1. 21-1 Benzene and and the Concept of Aromaticity Chapter 21

2. 21-2 21.1 A. Benzene - Kekulé  Discovered by Michael Faraday in 1825. (#C = #H)  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. 21-3 Benzene  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 all C-C-C and H-C-C bond angles 120°.

4. 21-4 B. Benzene - Molecular Orbital Model  The linear combination of six overlapping p orbitals must form six molecular orbitals.  Three will be bonding, three antibonding.  Lowest energy MO will have all bonding interactions, no nodes.  As energy of MO increases, the number of nodes increases.

5. 21-5 Benzene - Molecular Orbital Model Orbitals of equal energy are degenerate. Antibonding orbitals are starred (*). Electrons in the orbitals of lowest energy are in the “ground state”.

6. 21-6 Benzene - Molecular Orbital Model  The pi system of benzene: (a) the carbon framework with the six 2p orbitals. (b) overlap of the parallel 2p orbitals forms one torus above the plane of the ring and another. below it. this orbital represents the lowest-lying pi-bonding molecular orbital. Figure 21.2

7. 21-7 Benzene - Molecular Orbitals Viewed from the top of each carbon atom

8. 21-8 C. Benzene - Resonance Model  We often represent benzene as a hybrid of two equivalent Kekulé structures. each makes an equal contribution to the hybrid and thus the C-C bonds are neither double nor single, but something in between.

9. 21-9 Benzene - Resonance  Resonance energy:  Resonance energy: the difference in energy between a resonance hybrid and the most stable of its hypothetical contributing structures 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.

10. 21-10 Benzene, Fig 21.3

11. 21-11 21.2 A. Concept of Aromaticity aromaticity  The underlying criteria for aromaticity were recognized in the early 1930s by Erich Hückel, based on molecular orbital (MO) calculations.  To be aromatic, a compound must: 1. be cyclic. 2. have one p orbital on each atom of the ring (sp 2 ). 3. be planar or nearly planar so that there is continuous or nearly continuous overlap of all p orbitals of the ring. 4. have a closed loop of (4n + 2) pi electrons in the cyclic arrangement of p orbitals.

12. 21-12 Frost Circles (Polygon Rule)  Frost circle:  Frost circle: a graphic method for determining the relative order of pi MOs in planar, fully conjugated monocyclic compounds. inscribe a polygon of the same number of sides as the ring to be examined such that one of the vertices is at the bottom of the ring. the relative energies of the MOs in the ring are given by where the vertices touch the circle  Those MOs: below the horizontal line through the center of the ring are bonding MOs. on the horizontal line are nonbonding MOs. above the horizontal line are antibonding MOs.

13. 21-13 Frost Circles, Fig 21.4 following are Frost circles describing the MOs for monocyclic, planar, fully conjugated four-, five-, and six-membered rings.

14. 21-14 B. Aromatic Hydrocarbons  Annulene:  Annulene: a cyclic hydrocarbon with a continuous alternation of single and double bonds. [14]annulene is aromatic according to Hückel’s criteria.

15. 21-15 Aromatic Hydrocarbons [18]annulene is also aromatic.

16. 21-16 Aromatic Hydrocarbons according to Hückel’s criteria, [10]annulene should be aromatic; it has been found, however, that it is not. nonbonded interactions between the two hydrogens that point inward toward the center of the ring force the ring into a nonplanar conformation in which overlap of the ten 2p orbitals is no longer continuous.

17. 21-17 Aromatic Hydrocarbons what is remarkable relative to [10]annulene is that if the two hydrogens facing inward toward the center of the ring are replaced by a methylene (CH 2 ) group, the ring is able to assume a conformation close enough to planar that it becomes aromatic.

18. 21-18 C. Antiaromatic Hydrocarbons  Antiaromatic hydrocarbon:  Antiaromatic hydrocarbon: a monocyclic, planar, fully conjugated hydrocarbon with 4n pi electrons (4, 8, 12, 16, 20...), it does not obey Huckel’s rule. an antiaromatic hydrocarbon is especially unstable relative to an open-chain fully conjugated hydrocarbon of the same number of carbon atoms.  Cyclobutadiene is antiaromatic. in the ground-state electron configuration of this molecule, two electrons fill the  1 bonding MO. the remaining two electrons lie in the  2 and  3 nonbonding MOs.

19. 21-19 Cyclobutadiene, Fig. 21.5 planar cyclobutadiene has two unpaired electrons, which make it highly unstable and reactive.

20. 21-20 Cyclooctatetraene cyclooctatetraene, with 8 pi electrons is not aromatic; it shows reactions typical of alkenes. x-ray studies show that the most stable conformation is a nonplanar “tub’ conformation.

21. 21-21 Cyclooctatetraene 2p orbital overlap forms each pi bond, there is essentially no overlap between adjacent alkenes.

22. 21-22 Cyclooctatetraene, Fig. 21.6 planar cyclooctatetraene, if it existed, would be antiaromatic. it would have unpaired electrons in the  4 and  5 nonbonding MOs.

23. 21-23 D. Heterocyclic Aromatics  Heterocyclic compound:  Heterocyclic compound: a compound that contains more than one kind of atom in a ring. in organic chemistry, the term refers to a ring with one or more atoms are other than carbon.  Pyridine and pyrimidine are heterocyclic analogs of benzene; each is aromatic.

24. 21-24 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 system. pyridine has a resonance energy of 134 kJ (32 kcal)/mol, slightly less than that of benzene.

25. 21-25 Furan, Fig. 21.7 the oxygen atom of furan is sp 2 hybridized. one unshared pairs of electrons on oxygen lies in an unhybridized 2p orbital and is a part of the aromatic sextet. the other unshared pair lies in an sp 2 hybrid orbital and is not a part of the aromatic system. the resonance energy of furan is 67 kJ (16 kcal)/mol.

26. 21-26 Other Heterocyclics

27. 21-27 Indole N H N H CH 2 CH 2 NH 2 Serotonin (a neurotransmitter) HO Caffeine N N N N O O H 3 C CH 3 CH 3 Purine N N N N H

28. 21-28 E. Aromatic Hydrocarbon Ions  Any neutral, monocyclic unsaturated hydrocarbon with an odd number of carbons must have at least one CH 2 group and, therefore, cannot be aromatic. cyclopropene, for example, has the correct number of pi electrons to be aromatic, 4(0) + 2 = 2, but does not have a closed loop of 2p orbitals.

29. 21-29 Cyclopropenyl Cation if, however, the CH 2 group of cyclopropene is transformed into a CH + group in which carbon is sp 2 hybridized and has a vacant 2p orbital, the overlap of orbitals is continuous and the cation is aromatic.

30. 21-30 Cyclopropenyl Cation when 3-chlorocyclopropene is treated with SbCl 5, it forms a stable salt. this chemical behavior is to be contrasted with that of 5-chloro-1,3-cyclopentadiene, which cannot be made to form a stable salt.

31. 21-31 Cyclopentadienyl Cation if planar cyclopentadienyl cation existed, it would have 4 pi electrons and be antiaromatic. note that we can draw five equivalent contributing structures for the cyclopentadienyl cation; yet this cation is not aromatic because it has only 4 pi electrons.

32. 21-32 Cyclopentadienyl Anion  To convert cyclopentadiene to an aromatic ion, it is necessary to convert the CH 2 group to a CH group in which carbon becomes sp 2 hybridized and has 2 electrons in its unhybridized 2p orbital. pKa = ~16

33. 21-33 Cyclopentadienyl Anion as seen in the Frost circle, the six pi electrons occupy the  1,  2, and  3 molecular orbitals, all of which are bonding.

34. 21-34 Cyclopentadienyl Anion  The pK a of cyclopentadiene is 16. in aqueous NaOH, it is in equilibrium with its sodium salt. it is converted completely to its anion by very strong bases such as NaNH 2, NaH, and LDA.

35. 21-35 MOs of Aromatic Ions  Cyclopropenyl cation and cyclopentadienyl anion.

36. 21-36 pKa's of some hydrogens

37. 21-37 Cycloheptatrienyl Cation  Cycloheptatriene forms an aromatic cation by conversion of its CH 2 group to a CH + group with its sp 2 carbon having a vacant 2p orbital.

38. 21-38 21.3 A. Nomenclature  Monosubstituted alkylbenzenes are named as derivatives of benzene. many common names are retained. TolueneCumeneEthylbenzeneStyrene PhenolAnilineBenzoic acidAnisole COOHNH 2 OCH 3 OH Benzaldehyde CHO

39. 21-39 Nomenclature  Benzyl and phenyl groups. (Z)-2-Phenyl- 2-butene 4-(3-Methoxyphenyl)- 2-butanone 1-Phenyl-1pentanone OO H 3 CO Ph BenzenePhenyl group, Ph-TolueneBenzyl group, Bn- CH 3 CH 2 -

40. 21-40 B. Disubstituted Benzenes orthometapara  Locate two groups by numbers or by 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.

41. 21-41 Disubstituted Benzenes where neither group imparts a special name, locate the groups and list them in alphabetical order.

42. 21-42 C. Polysubstituted Derivatives if one group imparts a special name, name the molecule as a derivative of that compound. if no group imparts a special name, list them in alphabetical order, giving them the lowest set of numbers. CH 3 NO 2 OH Br Br NO 2 CH 2 CH 3 Br 4 2 1 6 4 2 1 4 1 2 4-Chloro-2-nitro- toluene 2,4,6-Tribromo- phenol 2-Bromo-1-ethyl-4- nitrobenzene Br Cl

43. 21-43 21.4 A. Phenols phenol  The functional group of a phenol is an -OH group bonded to a benzene ring. 1,2-Benzenediol (Catechol) 1,4-Benzenediol (Hydroquinone) 3-Methylphenol (m-Cresol) Phenol OH OH OH OH OH CH 3 OH 1,3-Benzenediol is resorcinol

44. 21-44 Phenols hexylresorcinol is a mild antiseptic and disinfectant. eugenol is used as a dental antiseptic and analgesic. urushiol is the main component of the oil of poison ivy.

45. 21-45 B. Acidity of Phenols  Phenols are significantly more acidic than alcohols, compounds that also contain the OH group.

46. 21-46 Acidity of Phenols the greater acidity of phenols compared with alcohols is due to the greater stability of the phenoxide ion relative to an alkoxide ion.

47. 21-47 Acidity of Phenols  Alkyl and halogen substituents effect acidities by inductive effects. alkyl groups are electron-releasing. halogens are electron-withdrawing.

48. 21-48 Acidity of Phenols nitro groups increase the acidity of phenols by both an electron-withdrawing inductive effect and a resonance effect.

49. 21-49 Acidity of Phenols part of the acid-strengthening effect of -NO 2 is due to its electron-withdrawing inductive effect. in addition, -NO 2 substituents in the ortho and para positions help to delocalize the negative charge.

50. 21-50 C. Acid-Base Reactions of Phenols  Phenols are weak acids and react with strong bases to form water-soluble salts. water-insoluble phenols dissolve in NaOH(aq).

51. 21-51 Acid-Base Reactions of Phenols most phenols do not react with weak bases such as NaHCO 3 ; they do not dissolve in aqueous NaHCO 3 No reaction

52. 21-52 D. Alkyl-Aryl Ethers  Alkyl-aryl ethers can be prepared by the Williamson ether synthesis. but only using phenoxide salts and haloalkanes. haloarenes are unreactive to S N 2 reactions.  The following two examples illustrate: the use of a phase-transfer catalyst. the use of dimethyl sulfate as a methylating agent. no reaction + X RO - Na +

53. 21-53 Alkyl-Aryl Ethers

54. 21-54 E. Kolbe Carboxylation  Phenoxide ions react with carbon dioxide to give a carboxylic salt.

55. 21-55 Kolbe Carboxylation the mechanism begins by nucleophilic addition of the phenoxide ion to a carbonyl group of CO 2.

56. 21-56 F. Quinones  Because of the presence of the electron-donating -OH group, phenols are susceptible to oxidation by a variety of strong oxidizing agents.  Quinones  Quinones are six-membered rings with two C=O. H 2 CrO 4 Phenol 1,4-Benzoquinone (p-Quinone) O O OH

57. 21-57 Quinones

58. 21-58 Quinones  Perhaps the most important chemical property of quinones is that they are readily reduced to hydroquinones by sodium hydrosulfite.

59. 21-59 Coenzyme Q  Coenzyme Q is a carrier of electrons in the respiratory chain.

60. 21-60 Vitamin K both natural and synthetic vitamin K (menadione) are 1,4-naphthoquinones.

61. 21-61 21.5 A. Benzylic Oxidation  Benzene is unaffected by strong oxidizing agents such as H 2 CrO 4 and KMnO 4. halogen and nitro substituents are also unaffected by these reagents. benzyliccarbonan alkyl group with at least one hydrogen on its benzylic carbon is oxidized to a carboxyl group.

62. 21-62 Benzylic Oxidation if there is more than one alkyl group on the benzene ring, each is oxidized to a -COOH group. an aryl-COOH is the oxidation product regardless of the alkyl group that was attached to the aromatic ring (may be Me, Et, Pr, Bu, vinyl, etc.). 1,4-Dimethylbenzene (p-xylene) 1,4-Benzenedicarboxylic acid (terephthalic acid) CH 3 H 2 SO 4 K 2 Cr 2 O 7 H 3 C COH O HOC O

63. 21-63 B. Benzylic Chlorination  Chlorination (and bromination) occurs by a radical mechanism.

64. 21-64 Benzylic Reactions  Benzylic radicals (and cations also) are easily formed because of the resonance stabilization of these intermediates. the benzyl radical is a hybrid of five contributing structures.

65. 21-65 Benzylic Halogenation benzylic bromination is highly regioselective. benzylic chlorination is less regioselective.

66. 21-66 C. Hydrogenolysis  Hydrogenolysis: Cleavage of a single bond by H 2 : among ethers, benzylic ethers are unique in that they are cleaved under conditions of catalytic hydrogenation.

67. 21-67 Benzyl Ethers  Benzyl ethers are used as protecting groups for the OH groups of alcohols and phenols. to carry out hydroboration/oxidation of this alkene, the phenolic -OH must first be protected; it is acidic enough to react with BH 3 and destroy the reagent.

68. 21-68 End of Chapter 21 Benzene and and the Concept of Aromaticity