1. Arenes and Aromaticity (Benzene)3

2. All C—C bond distances = 140 pm
Benzene
empirical formula = CH 9

3. All C—C bond distances = 140 pm
140 pm is the average between the C—C single bond distance and the double bond distance in 1,3-butadiene. 9

4. Figure: UN
Caption:
Benzene is actually a resonance hybrid between the two Kekule structures. This representation implies that the pi electrons are delocalized, with a bond order of 1.5 between adjacent carbon atoms. The carbon-carbon bond lengths in benzene are shorter than typical single-bond lengths, yet longer than typical double-bond lengths.

5. Resonance & Benzene 5

6. Thermochemical Measures of Stability
heat of hydrogenation: compare experimental value with "expected" value for hypothetical "cyclohexatriene" Pt +
3H2
H°= – 208 kJ 17

7. Cyclic conjugation versus noncyclic conjugation
3H2 Pt
heat of hydrogenation = -208 kJ/mol (-49 kcal/mol)
3H2 Pt
heat of hydrogenation = -337 kJ/mol (-85.8 kcal/mol) 20

8. Figure: 16-02
Caption:
The molar heats of hydrogenation and the relative energies of cyclohexene, 1,4-cyclohexadiene, 1,3-cyclohexadiene, and benzene.

9. Resonance & Benzene
express the structure of benzene as a resonance hybrid of the two Lewis structures. Electrons are not localized in alternating single and double bonds, but are delocalized over all six ring carbons. H H 6

10. Resonance & Benzene
Circle-in-a-ring notation stands for resonance description of benzene (hybrid of two resonance structures) 14

11. Question
Which of the following compounds has a double bond that is conjugated with the p system of the benzene ring?
A) p-Benzyltoluene
B) 2-Phenyl-1-decene
C) 3-Phenylcyclohexene
D) 3-Phenyl-1,4-pentadiene
E) 2,4,6-trichloroanisole

12. Figure: 16-01
Caption:
Benzene is a flat ring of sp2 hybrid carbon atoms with their unhybridized p orbitals all aligned and overlapping. The carbon-carbon bond lengths are all Å, and all the bond angles are exactly 120o.

13. Question Which of the following has the smallest heat of combustion?
A) B)
C) D)

14. The  Molecular Orbitals of Benzene26

15. Orbital Hybridization Model of Bonding in Benzene
High electron density above and below plane of ring 23

16. Figure: 16-05
Caption:
Energy diagram of the molecular orbitals of benzene. Benzene's six pi electrons fill the three bonding orbitals, leaving the antibonding orbitals vacant.

17. Hückel's Rule
Frost's circle is a mnemonic that allows us to draw a diagram showing the relative energies of the p orbitals of a cyclic conjugated system.
1) Draw a circle.
2) Inscribe a regular polygon inside the circle so that one of its corners is at the bottom.
3) Every point where a corner of the polygon touches the circle corresponds to a p electron energy level.
4) The middle of the circle separates bonding and antibonding orbitals. 7

18. Frost's Circle
p MOs of Benzene
Bonding
Antibonding 8

19. Benzene MOs 6 p AOs combine to give 6  MOs
Antibonding orbitals
Energy
Bonding orbitals
6 p AOs combine to give 6  MOs
3 MOs are bonding; 3 are antibonding 6

20. Benzene MOs All bonding MOs are filled
Antibonding orbitals
Energy
Bonding orbitals
All bonding MOs are filled
No electrons in antibonding orbitals 6

21. Benzene MOs 6

22. Figure: 16-04
Caption:
The six pi molecular orbitals of benzene viewed from above. The number of nodal planes increases with energy, and there are two degenerate MOs at each intermediate energy level.

23. Figure: 16-09
Caption:
In a cyclic conjugated system, the lowest-lying MO is filled with two electrons. Each of the additional shells consists of two degenerate MOs, with space for four electrons. If a molecule has (4N + 2) pi electrons, it will have a filled shell. If it has 4N electrons, there will be two unpaired electrons in two degenerate orbitals.

24. Hückel's Rule: Annulenes
the additional factor that influences aromaticity is the number of p electrons 6

25. Figure: UN
Caption:
Hydrocarbons with alternating double and single bonds are called annulenes. For example, benzene is a six-membered annulene, so it can be named [6]-annulene. Cylobutadiene is [4]-annulene, cyclooctatetraene is [8]-annulene.

26. Aromatic vs. “anti-aromatic” A. Deniz, et. al
Aromatic vs. “anti-aromatic” A. Deniz, et. al., Science, 5 November 1999

27. Question How many p electrons does the compound shown have? A) 5 B) 8

28. Hückel's Rule
Among planar, monocyclic, completely conjugated polyenes, only those with 4N p electrons have resonance stability (i.e. They are aromatic; and they are also planar.)
N 4N+2
0 2
1 6
2 10
3 14
4 18 7

29. Hückel's Rule
Among planar, monocyclic, completely conjugated polyenes, only those with 4N p electrons have resonance stability (i.e. They are aromatic; and they are also planar.)
N 4N+2
0 2
1 6 benzene!
2 10
3 14
4 18 7

30. Question
What is the value of N of Huckel's rule for the cyclopentadienyl cation (shown)?
A) N=1/2
B) N=1/4
C) N=1
D) N=2

31. Question Which is a true statement based on Huckel’s Rule.II
Which is a true statement based on Huckel’s Rule.
(Assume that both are planar and that vacant p-orbitals do not interupt conjugation.)
A) I is aromatic and II is not.
B) II is aromatic and I is not.
C) I and II are aromatic.
D) I and II are not aromatic.

32. Hückel's Rule & molecular orbitals
Hückel”s rule applies to: cyclic, planar, conjugated, polyenes
the p molecular orbitals of these compounds have a distinctive pattern
one p orbital is lowest in energy, another is highest in energy, and the others are arranged in pairs between the highest and the lowest 7

33. p-MOs of Benzene 6 p orbitals give 6 p orbitals
Antibonding
Non-bonding
Benzene
Bonding
6 p orbitals give 6 p orbitals
3 orbitals are bonding; 3 are antibonding 8

34. p-MOs of Benzene 6 p electrons fill all of the bonding orbitals
Antibonding
Non-bonding
Benzene
Bonding
6 p electrons fill all of the bonding orbitals
all p antibonding orbitals are empty 8

35. Figure: 16-19
Caption:
Ultraviolet spectra of benzene and styrene

36. http://www.youtube.com/watch?v=HPGE_GJkLJA Figure: 16-17-03UN1-3
Caption:
Disubstituted benzenes are named using the prefixes ortho-, meta-, and para- to specify the substitution patterns. These terms are abbreviated o-, m-, and p-.

37. Question How many isomers of dibromophenol are aromatic? A) 3 B) 4
E) none

38. p-MOs of Cyclobutadiene (square planar)
Antibonding
Non-bonding
Cyclo- butadiene
Bonding
4 p orbitals give 4p orbitals
1 orbital is bonding, one is antibonding, and 2 are nonbonding 8

39. p-MOs of Cyclobutadiene (square planar)
Antibonding
Non-bonding
Cyclo- butadiene
Bonding
4 p electrons; bonding orbital is filled; other 2 p electrons singly occupy two nonbonding orbitals 8

40. p-MOs of Cyclooctatetraene (square planar)
Antibonding
Non-bonding
Cyclo- octatetraene
Bonding
8 p orbitals give 8 p orbitals
3 orbitals are bonding, 3 are antibonding, and 2 are nonbonding 8

41. p-MOs of Cyclooctatetraene (square planar)
Antibonding
Non-bonding
Cyclo- octatetraene
Bonding
8 p electrons; 3 bonding orbitals are filled; 2 nonbonding orbitals are each half-filled 8

42. p-Electron Requirement for Aromaticity
4 p electrons
6 p electrons
8 p electrons
not aromatic
not aromatic
aromatic 2

43. Anti-aromatic (planar) Non-aromatic (non-planar)
Figure: 16-08
Caption:
The polygon rule predicts that the MO energy diagram for these annulenes will resemble the polygonal shapes of the annulenes.
Non-aromatic
(non-planar)
Aromatic

44. Completely Conjugated Polyenes
6 p electrons; not completely conjugated
6 p electrons; completely conjugated H
not aromatic
aromatic 2

45. Question The planar compound shown below is classified as
A) non-aromatic
B) aromatic
C) anti-aromatic
D) none of the above

46. [10]Annulene
predicted to be aromatic by Hückel's rule, but too much angle strain when planar and all double bonds are cis (therefore non-planar)
10-sided regular polygon has angles of 144° 16

47. [10]Annulene
incorporating two trans double bonds into the ring relieves angle strain but introduces van der Waals strain into the structure and causes the ring to be distorted from planarity 16

48. van der Waals strain between these two hydrogens
[10]Annulene
van der Waals strain between these two hydrogens
incorporating two trans double bonds into the ring relieves angle strain but also introduces van der Waals strain into the structure and causes the ring to be non-planar 16

49. [14]Annulene 14 p electrons satisfies Hückel's rule
van der Waals strain between hydrogens inside the ring & thererfore non-planar 16

50. [16]Annulene 16 p electrons does not satisfy Hückel's rule
alternating short (134 pm) and long (146 pm) bonds
not aromatic 16

51. [18]Annulene 18 p electrons satisfies Hückel's rule
resonance energy = kJ/mol 16

52. Figure: 16-15
Caption:
Structures of diamond and graphite. Diamond is a lattice of tetrahedral carbon atoms linked in a rigid three-dimensional array. Graphite consists of planar layers of fused aromatic rings.

53. Figure: 16-16
Caption:
Structure of C60 and a carbon nanotube. Each carbon in C60 is a bridgehead carbon for a five-membered ring and two six-membered rings.

54. Aromatic Ions 21

55. Figure: UN
Caption:
We can draw a five-membered ring of sp2 hybrid carbon atoms with all the unhybridized p orbitals lined up to form a continuous ring. With five pi electrons, this system would be neutral, but it would be a radical because of an odd number of electrons cannot all be paired. With four pi electrons Huckel's rule predicts the system to be antiaromatic. With six pi electrons, Huckel's rule predicts aromaticity.

56. Figure: UN
Caption:
Cyclopentadiene is unusually acidic because loss of a proton converts the nonaromatic diene to the aromatic cyclopentadienyl anion.

57. Figure: UN
Caption:
Huckel's rule predicts that the cyclopentadienyl cation, with four pi electrons, is antiaromatic. In agreement with this prediction, the cyclopentadienyl cation is not easily formed.

58. Figure: UN
Caption:
With conjugated systems such as cyclopendadienyl cation and anion, the resonance approach is a poor predictor of stability. The Huckel rule, based on molecular orbital theory, is a much better predictor of stability for these aromatic and antiaromatic systems.

59. Figure: UN
Caption:
The cycloheptatrienyl cation is easily formed by treating the corresponding alcohol with dilute (0.01N) aqueous sulfuric acid.

60. Figure: 16-18
Caption:
The mass spectrum of n-butylbenzene has its base peak at m/z = 91, corresponding to cleavage of a benzylic bond. The fragments are a benzyl cation and a propyl radical. The cation rearranges to the tropylium ion, detected at m/z = 91.

61. Figure: UN
Caption:
Dianions of hydrocarbons are rare and are usually much more difficult to form. Cyclooctatetraene reacts with potassium metal, however, to form an aromatic dianion.

62. Heterocyclic Aromatic Compounds30

63. Aromatic Heterocyclic Compounds
A heterocycleis a cyclic compound in which
one or more of the ring atoms is an atom other than
carbon.

64. Figure: 16-14
Caption:
Pyrrole, furan, and thiophene are isoelectronic. In furan and thiophene, the pyrrole N-H bond is replaced by a nonbonding pair of electrons in the sp2 hybrid orbital.

65. Furan Is Aromatic

66. Pyridine Is Aromatic

67. Figure: 16-10
Caption:
Pyridine has six delocalized electrons in its pi system. The two non-bonding electrons on nitrogen are in an sp2 orbital, and they do not interact with the pi electrons of the ring.

68. Figure: 16-11
Caption:
Pyridine is basic, with non-bonding electrons available to abstract a proton. The protonated pyridine (a pyridinium ion) is still aromatic.

69. Pyrrole Is Aromatic

70. Figure: 16-12
Caption:
The pyrrole nitrogen atom is sp2 hybridized with a lone pair of electrons in the p orbital. This p orbital overlaps with the p orbitals of the carbon atoms to form a continuous ring.

71. Figure: 16-13
Caption:
The pyrrole nitrogen atom must become sp3 hybridized to abstract a proton. This eliminates the unhybridized p orbital needed for aromaticity.

72. Question
Which of the following compounds is best classified as an aromatic heterocycle?
A) B)
C) Aniline D) Pyridine
E) All of them

75. Quinine

77. Examples of Important Nitrogen Hetero-bicyclic Aromatic Compounds

79. Aromatic Compounds & Cancer30

80. Figure: UN
Caption:
Naphthalene is the simplest fused aromatic hydrocarbon, consisting of two fused benzene rings.

81. Figure: UN
Caption:
As the number of aromatic ring increases, the resonance energy per ring continues to decrease and the compounds become more reactive.

82. Benzene:
Benzene is classified as a Group A, human carcinogen by the EPA.
Increased incidence of leukemia has been observed in humans occupationally exposed to benzene.
Chronic inhalation has caused various blood disorders, including reduced red blood cell count and aplastic anemia.
Reproductive effects have been reported for women exposed to high levels by inhalation.
Adverse effects on the developing fetus have been observed in animal tests.
Benzene is an established human leukemogen, but the mechanism by which it produces leukemia remains unclear.
Epidemiologic studies and case studies provide clear evidence of a causal association between exposure to benzene and acute nonlymphocytic leukemia (ANLL) and also suggest evidence for chronic nonlymphocytic leukemia (CNLL) and chronic lymphocytic leukemia (CLL). Other neoplastic conditions that are associated with an increased risk in humans are hematologic neoplasms, blood disorders such as preleukemia and aplastic anemia, Hodgkin's lymphoma, and myelodysplastic syndrome (MDS).
Benzene is used as a constituent in motor fuels; as a solvent for fats, waxes, resins, oils, inks, paints, plastics, and rubber; in the extraction of oils from seeds and nuts; and in photogravure printing. It is also used as a chemical intermediate. Benzene is also used in the manufacture of detergents, explosives, pharmaceuticals, and dyestuffs.
"Classic" carcinogens, such as benzo[a]pyrene, aromatic amines, and aflatoxin, are thought to be activated to a single, ultimate carcinogenic metabolite. Typically these metabolites are highly electrophilic and bind strongly to DNA in a covalent fashion. Covalent binding of this type is readily measured as bound radioactivity. Classic carcinogens and their metabolites are also highly mutagenic in the Ames Salmonella test producing point mutations and small deletions.
Benzene presents the exact opposite scenario. It does not form a single highly electrophilic metabolite. Its epoxide, benzene oxide, is highly unstable and rapidly rearranges to the major metabolite phenol.

83. Benzene: Toxic Metabolites
Benzene is an established human leukemogen, but the mechanism by which it produces leukemia remains unclear.
"Classic" carcinogens, such as benzo[a]pyrene, aromatic amines, and aflatoxin, are thought to be activated to a single, ultimate carcinogenic metabolite. Typically these metabolites are highly electrophilic and bind strongly to DNA in a covalent fashion. Covalent binding of this type is readily measured as bound radioactivity. Classic carcinogens and their metabolites are also highly mutagenic in the Ames Salmonella test producing point mutations and small deletions.
Benzene presents the exact opposite scenario. It does not form a single highly electrophilic metabolite. Its epoxide, benzene oxide, is highly unstable and rapidly rearranges to the major metabolite phenol.

84. Carcinogenic Mechanism
Cooked meat products, regular consumption of which has been epidemiologically associated with increased levels of colon cancer[1] (although this in itself does not prove carcinogenicity)[2], have been shown to contain up to 4 ng/g of benzo[a]pyrene,[3] and up to 5.5ng/g in fried chicken[4] and 62.6ng/g in overcooked charcoal barbecued beef.[5]
Caption:
Benzo[a]pyrene, one of the most thoroughly studied carcinogens, is formed whenever organic compounds undergo incomplete combustion.
Carcinogenic Mechanism