As a result, hydrogenation of its four double bonds should give off four times the energy that hydrogenating one double bond releases. The data indicate that this is approximately the case. 39. Rule: Aromaticity requires [a] (4n 2) electrons contained in [b] a complete, unbroken circle of p orbitals. (a) No. 3 electrons. (b) Yes. Benzene is intact; extra double bond is an irrelevant substituent, not being part of the circle. (c) No. The saturated carbon is sp3 , breaking the circle of p orbitals; without a circle of p orbitals, the number of electrons is irrelevant. (d) Yes. 10 electrons; the sp3 carbon here is bridging and does not interrupt the p orbital circle. (e) No. 12 electrons; wrong number. (f) No. 9 bonds 18 electrons, which would be fine, except that the 2 charge adds 2 more for a total of 20 electrons; wrong number. (g) No. Saturated ring fusion carbons interrupt circle. 40. (a) UV spectrum supports the presence of benzene ring. 13C NMR: three peaks, so the molecule must have symmetry. 1 H NMR: two sets of signals in equal intensity. Look at the three possible dibromobenzenes. The answer is clear: The IR (single band at 745 cm1 ) agrees with the conclusion. (b) 1 H NMR: four benzene H’s (one quite different from the other three), CH3OO ( 3.7). IR: meta disubstituted benzene. The answer is (c) 1 H NMR: two benzene H’s, three CH3’s (two equivalent; note just two methyl carbons in 13C NMR). Also, there are just four benzene carbons in 13C spectrum, so the molecule has some symmetry to it. The answer (use trial and error) is Br CH3 CH3 CH3 Br OCH3 Br Br 13C: 2 kinds of carbons 1 H: All equivalent 4 kinds of carbons 3 kinds of hydrogens p Br Br m Br o Br Ortho isomer is the answer. 3 kinds of carbons 2 kinds of hydrogens 280 • Chapter 15 BENZENE AND AROMATICITY: ELECTROPHILIC AROMATIC SUBSTITUTION 1559T_ch15_276-290 11/03/05 20:24 Page 280
1559rch15276-29011/03/0520:24Page282 EQA 282.Chapter 15 BENZENE AND AROMATICITY:ELECTROPHILIC AROMATIC SUBSTITUTION Assign the signals at 136.9 and 1866 ppm more precisely by looking at the resonance forms ǒ-ǒ-ǒ erefore correspon eventually (Compare Exercise 15-22) CH), olving ⊕ n6aiandtg-aa (CH)C-CH-CHs
Assign the signals at 136.9 and 186.6 ppm more precisely by looking at the resonance forms: In the resonance forms, positive charges are located at only three of the carbons: They should be the most deshielded. That explains the 178.1 chemical shift of the “bottom” carbon. The 186.6 signal must therefore correspond to the positively charged “end” carbons of the delocalized cation: 44. (a) (b) initially; eventually (Compare Exercise 15-22) (c), (e) (d) (f) (g) (h) CH3 C O CH3 CH3 CH3 CH3 (CH3)2C CH(CH3)2 (CH3)2CH CH3 CH3 C (CH3)2CH CH3 CH3 C H H (CH3)2C CH CH3 CH3 AlCl4 CH3 shift Careful! (CH3)3CCH CH2 Cl AlCl3 H H shift NO2 C(CH3)3 Friedel-Crafts alkylation involving (CH3)3C cation T T T T T T Cl T 186.6 So this is 136.9 H H H H H H 282 • Chapter 15 BENZENE AND AROMATICITY: ELECTROPHILIC AROMATIC SUBSTITUTION 1559T_ch15_276-290 11/03/05 20:24 Page 282
1559T_ch15_276-29011/03/0520:24Pa9e283 EQA Solutions to Problems.283 45.ac-一ac4ac C(CH3) (f)is shown in the answer to Problem 44. present methods to replace groups on ben zene nngs benzene rings (in that case,benzer esufonc cid,to replacement of the-SO,H group byH)We could presented to show how to get rid of theH grou.But do we need thatg up in the first place for what ⊕ the n Hwith aD!But the reaction:We eed to most of the benzene C -H bonds with 47.Identify a likely electrophilicatom and follow a reasonable mechanistic patter 0+*一 了o You need to make an OH into a leaving group. H,0+ then ad
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
1559r.ah15276-29011/02/0520:24Page284 284.Chapter 15 BENZENE AND AROMATICITY:ELECTROPHILIC AROMATIC SUBSTITUTION 48.Consider a mechanism similar to Friedel-Crafts alkylation.Twice. a-s-aa=a--ia,- C◇-C℃ 49.(a)Think mechanistically.Then see if what you come up with matches the data CoHoClO -O 益
48. Consider a mechanism similar to Friedel-Crafts alkylation. Twice. 49. (a) Think mechanistically. Then see if what you come up with matches the data. (b) H2, Pd-C, CH3CH2OH O OH NaBH4, CH3CH2OH Conc. H2SO4, 100°C C9H8O O H O C9H9ClO AlCl3 Cl O O AlCl3; think Friedel-Crafts akanoylation S S H AlCl4 S Cl AlCl3 H AlCl4 ClS ClS AlCl3 Cl S Cl AlCl 3 Cl S Cl AlCl3 284 • Chapter 15 BENZENE AND AROMATICITY: ELECTROPHILIC AROMATIC SUBSTITUTION 1559T_ch15_276-290 11/03/05 20:24 Page 284
1559T_ch15_276-29011/7/0523:21Pa9e285 ⊕ EQA Soutions to Problems285 50 CH+E CH3-E+H Reaction coordinate- OH 51.(a)CoHsCHCH3 (b)CoHsCH2CH2OH a)C. (h) IO CH Mg.(CHCH)O
50. The reaction of methylbenzene should proceed with a lower energy of activation than that of benzene (Ea methylbenzene Ea benzene), and the intermediate cation should be more stable. OH A 51. (a) C6H5CHCH3 (b) C6H5CH2CH2OH 52. More than one approach can be used for both (a) and (b): one based on Friedel-Crafts alkanoylation (acylation), which uses an alkanoyl chloride, and another using Grignard addition to an aldehyde or a ketone. If you got one but not the other, try now to give a second answer before peeking at the solutions below. (a) (b) 1. CH3CH2CCl, AlCl3 2. H, H2O 1. CH3MgI, (CH3CH2)2O 2. H, H2O Br2, FeBr3 O Mg, (CH3CH2)2O O 1. CH3CH2CCH3 2. H, H2O O CH2CH3 C CH2CH3 C HO CH3 CH2CH3 C HO CH3 Br MgBr 1. CH3(CH2)5CCl, AlCl3 2. H, H2O NaBH4, CH3CH2OH Br2, FeBr3 C O (CH2)5CH3 (CH2)5CH3 C HO H Mg, (CH3CH2)2O Br MgBr O 1. CH3(CH2)5CH 2. H, H2O O (CH2)5CH3 C HO H Solutions to Problems • 285 1559T_ch15_276-290 11/7/05 23:21 Page 285
1559r.eh15276-29011/03/0520:24Pag8286 EQA 286.chapter 15 BENZENE AND AROMATICITY:ELECTROPHILIC AROMATIC SUBSTITUTION (e)Friedel-Crafts akyation is liable to lead to rearranged products.Use alkanoylation-reduction a OH ICH-CH CHCH SCH2 CH-CH-CH2 -H-O CH O OH CH-CHCHs H-O OH 54.Cyclooctatetraene lacks resonance stabilization;its double bonds behave as if they were isolated,not cule (wnic CH ,8-dimetnylcyclooctaletraene,respectively
(c) Friedel-Crafts alkylation is liable to lead to rearranged products. Use alkanoylation-reduction. 53. (a) (b) (c) 54. Cyclooctatetraene lacks resonance stabilization; its double bonds behave as if they were isolated, not conjugated. This is in fact the case. As a result of the geometry of the molecule (which is not planar; see Figure 15-17), the double bonds do not overlap into conjugated systems. So, resonance does not occur: The two structures above actually represent different molecules! Their names are 1,2-dimethylcyclooctatetraene and 1,8-dimethylcyclooctatetraene, respectively. CH3 CH3 CH3 CH3 CH CHCH3 OH CH3O H2O CH CH CH2 OH CH3O CH2 CH CH OH CH3O CHCH CH2 OH CH3O HO H H2O O OCH3 CH3O H CH2OH OH CH3O 1. NaBH4 2. HBr 3. LiAlH4 1. CH3(CH2)6CCl, AlCl3 2. H, H2O O O (CH2)7CH3 (CH2)6CH3 C 286 • Chapter 15 BENZENE AND AROMATICITY: ELECTROPHILIC AROMATIC SUBSTITUTION 1559T_ch15_276-290 11/03/05 20:24 Page 286