1. William Brown Thomas Poon www.wiley.com/college/brown Chapter Nine Benzene and Its Derivatives

2. 9-2 Benzene - Kekulé The first structure for benzene was proposed by August Kekulé in 1872. This structure, however, did not account for the unusual chemical reactivity of benzene.

3. 9-3 Benzene - Molecular Orbital Model The concepts of hybridization of atomic orbitals and the theory of resonance, developed in the 1930s, provided the first adequate description of benzene’s structure. The carbon skeleton is a regular hexagon, with all C-C-C and H-C-C bond angles 120°.

4. 9-4 Benzene - Molecular Orbital Model (a) The carbon framework; the six parallel 2p orbitals, each with one electron, are shown uncombined. (b) Overlap of the six 2p orbitals forms a continuous pi cloud, shown here as one torus above the plane of the ring, the other below it.

5. 9-5 Benzene - Resonance Model We often represent benzene as a hybrid of two equivalent Kekulé structures. Each Kekulé structure makes an equal contribution to the hybrid. The C-C bonds are neither double nor single but something in between.

6. 9-6 Benzene - Resonance Energy Resonance energy:Resonance energy: The difference in energy between a resonance hybrid and its most stable hypothetical contributing structure in which electrons are localized on particular atoms and in particular bonds. One way to estimate the resonance energy of benzene is to compare the heats of hydrogenation of benzene and cyclohexene.

7. 9-7 Benzene - Resonance Energy Heats of hydrogenation for both cyclohexene and benzene are negative (heat is liberated). These results are shown graphically on the next slide.

8. 9-8 Benzene - Resonance Energy Figure 9.2 The resonance energy of benzene, as determined by heats of hydrogenation, is approximately 36 kcal/mol.

9. 9-9 Resonance Energy resonance energies for several other aromatic hydrocarbons

10. 9-10 Concept of Aromaticity The underlying criteria for aromaticity were recognized in the early 1930s by Erich Hückel. To be aromatic, a ring must have one 2p orbital on each atom of the ring. be planar or nearly planar, so that overlap of all 2p orbitals of the ring is continuous or nearly continuous. have 2, 6, 10, 14, 18, and so forth pi electrons in the cyclic arrangement of 2p orbitals. Benzene meets these criteria It is cyclic, planar, has one 2p orbital on each atom of the ring, and has 6 pi electrons (the aromatic sextet) in the cyclic arrangement of its 2p orbitals.

11. 9-11 Heterocyclic Aromatics Heterocyclic compound:Heterocyclic compound: A compound that contains one or more atoms other than carbon in its ring. Heterocyclic aromatic compound:Heterocyclic aromatic compound: A heterocyclic compound whose ring is aromatic. Pyridine and pyrimidine are heterocyclic analogs of benzene; each is aromatic.

12. 9-12 Pyridine The nitrogen atom of pyridine is sp 2 hybridized. The unshared pair of electrons lies in an sp 2 hybrid orbital and is not a part of the six pi electrons of the aromatic sextet. Pyridine has a resonance energy of 32 kcal (134 kJ)/mol, slightly less than that of benzene.

13. 9-13 Concept of Aromaticity The five-membered-ring compounds furan, pyrrole, and imidazole are also aromatic.

14. 9-14 Concept of Aromaticity Nature abounds in compounds with a heterocyclic aromatic ring fused to another aromatic ring.

15. 9-15 Nomenclature Monosubstituted alkylbenzenes are named as derivatives of benzene. Many common names are retained.

16. 9-16 Nomenclature Phenyl and benzyl groups

17. 9-17 Nomenclature Disubstituted benzenes Locate substituents by numbering or Use the locators ortho (1,2-), meta (1,3-), and para (1,4-) Where one group imparts a special name, name the compound as a derivative of that molecule.

18. 9-18 Nomenclature Polysubstituted benzenes With three or more substituents, number the atoms of the ring. If one group imparts a special name, it becomes the parent name. If no group imparts a special name, number to give the smallest set of numbers, and then list alphabetically.

19. 9-19 Polynuclear Aromatic Hydrocarbons Polynuclear aromatic hydrocarbons (PAHs)Polynuclear aromatic hydrocarbons (PAHs) Contain two or more fused aromatic rings.

20. 9-20 Carcinogenic PAHs Benzo[a]pyrene is a carcinogen. Once absorbed, the body oxidizes it to a more soluble compound that can be excreted. The diol epoxide contains a highly reactive epoxide ring and can bind to DNA, thereby altering the structure of DNA and producing a cancer-causing mutation.

21. 9-21 Benzylic Oxidation Benzene is unaffected by strong oxidizing agents such as H 2 CrO 4 and KMnO 4. Halogen and nitro substituents are unaffected by these reagents. An alkyl group with at least one hydrogen on the benzylic carbon is oxidized to a carboxyl group.

22. 9-22 Benzylic Oxidation If there is more than one alkyl group, each is oxidized to a -COOH group. Terephthalic acid is one of the two monomers required for the synthesis of poly(ethylene terephthalate), a polymer that can be fabricated into Dacron polyester fibers and into Mylar films.

23. 9-23 Reactions of Benzene The most characteristic reaction of aromatic compounds is substitution at a ring carbon.

24. 9-24 Reactions of Benzene

25. 9-25 Electrophilic Aromatic Substitution Electrophilic aromatic substitution (EAS):Electrophilic aromatic substitution (EAS): a reaction in which an electrophile, E +, substitutes for an H on an aromatic ring. We study several common types of electrophiles. how each is generated. the mechanism by which it replaces hydrogen.

26. 9-26 Electrophilic Aromatic Substitution All occur by a common three-step mechanism Step 1:Step 1: Generation of the electrophile. Step 2:Step 2: Reaction of the electrophile with the ring. Step 3:Step 3: Proton transfer to regenerate the aromatic ring.

27. 9-27 Chlorination and Bromination Step 1Step 1: Formation of a chloronium ion.

28. 9-28 Chlorination and Bromination Step 2:Step 2: Reaction of the chloronium ion with the aromatic ring.

29. 9-29 Chlorination and Bromination Step 3:Step 3: Proton transfer to FeCl 4 - forms HCl, regenerates the Lewis acid catalyst, and gives chlorobenzene.

30. 9-30 Nitration The electrophile is NO 2 +, generated as follows.

31. 9-31 Friedel-Crafts Alkylation Friedel-Crafts alkylation forms a new C-C bond between an aromatic ring and an alkyl group.

32. 9-32 Friedel-Crafts Alkylation Step 1:Step 1: Formation of an alkyl cation as an ion pair.

33. 9-33 Friedel-Crafts Alkylation Step 2:Step 2: Reaction of the cation with the aromatic ring. Step 3:Step 3: Proton transfer regenerates the aromatic ring.

34. 9-34 Friedel-Crafts Alkylations There are two major limitations on F-C alkylations. It is practical only with stable carbocations, such as 2° and 3° carbocations. It fails on benzene rings bearing one or more strongly electron-withdrawing groups.

35. 9-35 Friedel-Crafts Acylations Treating an aromatic ring with an acid chloride in the presence of AlCl 3. Acid (acyl) chloride:Acid (acyl) chloride: a derivative of a carboxylic acid in which the -OH is replaced by a chlorine.

36. 9-36 Friedel-Crafts Acylations The electrophile is an acylium ion generated by reaction of the acid chloride and the Lewis acid catalyst.

37. 9-37 Other Benzene Alkylations Generation of carbocations Treating an alkene with a protic acid, most commonly H 2 SO 4 or H 3 PO 4. Treating an alcohol with H 2 SO 4 or H 3 PO 4.

38. 9-38 Di- and Polysubstitution orientationrateExisting groups on a benzene ring influence further substitution in both orientation and rate. Orientation ortho-para positions, metaCertain substituents direct new substitution preferentially toward ortho-para positions, others direct preferentially toward meta positions. Rate activating deactivating toward further substitution.Certain substituents are activating toward further substitution, others are deactivating toward further substitution.

39. 9-39 Di- and Polysubstitution -OCH 3 is ortho-para directing.

40. 9-40 Di- and Polysubstitution -NO 2 is meta directing.

41. 9-41 Di- and Polysubstitution Table 9.1 Effects of Substituents on Further Electrophilic Aromatic Substitution

42. 9-42 Di- and Polysubstitution Generalizations 1. Alkyl groups, phenyl groups, and substituents in which the atom bonded to the ring has an unshared pair of electrons are ortho-para directing; all other substituents are meta directing. 2. All ortho-para directing groups are activating toward further substitution; the exception to this generalization are the halogens, which are weakly deactivating. 3. All meta directing groups carry either a partial or full positive charge on the atom bonded to the ring.

43. 9-43 Di- and Polysubstitution The order of steps is important.

44. 9-44 Theory of Directing Effects The rate of electrophilic aromatic substitution The rate of EAS is determined by the slowest step in the reaction. For almost every EAS, the rate-determining step is attack of the pi bond of the aromatic ring on E + to give a resonance-stabilized cation intermediate. The more stable this cation intermediate, the faster the rate-determining step and the faster the overall reaction.

45. 9-45 Theory of Directing Effects For ortho-para directors, ortho-para attack forms a more stable cation than meta attack. Ortho-para products are formed faster than meta products. For meta directors, meta attack forms a more stable cation than ortho-para attack. Meta products are formed faster than ortho-para products.

46. 9-46 Theory of Directing Effects -OCH 3 ; assume meta attack.

47. 9-47 Theory of Directing Effects -OCH 3 ; assume para (or ortho) attack.

48. 9-48 Theory of Directing Effects -NO 2 ; assume meta attack.

49. 9-49 Theory of Directing Effects -NO 2 ; assume para attack. Contributor (e) places positive charge on adjacent atoms.

50. 9-50 Activating-Deactivating Any resonance effectAny resonance effect, such as those of -NH 2, -OH, and -OR, which delocalizes the positive charge on the cation intermediate, lowers the activation energy for its formation and activates the ring toward further EAS. Any resonance or inductive effectAny resonance or inductive effect, such as those of -NO 2, -C=O, -SO 3 H, -NR 3 +, -CCl 3, and -CF 3, which decreases electron density on the ring, deactivates the ring toward further EAS.

51. 9-51 Halogens Halogens: the resonance and inductive effects operate in opposite directions. The inductive effect:The inductive effect: the halogens are more electronegative than carbon and have an electron- withdrawing inductive effect; aryl halides, therefore, react more slowly in electrophilic aromatic substitution than benzene. The resonance effect:The resonance effect: a halogen ortho or para to the site of electrophilic attack stabilizes the cation intermediate by delocalizing the positive charge; halogen, therefore, is ortho-para directing.

52. 9-52 Phenols The functional group of a phenol is an -OH group bonded to a benzene ring.

53. 9-53 Phenols some phenols Oil of clovesPoison oak Oil of thyme

54. 9-54 Acidity of Phenols Phenols are significantly more acidic than alcohols.

55. 9-55 Acidity of Phenols The greater acidity of phenols compared with alcohols is the result of the greater stability of the phenoxide ion relative to an alkoxide ion.

56. 9-56 Acidity of Phenols Ring substituents, particularly halogens and nitro groups, increase the acidity of phenols by a combination of resonance and inductive effects.

57. 9-57 Acidity of Phenols Phenols are weak acids. They react with strong bases to form water-soluble salts. They do not react with weak bases, such as sodium bicarbonate.

58. 9-58 Acidity of Phenols Separations

59. 9-59 Phenols as Antioxidants AutoxidationAutoxidation An oxidation requiring O 2 and no other reactant. A radical chain process. hydroperoxideConverts an R-H to R-O-O-H, a hydroperoxide. We are concerned with allylic autoxidation. Allylic carbon:Allylic carbon: a carbon adjacent to a C=C.

60. 9-60 Phenols as Antioxidants Chain initiation: Chain initiation: Formation of radicals from nonradicals.

61. 9-61 Phenols as Antioxidants Chain propagation: Reaction of a radical to form a new radical. Step 2aStep 2a Step 2b:Step 2b:

62. 9-62 Phenols as Antioxidants Vitamin E is a natural antioxidant. BHT and BHA are synthetic antioxidants.

63. 9-63 Benzene and its Derivatives End Chapter 9