1. Chapter 11 Arenes and Aromaticity
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 3

2. Examples of Aromatic Hydrocarbons
Benzene H
Toluene
Naphthalene 2

3. Some history of Benzene
1825 Michael Faraday isolated a new hydrocarbon from illuminating gas.
1834 Eilhardt Mitscherlich isolated same substance and determines its empirical formula to be CnHn. Compound comes to be called benzene.
1845 August W. von Hofmann isolated benzene from coal tar.
1866 August Kekulé proposed structure of benzene. 4

4. The structure of Benzene: Kekulé
Kekulé (1866) proposed a cyclic structure for C6H6 with alternating single and double bonds. H 6

5. Kekulé Formulation of Benzene
Later, Kekulé revised his proposal by suggesting a rapid equilibrium between two equivalent structures. H H
Proposed 6

6. Kekulé Formulation of Benzene
However, this proposal suggested isomers of the kind shown were possible. Yet, none were ever found.
Never
Observed H X H X 6

7. Benzene has the shape of a regular hexagon.
Structure of Benzene
Structural studies of benzene do not support the Kekulé formulation. Instead of alternating single and double bonds, all of the C—C bonds in benzene are the same length, 140 pm.
Benzene has the shape of a regular hexagon. 6

8. 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

9. Kekulé Formulation of Benzene
So, instead of Kekulé's suggestion of a rapid equilibrium between two structures:
Equilibrium
not valid H H 6

10. Resonance Formulation of Benzene
The structure of benzene is expressed 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
Resonance
model 6

11. Resonance Formulation of Benzene
The "circle-in-a-ring" notation is an acceptable resonance description of benzene (a hybrid of two Kekulé structures). But it does not show the Lewis structure electrons. 14

12. 11.3 The Stability of Benzene
Benzene is the best and most familiar example of a substance that possesses "special stability" or "aromaticity ".
Aromaticity is a level of stability that is substantially greater for a molecule than would be expected on the basis of any of the Lewis structures written for it. 15

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

14. Figure 11.2 (p 433) 3 x cyclohexene 360 kJ/mol 231 kJ/mol 208 kJ/mol18

15. Figure 11.2 (p 433)
3 x cyclohexene
"Expected" heat of hydrogenation of benzene is 3x the heat of hydrogenation of cyclohexene.
360 kJ/mol
120 kJ/mol 18

16. Figure 11.2 (p 433)
3 x cyclohexene
The observed heat of hydrogenation is kJ/mol (36 kcal/mol) less than "expected", so benzene is kJ/mol more stable than expected.
This 152 kJ/mol is the resonance energy of benzene.
360 kJ/mol
208 kJ/mol 18

17. Figure 11.2 (p 433)
Hydrogenation of 1,3- cyclohexadiene (using + 2 H2) gives off more heat than hydrogenation of benzene (using + 3 H2) !
231 kJ/mol
208 kJ/mol 18

18. Cyclic conjugation versus noncyclic conjugation
3H2 Pt
heat of hydrogenation = 208 kJ/mol
3H2 Pt
heat of hydrogenation = 337 kJ/mol 20

19. Resonance Energy of Benzene
Compared to localized 1,3,5-cyclohexatriene
benzene is 152 kJ/mol more stable.
Compared to 1,3,5-hexatriene
benzene is 129 kJ/mol more stable.
Exact value of resonance energy of benzene depends on what it is compared to, but regardless of model, benzene is more stable than expected by a substantial amount. 21

20. 11.4 An Orbital Hybridization View of Bonding in Benzene22

21. Orbital Hybridization Model of Bonding in Benzene
Planar ring of 6 sp2 hybridized carbons.
Figure 11.3 23

22. Orbital Hybridization Model of Bonding in Benzene
Each carbon contributes a p orbital.
Six p orbitals overlap to give cyclic  system; six  electrons are delocalized throughout the  system.
Figure 11.3 23

23. Orbital Hybridization Model of Bonding in Benzene
High electron density above and below plane of ring.
Figure 11.3 23

24. Requirements for Aromaticity26

25. Requirements for Aromaticity
The compound must be cyclic.
Each ring atom must have a p-orbital.
The ring must be planar. ( electrons are completely conjugated)
An additional factor that is required for a molecule to be aromatic is Huckel’s Rule. (4n + 2  electrons). 5

26. Aromatic compounds meet all of these four conditions.
Definitions
Aromatic compounds meet all of these four conditions.
A nonaromatic compound does not obey one or more of conditions 1-3.
An antiaromatic obeys conditions 1-3 but does not meet condition 4.
Annulenes: cyclic compounds that have completely alternating double bonds and may or may not be aromatic. 5

27. Heats of Hydrogenation
To give cyclohexane (kJ/mol):
120
231
208
Heat of hydrogenation of benzene is 152 kJ/mol, less than 3 times heat of hydrogenation of cyclohexene (resonance energy). 3

28. Heats of Hydrogenation
To give cyclooctane (kJ/mol): 97
205
303
410
Heat of hydrogenation of cyclooctatetraene is roughly four times the heat of hydrogenation of cyclooctene (no stabilization). 4

29. Structure of Cyclooctatetraene
Cyclooctatetraene is not planar. It has alternating long (146 pm) and short (133 pm) bonds. 5

30. Structure of Cyclobutadiene
MO calculations give alternating short and long bonds for cyclobutadiene (no resonance or bonds would all be equal). H
135 pm
156 pm 5

31. Structure of Cyclobutadiene
The actual structure of a stabilized derivative is characterized by alternating short bonds and long bonds.
C(CH3)3
(CH3)3C
CO2CH3
138 pm
151 pm 5

32. Stability of Cyclobutadiene
Cyclobutadiene is observed to be highly reactive, and too unstable to be isolated and stored in the customary way.
Not only is cyclobutadiene not aromatic, it is antiaromatic when meeting rules 1-3.
An antiaromatic substance is one that is destabilized by cyclic conjugation. 5

33. Requirements for Aromaticity
Cyclic conjugation is necessary, but not sufficient.
not aromatic
aromatic
Antiaromatic when square.
Antiaromatic when planar. 2

34. 11.19 Huckel’s Rule
The additional factor that influences aromaticity is the number of  electrons. 26

35. Hückel's Rule
Among planar, monocyclic, completely conjugated polyenes, only those with 4n  electrons possess special stability (are aromatic).
n 4n+2
0 2
1 6 Benzene!
2 10
3 14
4 18 7

36. Hückel's Rule
Hückel restricted his analysis to planar, completely conjugated, monocyclic polyenes.
He found that the  molecular orbitals of these compounds had a distinctive pattern.
One  orbital was lowest in energy, another was highest in energy, and the others were arranged in pairs between the highest and the lowest. 7

37. -Electron Requirement for Aromaticity
4  electrons
6  electrons
8  electrons
not aromatic
not aromatic
aromatic 2

38. Completely Conjugated Polyenes
6  electrons; not completely conjugated, one C lacks a p orbital.
6  electrons; completely conjugated. H
not aromatic
aromatic 2

39. 11.20 Annulenes 26

40. They may or may not be aromatic.
Annulenes
Annulenes are planar, monocyclic, completely conjugated polyenes (alternating double and single bonds) kind of hydrocarbons treated by Hückel's rule.
They may or may not be aromatic. 15

41. 10-sided regular polygon has angles of 144°.
[10]Annulene
Not planar
Predicted to be aromatic by Hückel's rule, but too much angle strain when planar and all double bonds are cis.
10-sided regular polygon has angles of 144°. 16

42. van der Waals strain between these two hydrogens
[10]Annulene
van der Waals strain between these two hydrogens
Not planar
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

43. 14  electrons satisfies Hückel's rule.
[14]Annulene H H
Aromatic H H
14  electrons satisfies Hückel's rule.
van der Waals strain between hydrogens inside the ring. 16

44. 16  electrons does not satisfy Hückel's rule.
[16]Annulene
Antiaromatic
16  electrons does not satisfy Hückel's rule.
Alternating short (134 pm) and long (146 pm) bonds.
Is an antiaromatic 4n p-electron system. 16

45. 18  electrons satisfies Hückel's rule. Resonance energy = 418 kJ/mol.
[18]Annulene H
Aromatic
18  electrons satisfies Hückel's rule.
Resonance energy = 418 kJ/mol.
Bond distances range between pm. 16

46. 11.21 Aromatic Ions 26

47. Cycloheptatrienyl Cation (the tropylium ion)
It has 6  electrons (4n + 2) delocalized over 7 carbons.
The positive charge is dispersed over 7 carbons.
Is a very stable carbocation (aromatic) also called a tropylium cation. H + + H 22

48. Cycloheptatrienyl CationBr +
Br–
Ionic
Covalent
The tropylium cation is so stable that tropylium bromide is ionic rather than covalent. mp 203°C; soluble in water; insoluble in diethyl ether 23

49. Cyclopentadienide Anion
It has 6  electrons delocalized over 5 carbons.
The negative charge is dispersed over 5 carbons.
It is a stabilized anion (aromatic). H •• – H – 22

50. Acidity of CyclopentadieneH H •• – H+ +
pKa = 16, Ka = 10-16
Cyclopentadiene is unusually acidic for a hydrocarbon.
Increased acidity is due to stability of cyclopentadienyl anion (becomes aromatic). 22

51. Electron Delocalization in Cyclopentadienide AnionH H •• – •• – H •• – H –
Resonance structures H – •• •• 26

52. Compare Acidities of Cyclopentadiene and Cycloheptatriene
pKa = 16
Ka = 10-16
pKa = 36
Ka = 10-36
By losing H+ it
becomes aromatic. 27

53. Compare Acidities of Cyclopentadiene and Cycloheptatriene•• H •• – –
An aromatic anion
6  electrons
An antiaromatic anion 8  electrons 27

54. Cyclopropenyl Cation + H H + also written as: An aromatic cation.
4n +2 = 2  electrons 28

55. Cyclooctatetraene DianionH H H H – H •• H H H
also written as 2– •• H H H H – H H
An aromatic dianion. H H
n = 2
4n +2 = 10 electrons 29

56. 11.5 The  Molecular Orbitals of Benzene26

57. 6 p AOs combine to give 6  MOs 3 MOs are bonding; 3 are antibonding
Benzene MOs
Antibonding orbitals
Energy
Bonding orbitals
6 p AOs combine to give 6  MOs
3 MOs are bonding; 3 are antibonding 6

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

59. Benzene MOs
Figure 11.4
p 435 6

60. Frost's Circle (Polygon Rule)
p MOs of Benzene
Bonding
Antibonding 8

61. Frost's Circle (Polygon Rule)
2  es
4  es
6  es
6  es
6  es
8  es 7

62. 6 p orbitals give 6  orbitals.
-MOs of Benzene
Antibonding
Benzene
Bonding
6 p orbitals give 6  orbitals.
3 orbitals are bonding; 3 are antibonding. 8

63. 6  electrons fill all of the bonding orbitals.
-MOs of Benzene
Antibonding
Benzene
Bonding
6  electrons fill all of the bonding orbitals.
All  antibonding orbitals are empty. 8

64. -MOs of Cyclobutadiene (Square Planar)
Antibonding
Cyclo- butadiene
Bonding
4 p orbitals give 4 orbitals.
1 orbital is bonding, one is antibonding, and 2 are nonbonding. 8

65. -MOs of Cyclobutadiene (Square Planar)
Antibonding
Cyclo- butadiene
Bonding
4  electrons; bonding orbital is filled; other 2  electrons singly occupy two nonbonding orbitals. 8

66. -MOs of Cyclooctatetraene (Planar)
Antibonding
Cyclo- octatetraene
Bonding
8 p orbitals give 8  orbitals.
3 orbitals are bonding, 3 are antibonding, and 2 are nonbonding. 8

67. -MOs of Cyclooctatetraene (Planar)
Antibonding
Cyclo- octatetraene
Bonding
8  electrons; 3 bonding orbitals are filled; 2 nonbonding orbitals are each half-filled. 8

68. 11.6 Substituted Derivatives of Benzene and their Nomenclature26

69. General Points on Naming
1. Benzene is considered as the parent and comes last in the name.
2. Substituents are listed in alphabetical order.
3. Number ring in direction that gives lowest numbering. 2

70. General Points on Naming
Benzene is considered as the parent and comes last in the name. Br
C(CH3)3
NO2
Bromobenzene
tert-Butylbenzene
Nitrobenzene 2

71. Substituents are listed in alphabetical order.
Example
Substituents are listed in alphabetical order.
Number ring in direction that gives lowest numbering. Cl Br F
2-bromo-1-chloro-4-fluorobenzene 3

72. Can use ortho, meta and para for disubstituted derivatives of benzene.
1,2 = ortho (abbreviated o-)
1,3 = meta (abbreviated m-)
1,4 = para (abbreviated p-)
o-xylene
m-xylene
p-xylene 4

73. (1-ethyl-2-nitrobenzene)
Examples
NO2 Cl
CH2CH3
o-ethylnitrobenzene
m-dichlorobenzene
(1-ethyl-2-nitrobenzene)
(1,3-dichlorobenzene) 4

74. Certain monosubstituted derivatives of benzene have unique names.
Table 11.1
Certain monosubstituted derivatives of benzene have unique names. CH O
COH O
Benzaldehyde
Benzoic acid
1 2

75. Table 11.1
CCH3 O OH
Phenol
Acetophenone
CH2 CH
Styrene 8

76. p-Nitroanisole or 4-Nitroanisole
Table 11.1
OCH3
NO2
OCH3
OCH3
Anisole
NH2
p-Nitroanisole or 4-Nitroanisole
Aniline 8

77. Easily Confused Names phenyl phenol benzyl OH CH2— a group
(substituent)
a compound 14

78. 11.7 Polycyclic Aromatic Hydrocarbons26

79. Naphthalene
Napthtalene resonance energy = 255 kJ/mol (~127 kJ/mol per ring). The resonance energy per ring decreases as the number of rings increases or with fewer possible Kekule structures (benzene = 152 kJ/m).
Most stable Lewis structure; here both rings correspond to Kekulé benzene. 16

80. Anthracene and Phenanthrene
resonance energy:
347 kJ/mol (~116 kJ/mol per ring)
381 kJ/mol (~127 kJ/mol per ring) 17

81. 11.8 Physical Properties of Arenes26

82. Arenes (aromatic hydrocarbons) resemble other hydrocarbons. They are:
Physical Properties
Arenes (aromatic hydrocarbons) resemble other hydrocarbons. They are:
nonpolar,
insoluble in water and
less dense than water. 2

83. 11.9 Reactions of Arenes: A Preview
1. Some reactions involve the ring.
2. Some reactions involve groups attached to the ring. 26

84. Reactions of Aromatics
1. Reactions involving the ring:
a) Reduction:
Catalytic hydrogenation (Section 11.3) Birch reduction (Section 11.10)
b) Electrophilic aromatic substitution: (Chapter 12)
c) Nucleophilic aromatic substitution: (Chapter 12)
2. Reactions involving a substituent: (Sections ) 2

85. Reduction of Benzene Rings
catalytic hydrogenation
(Section 11.3) H
Birch reduction
(Section 11.10) H
High T High P
Dissolving Metal H 3

86. 11.10 The Birch Reduction
A reaction involving the ring. 26

87. Birch Reduction of Benzene
Na, NH3
CH3OH
(80%)
Product is non-conjugated diene.
Reaction stops here. There is no further reduction.
Reaction is not hydrogenation (H2 is not involved),
it is a dissolving metal reduction. 5

88. Mechanism of the Birch Reduction
Step 1: Electron transfer from sodium H +
Na+ H • •• – + Na • 6

89. Mechanism of the Birch Reduction
Step 2: Proton transfer from methanol H H H H H H • • •• •
OCH3 H •• H H – H H H H H –
OCH3 •• • 6

90. Mechanism of the Birch Reduction
Step 3: Electron transfer from sodium H • + Na H •• +
Na+ – 6

91. Mechanism of the Birch Reduction
Step 4: Proton transfer from methanol •
OCH3 H •• H – •
OCH3 •• H – H H •• H H H H 6

92. Birch Reduction of an Alkylbenzene
C(CH3)3 H
C(CH3)3
Na, NH3
CH3OH
(86%)
If an alkyl group is present on the ring, it ends up as a substituent on the double bond. 5

93. 11.11 Free-Radical Halogenation of Alkylbenzenes
A reaction involving an alkyl substituent. 26

94. A Substituent on the Benzene Ring• C • C
allylic radical
benzylic radical
Benzylic carbon is analogous to allylic carbon. 2

95. Bond-dissociation energy for C—H bond is equal to H° for:
Recall:
Bond-dissociation energy for C—H bond is equal to H° for:
R—H R• + •H
and is about 400 kJ/mol for alkanes.
The more stable the free radical R•, the weaker the bond, and the smaller the bond-dissociation energy (allylic is more stable than 3o, Chapter 4). 13

96. Bond-dissociation Energies of Propene and TolueneH
H2C CH C H C •
368 kJ/mol
H2C CH
-H• H C H C •
356 kJ/mol
-H•
Low BDEs indicate allyl and benzyl radical are more stable than simple alkyl radicals. 14

97. Resonance in Benzyl RadicalH H C H • • C H H H H H
Unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it. 18

98. Resonance in Benzyl RadicalH • H H C H H • H H H
Unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it. 18

99. Free-radical Chlorination of Toluene
Cl2
light or heat
CH2Cl
Toluene
Benzyl chloride
An industrial process that is highly regioselective for benzylic position. Bromine is commonly used in lab. 16

100. Free-radical Chlorination of Toluene
Similarly, dichlorination and trichlorination are selective for the benzylic carbon. Further chlorination gives:
CHCl2
CCl3
(Dichloromethyl) benzene
(Dichloromethyl) benzene 16

101. Benzylic Bromination
Br2 is used in the laboratory to introduce a halogen at the benzylic position.
CH3
NO2
+ HBr
NO2
CH2Br
p-Nitrobenzyl
bromide (71%)
CCl4, 80°C
light
+ Br2
p-Nitrotoluene 22

102. N-Bromosuccinimide (NBS)
As was the case in allylic bromination, NBS is a convenient reagent for benzylic bromination.
CCl4
benzoyl
peroxide,
heat
CH2CH3 +
CHCH3 NH O Br
(87%)
NBr O 23

103. 11.12 Oxidation of Alkylbenzenes
A reaction involving an alkyl substituent. 26

104. Site of Oxidation is Benzylic Carbon
CH3 or
Na2Cr2O7
H2SO4
COH O
CH2R
H2O
heat or
CHR2 25

105. p-Nitrobenzoic acid (82-86%)
Example
COH O
CH3
NO2
Na2Cr2O7
H2SO4
H2O
heat
NO2
p-Nitrotoluene
p-Nitrobenzoic acid (82-86%) 26

106. Basic KMnO4 is also an effective oxidant for this reaction. (45%)
Example
COH O
CH(CH3)2
CH3
1. conc. KMnO4
NaOH, heat
2. H+ , HOH
Basic KMnO4 is also an effective oxidant for this reaction.
(45%) 26

107. 11.13 SN1 Reactions of Benzylic Halides26

108. Relative solvolysis rates in aqueous acetone:
Benzylic SN1 Reactions
Relative solvolysis rates in aqueous acetone: C
CH3 Cl C
CH3 Cl
620 1
Tertiary benzylic carbocation is formed more rapidly than tertiary carbocation; therefore, more stable. 30

109. Relative rates of formation:
Benzylic SN1 Reactions
Relative rates of formation:
CH3 +
CH3 + C
CH3 C
more stable
less stable 30

110. Benzylic carbon is analogous to allylic carbon.
Compare + C + C
allylic carbocation
benzylic carbocation
Benzylic carbon is analogous to allylic carbon. 2

111. Resonance in Benzyl CationH + H H H C C H H H H + H H H H H H
Positive charge is delocalized between benzylic carbon and the ring carbons that are ortho and para to it as shown by resonance. 18

112. Resonance in Benzyl CationH H H H C C H H H H + + H H H H H H
Additional resonance structures give this benzylic carbocation greater stability. 18

113. Solvolysis C CH3 Cl Mechanism is SN1
Solvolysis of a 3o benzylic halide is faster than a normal tertiary halide.
CH3CH2OH C
CH3
OCH2CH3
(87%) 37

114. 11.14 SN2 Reactions of Benzylic Halides26

115. Primary Benzylic Halides
CH2Cl
O2N
Mechanism is SN2
Like allylic halide, substitution is faster than a normal primary halide.
NaOCCH3 O
acetic acid
CH2OCCH3
O2N O
(78-82%) 29

116. 11.15 Preparation of Alkenyl Benzenes
dehydrogenation
dehydration
dehydrohalogenation 26

117. Industrial preparation of styrene
Dehydrogenation
Industrial preparation of styrene
Almost 12 billion lbs. produced annually in U.S.
CH2CH3
630°C
CH2 CH
ZnO
+ H2 2

118. Acid-Catalyzed Dehydration of Benzylic Alcohols
CHCH3 OH Cl Cl
KHSO4
CH2 CH
heat
(80-82%)
CHCH3 Cl + 3

119. Dehydrohalogenation H3C CH2CHCH3 Br NaOCH2CH3 ethanol, 50°C (99%) CH

120. 11.16 Addition Reactions of Alkenyl Benzenes
hydrogenation
halogenation
addition of hydrogen halides 26

121. Hydrogenation Br C
CH3
CHCH3 Br
CHCH2CH3
CH3 H2 Pt
(92%) 7

122. Halogenation
Br2
CH2 CH CH
CH2 Br Br
(82%) 8

123. Addition of Hydrogen HalidesCl
HCl
(75-84%)
via benzylic carbocation + 9

124. Free-Radical Addition of HBr
CH2 CH
CH2CH2Br
peroxides
via benzylic radical
CH2Br CH • 10

125. 11.22 Heterocyclic Aromatic Compounds26

126. Examples N N H O S Pyridine Pyrrole Furan Thiophene N N Quinoline•• N H •• •• O •• S
Pyridine
Pyrrole
Furan
Thiophene N •• N •
Quinoline
Isoquinoline 31

127. 11.23 Heterocyclic Aromatic Compounds and Huckel’s Rule26

128. Pyridine Pyrrole Furan N H N O •• •• •• 6  electrons in ring.
Lone pair on nitrogen is in an sp2 hybridized orbital; not part of system of ring.
Lone pair on nitrogen must be part of ring  system if ring is to have 6  electrons.
Lone pair must be in a p orbital in order to overlap with ring  system.
Two lone pairs on oxygen.
One pair is in a p orbital and is part of ring  system; other is in an sp2 hybridized orbital and is not part of ring  system. 31

129. End of Chapter 11 Arenes and Aromaticity26