1. Chapter 11 Arenes and Aromaticity
Dr. Wolf's CHM 201 & 202 3

2. Examples of Aromatic Hydrocarbons
Benzene
Toluene
Naphthalene
Dr. Wolf's CHM 201 & 202 2

3. Benzene
Dr. Wolf's CHM 201 & 202 3

4. 1845 August W. von Hofmann isolates benzene from coal tar.
Some history
1834 Eilhardt Mitscherlich isolates a new hydrocarbon and determines its empirical formula to be CnHn. Compound comes to be called benzene.
1845 August W. von Hofmann isolates benzene from coal tar.
1866 August Kekulé proposes structure of benzene.
Dr. Wolf's CHM 201 & 202 4

5. Kekulé and the Structure of Benzene
Dr. Wolf's CHM 201 & 202 5

6. Kekulé Formulation of Benzene
Kekulé proposed a cyclic structure for C6H6 with alternating single and double bonds. H
Dr. Wolf's CHM 201 & 202 6

7. Kekulé Formulation of Benzene
Later, Kekulé revised his proposal by suggesting a rapid equilibrium between two equivalent structures. H H
Dr. Wolf's CHM 201 & 202 6

8. Kekulé Formulation of Benzene
However, this proposal suggested isomers of the kind shown were possible. Yet, none were ever found. H X H X
Dr. Wolf's CHM 201 & 202 6

9. 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 are the same length.
Benzene has the shape of a regular hexagon.
Dr. Wolf's CHM 201 & 202 6

10. All C—C bond distances = 140 pm
Dr. Wolf's CHM 201 & 202 9

11. 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.
Dr. Wolf's CHM 201 & 202 9

12. A Resonance Picture of Bonding in Benzene
Dr. Wolf's CHM 201 & 202 5

13. Kekulé Formulation of Benzene
Instead of Kekulé's suggestion of a rapid equilibrium between two structures: H H
Dr. Wolf's CHM 201 & 202 6

14. Resonance Formulation of 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
Dr. Wolf's CHM 201 & 202 6

15. Resonance Formulation of Benzene
Circle-in-a-ring notation stands for resonance description of benzene (hybrid of two Kekulé structures)
Dr. Wolf's CHM 201 & 202 14

16. 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
Dr. Wolf's CHM 201 & 202 15

17. Thermochemical Measures of Stability
heat of hydrogenation: compare experimental value with "expected" value for hypothetical "cyclohexatriene" Pt +
3H2
DH°= – 208 kJ
Dr. Wolf's CHM 201 & 202 17

18. Figure 11.2 (p 404) 3 x cyclohexene 360 kJ/mol 231 kJ/mol 208 kJ/mol
Dr. Wolf's CHM 201 & 202 18

19. Figure 11.2 (p 404)
3 x cyclohexene
"expected" heat of hydrogenation of benzene is 3 x heat of hydrogenation of cyclohexene
360 kJ/mol
120 kJ/mol
Dr. Wolf's CHM 201 & 202 18

20. observed heat of hydrogenation is 152 kJ/mol less than "expected"
Figure 11.2 (p 404)
3 x cyclohexene
observed heat of hydrogenation is 152 kJ/mol less than "expected"
benzene is 152 kJ/mol more stable than expected
152 kJ/mol is the resonance energy of benzene
360 kJ/mol
208 kJ/mol
Dr. Wolf's CHM 201 & 202 18

21. Figure 11.2 (p 404)
hydrogenation of 1,3-cyclohexadiene (2H2) gives off more heat than hydrogenation of benzene (3H2)!
231 kJ/mol
208 kJ/mol
Dr. Wolf's CHM 201 & 202 18

22. Cyclic conjugation versus noncyclic conjugation
3H2 Pt
heat of hydrogenation = 208 kJ/mol
3H2 Pt
heat of hydrogenation = 337 kJ/mol
Dr. Wolf's CHM 201 & 202 20

23. Resonance Energy of Benzene
compared to localized 1,3,5-cyclohexatriene
152 kJ/mol
compared to 1,3,5-hexatriene
129 kJ/mol
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
Dr. Wolf's CHM 201 & 202 21

24. An Orbital Hybridization View of Bonding in Benzene
Dr. Wolf's CHM 201 & 202 22

25. Orbital Hybridization Model of Bonding in Benzene
Planar ring of 6 sp2 hybridized carbons
Figure 11.3
Dr. Wolf's CHM 201 & 202 23

26. Orbital Hybridization Model of Bonding in Benzene
Each carbon contributes a p orbital
Six p orbitals overlap to give cyclic p system; six p electrons delocalized throughout p system
Figure 11.3
Dr. Wolf's CHM 201 & 202 23

27. Orbital Hybridization Model of Bonding in Benzene
High electron density above and below plane of ring
Figure 11.3
Dr. Wolf's CHM 201 & 202 23

28. The p Molecular Orbitals of Benzene
Dr. Wolf's CHM 201 & 202 26

29. 6 p AOs combine to give 6 p MOs 3 MOs are bonding; 3 are antibonding
Benzene MOs
Antibonding orbitals
Energy
Bonding orbitals
6 p AOs combine to give 6 p MOs
3 MOs are bonding; 3 are antibonding
Dr. Wolf's CHM 201 & 202 6

30. 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
Dr. Wolf's CHM 201 & 202 6

31. The Three Bonding p MOs of Benzene
Dr. Wolf's CHM 201 & 202 28

32. Substituted Derivatives of Benzene and Their Nomenclature
Dr. Wolf's CHM 201 & 202 1

33. 1) Benzene is considered as the parent and comes last in the name.
General Points
1) Benzene is considered as the parent and comes last in the name.
Dr. Wolf's CHM 201 & 202 2

34. Examples Br C(CH3)3 NO2 Bromobenzene tert-Butylbenzene Nitrobenzene 3
Dr. Wolf's CHM 201 & 202 3

35. 1) Benzene is considered as the parent and comes last in the name.
General Points
1) Benzene is considered as the parent and comes last in the name.
2) List substituents in alphabetical order
3) Number ring in direction that gives lowest locant at first point of difference
Dr. Wolf's CHM 201 & 202 2

36. 2-bromo-1-chloro-4-fluorobenzene
Example Cl Br F
2-bromo-1-chloro-4-fluorobenzene
Dr. Wolf's CHM 201 & 202 3

37. alternative locants for disubstituted derivatives of benzene
Ortho, Meta, and Para
alternative locants for disubstituted derivatives of benzene
1,2 = ortho (abbreviated o-)
1,3 = meta (abbreviated m-)
1,4 = para (abbreviated p-)
Dr. Wolf's CHM 201 & 202 4

38. (1-ethyl-2-nitrobenzene)
Examples
NO2 Cl
CH2CH3
o-ethylnitrobenzene
m-dichlorobenzene
(1-ethyl-2-nitrobenzene)
(1,3-dichlorobenzene)
Dr. Wolf's CHM 201 & 202 4

39. Certain monosubstituted derivatives of benzene have unique names
Benzene Derivatives
Certain monosubstituted derivatives of benzene have unique names
Dr. Wolf's CHM 201 & 202 5

40. Benzene Derivatives CH O
Benzaldehyde
Dr. Wolf's CHM 201 & 202
1 2

41. Benzene Derivatives
COH O
Benzoic acid
Dr. Wolf's CHM 201 & 202
1 2

42. Benzene Derivatives
CH2 CH
Styrene
Dr. Wolf's CHM 201 & 202 6

43. Benzene Derivatives
CH3
Toluene
Dr. Wolf's CHM 201 & 202 9

44. Benzene Derivatives
CCH3 O
Acetophenone
Dr. Wolf's CHM 201 & 202 7

45. Benzene Derivatives OH
Phenol
Dr. Wolf's CHM 201 & 202 8

46. Benzene Derivatives
OCH3
Anisole
Dr. Wolf's CHM 201 & 202 8

47. Benzene Derivatives
NH2
Aniline
Dr. Wolf's CHM 201 & 202
1 0

48. Benzene derivative names can be used as parent
OCH3
OCH3
NO2
Anisole
p-Nitroanisole or 4-Nitroanisole
Dr. Wolf's CHM 201 & 202 13

49. Easily confused names OH CH2— phenyl phenol benzyl 14
Dr. Wolf's CHM 201 & 202 14

50. Polycyclic Aromatic Hydrocarbons
Dr. Wolf's CHM 201 & 202 15

51. resonance energy = 255 kJ/mol
Naphthalene
resonance energy = 255 kJ/mol
most stable Lewis structure; both rings correspond to Kekulé benzene
Dr. Wolf's CHM 201 & 202 16

52. Anthracene and Phenanthrene
resonance energy:
347 kJ/mol
381 kJ/mol
Dr. Wolf's CHM 201 & 202 17

53. Physical Properties of Arenes
Dr. Wolf's CHM 201 & 202 15

54. Resemble other hydrocarbons nonpolar insoluble in water
Physical Properties
Resemble other hydrocarbons
nonpolar
insoluble in water
less dense than water
Dr. Wolf's CHM 201 & 202 2

55. Reactions of Arenes: A Preview
1. Some reactions involve the ring.
2. In other reactions the ring is a substituent.
Dr. Wolf's CHM 201 & 202 1

56. 1. Reactions involving the ring
a) Reduction
Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11)
b) Electrophilic aromatic substitution (Chapter 12)
c) Nucleophilic aromatic substitution (Chapter 12b)
2. The ring as a substituent (Sections )
Dr. Wolf's CHM 201 & 202 2

57. Reduction of Benzene Rings
catalytic hydrogenation
(Section 11.4) H
Birch reduction
(Section 11.11) H H
Dr. Wolf's CHM 201 & 202 3

58. The Birch Reduction
Dr. Wolf's CHM 201 & 202 4

59. 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 in any way.
Dr. Wolf's CHM 201 & 202 5

60. Mechanism of the Birch Reduction (Figure 11.8)
Step 1: Electron transfer from sodium H H • •• – + +
Na+ Na •
Dr. Wolf's CHM 201 & 202 6

61. Mechanism of the Birch Reduction (Figure 11.8)
Step 2: Proton transfer from methanol H H H • •• H H –
OCH3 H •• • H
Dr. Wolf's CHM 201 & 202 6

62. Mechanism of the Birch Reduction (Figure 11.8)
Step 2: Proton transfer from methanol H H H H H H • • •• H H H H – H H
OCH3 H •• • H –
OCH3 •• •
Dr. Wolf's CHM 201 & 202 6

63. Mechanism of the Birch Reduction (Figure 11.8)
Step 3: Electron transfer from sodium H • + Na
Dr. Wolf's CHM 201 & 202 6

64. Mechanism of the Birch Reduction (Figure 11.8)
Step 3: Electron transfer from sodium H • + Na H – H H •• +
Na+ H H H H
Dr. Wolf's CHM 201 & 202 6

65. Mechanism of the Birch Reduction (Figure 11.8)
Step 4: Proton transfer from methanol •
OCH3 H •• H – H H •• H H H H
Dr. Wolf's CHM 201 & 202 6

66. Mechanism of the Birch Reduction (Figure 11.8)
Step 4: Proton transfer from methanol •
OCH3 H •• – •
OCH3 •• H H – H H •• H H H H
Dr. Wolf's CHM 201 & 202 6

67. 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.
Dr. Wolf's CHM 201 & 202 5

68. 1. Reactions involving the ring
a) Reduction
Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11)
b) Electrophilic aromatic substitution (Chapter 12)
c) Nucleophilic aromatic substitution (Chapter 23)
2. The ring as a substituent (Sections )
Dr. Wolf's CHM 201 & 202 2

69. Free-Radical Halogenation of Alkylbenzenes
Dr. Wolf's CHM 201 & 202 12

70. The Benzene Ring as a Substituent• C • C
allylic radical
benzylic radical
benzylic carbon is analogous to allylic carbon
Dr. Wolf's CHM 201 & 202 2

71. Bond-dissociation energy for C—H bond is equal to DH° for:
Recall:
Bond-dissociation energy for C—H bond is equal to DH° 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.
Dr. Wolf's CHM 201 & 202 13

72. 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.
Dr. Wolf's CHM 201 & 202 14

73. Resonance in Benzyl RadicalH 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
Dr. Wolf's CHM 201 & 202 18

74. Resonance in Benzyl RadicalH •
unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it
Dr. Wolf's CHM 201 & 202 18

75. Resonance in Benzyl RadicalH •
unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it
Dr. Wolf's CHM 201 & 202 18

76. Resonance in Benzyl RadicalH 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
Dr. Wolf's CHM 201 & 202 18

77. Free-radical chlorination of toluene
industrial process
highly regioselective for benzylic position
CH3
Cl2
CH2Cl
light or heat
Toluene
Benzyl chloride
Dr. Wolf's CHM 201 & 202 16

78. Free-radical chlorination of toluene
Similarly, dichlorination and trichlorination are selective for the benzylic carbon. Further chlorination gives:
CHCl2
CCl3
(Dichloromethyl)benzene
(Trichloromethyl)benzene
Dr. Wolf's CHM 201 & 202 16

79. Benzylic Bromination
is used in the laboratory to introduce a halogen at the benzylic position
CH3
NO2
CH2Br
CCl4, 80°C
light
+ Br2
+ HBr
NO2
p-Nitrotoluene
p-Nitrobenzyl
bromide (71%)
Dr. Wolf's CHM 201 & 202 22

80. N-Bromosuccinimide (NBS)
is a convenient reagent for benzylic bromination Br
NBr O
CH2CH3
CHCH3 NH O
CCl4 + +
benzoyl
peroxide,
heat
(87%)
Dr. Wolf's CHM 201 & 202 23

81. Oxidation of Alkylbenzenes
Dr. Wolf's CHM 201 & 202 24

82. Site of Oxidation is Benzylic Carbon
CH3 or
Na2Cr2O7
H2SO4
COH O
CH2R
H2O
heat or
CHR2
Dr. Wolf's CHM 201 & 202 25

83. p-Nitrobenzoic acid (82-86%)
Example
COH O
CH3
NO2
Na2Cr2O7
H2SO4
H2O
heat
NO2
p-Nitrotoluene
p-Nitrobenzoic acid (82-86%)
Dr. Wolf's CHM 201 & 202 26

84. Example O CH(CH3)2 COH Na2Cr2O7 H2SO4 H2O heat CH3 (45%) 26
Dr. Wolf's CHM 201 & 202 26

85. SN1 Reactions of Benzylic Halides
Dr. Wolf's CHM 201 & 202 24

86. Relative solvolysis rates in aqueous acetone
What about SN1?
Relative solvolysis rates in aqueous acetone C
CH3 Cl C
CH3 Cl
600 1
tertiary benzylic carbocation is formed more rapidly than tertiary carbocation; therefore, more stable
Dr. Wolf's CHM 201 & 202 30

87. Relative rates of formation:
What about SN1?
Relative rates of formation:
CH3 +
CH3 + C
CH3 C
more stable
less stable
Dr. Wolf's CHM 201 & 202 30

88. benzylic carbon is analogous to allylic carbon
Compare. + C + C
allylic carbocation
benzylic carbocation
benzylic carbon is analogous to allylic carbon
Dr. Wolf's CHM 201 & 202 2

89. Resonance in Benzyl CationH + 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
Dr. Wolf's CHM 201 & 202 18

90. Resonance in Benzyl CationH 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
Dr. Wolf's CHM 201 & 202 18

91. Resonance in Benzyl CationH 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
Dr. Wolf's CHM 201 & 202 18

92. Resonance in Benzyl CationH 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
Dr. Wolf's CHM 201 & 202 18

93. Solvolysis CH3 C Cl CH3CH2OH CH3 C OCH2CH3 (87%) 37
Dr. Wolf's CHM 201 & 202 37

94. SN2 Reactions of Benzylic Halides
Dr. Wolf's CHM 201 & 202 24

95. Primary Benzylic Halides
CH2Cl
O2N
NaOCCH3 O
Mechanism is SN2
acetic acid
CH2OCCH3
O2N O
(78-82%)
Dr. Wolf's CHM 201 & 202 29

96. Preparation of Alkenylbenzenes
dehydrogenation
dehydration
dehydrohalogenation
Dr. Wolf's CHM 201 & 202 1

97. Dehydrogenation industrial preparation of styrene 630°C CH2CH3 CH CH2
ZnO
+ H2
Dr. Wolf's CHM 201 & 202 2

98. Acid-Catalyzed Dehydration of Benzylic Alcohols
CHCH3 OH Cl Cl
KHSO4
CH2 CH
heat +
H2O
(80-82%)
Dr. Wolf's CHM 201 & 202 3

99. Acid-Catalyzed Dehydration of Benzylic Alcohols
CHCH3 OH Cl Cl
KHSO4
CH2 CH
heat
(80-82%)
CHCH3 Cl +
Dr. Wolf's CHM 201 & 202 3

100. Dehydrohalogenation H3C CH2CHCH3 Br NaOCH2CH3 ethanol, 50°C CH H3C
(99%)
Dr. Wolf's CHM 201 & 202 5

101. Addition Reactions of Alkenylbenzenes
hydrogenation
halogenation
addition of hydrogen halides
Dr. Wolf's CHM 201 & 202 6

102. Hydrogenation Br C CH3 CHCH3 Br CHCH2CH3 CH3 H2 Pt (92%) 7
Dr. Wolf's CHM 201 & 202 7

103. Halogenation
Br2
CH2 CH CH
CH2 Br Br
(82%)
Dr. Wolf's CHM 201 & 202 8

104. Addition of Hydrogen HalidesCl
HCl
(75-84%)
Dr. Wolf's CHM 201 & 202 9

105. Addition of Hydrogen HalidesCl
HCl +
via benzylic carbocation
Dr. Wolf's CHM 201 & 202 9

106. Free-Radical Addition of HBr
CH2 CH
CH2CH2Br
peroxides
Dr. Wolf's CHM 201 & 202 10

107. Free-Radical Addition of HBr
CH2 CH
CH2CH2Br
peroxides CH
CH2Br •
via benzylic radical
Dr. Wolf's CHM 201 & 202 10

108. Polymerization of Styrene
Dr. Wolf's CHM 201 & 202 11

109. Polymerization of Styrene
H2C
CHC6H5
CH2 CH
C6H5
polystyrene
Dr. Wolf's CHM 201 & 202 12

110. Mechanism •• • RO
H2C
CHC6H5 •
Dr. Wolf's CHM 201 & 202 16

111. Mechanism •• • RO
H2C
CHC6H5 •
Dr. Wolf's CHM 201 & 202 16

112. Mechanism •• • RO
H2C
CHC6H5 •
H2C
CHC6H5
Dr. Wolf's CHM 201 & 202 16

113. Mechanism •• • RO
H2C
CHC6H5
H2C
CHC6H5 •
Dr. Wolf's CHM 201 & 202 16

114. Mechanism •• • RO
H2C
CHC6H5
H2C
CHC6H5 •
Dr. Wolf's CHM 201 & 202 16

115. Mechanism •• • RO H2C CHC6H5 H2C CHC6H5 • H2C CHC6H5 16
Dr. Wolf's CHM 201 & 202 16

116. Mechanism •• • RO H2C CHC6H5 H2C CHC6H5 H2C CHC6H5 • 16
Dr. Wolf's CHM 201 & 202 16

117. Mechanism
H2C
CHC6H5 RO •• • •
Dr. Wolf's CHM 201 & 202 16

118. Mechanism
H2C
CHC6H5 RO •• • •
H2C
CHC6H5
Dr. Wolf's CHM 201 & 202 16

119. Mechanism
H2C
CHC6H5 RO •• • •
H2C
CHC6H5
Dr. Wolf's CHM 201 & 202 16

120. Cyclobutadiene and Cyclooctatetraene
Dr. Wolf's CHM 201 & 202 1

121. 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 3

122. Heats of Hydrogenation
to give cyclooctane (kJ/mol) 97
205
303
410
heat of hydrogenation of cyclooctatetraene is more than 4 times heat of hydrogenation of cyclooctene
Dr. Wolf's CHM 201 & 202 4

123. Structure of Cyclooctatetraene
cyclooctatetraene is not planar
has alternating long (146 pm) and short (133 pm) bonds
Dr. Wolf's CHM 201 & 202 5

124. Structure of Cyclobutadiene
structure of a stabilized derivative is characterized by alternating short bonds and long bonds
C(CH3)3
(CH3)3C
CO2CH3
138 pm
151 pm
Dr. Wolf's CHM 201 & 202 5

125. 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.
An antiaromatic substance is one that is destabilized by cyclic conjugation. 5

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

127. Conclusion
there must be some factor in addition to cyclic conjugation that determines whether a molecule is aromatic or not
Dr. Wolf's CHM 201 & 202 5

128. Hückel's Rule: Annulenes
the additional factor that influences aromaticity is the number of p electrons
Dr. Wolf's CHM 201 & 202 6

129. Hückel's Rule
among planar, monocyclic, completely conjugated polyenes, only those with 4n p electrons possess special stability (are aromatic)
n 4n+2
0 2
1 6
2 10
3 14
4 18
Dr. Wolf's CHM 201 & 202 7

130. Hückel's Rule
among planar, monocyclic, completely conjugated polyenes, only those with 4n p electrons possess special stability (are aromatic)
n 4n+2
0 2
1 6 benzene!
2 10
3 14
4 18
Dr. Wolf's CHM 201 & 202 7

131. Hückel's Rule
Hückel restricted his analysis to planar, completely conjugated, monocyclic polyenes
he found that the p molecular orbitals of these compounds had a distinctive pattern
one p orbital was lowest in energy, another was highest in energy, and the others were arranged in pairs between the highest and the lowest
Dr. Wolf's CHM 201 & 202 7

132. 6 p orbitals give 6 p orbitals
p-MOs of Benzene
Antibonding
Benzene
Bonding
6 p orbitals give 6 p orbitals
3 orbitals are bonding; 3 are antibonding
Dr. Wolf's CHM 201 & 202 8

133. 6 p electrons fill all of the bonding orbitals
p-MOs of Benzene
Antibonding
Benzene
Bonding
6 p electrons fill all of the bonding orbitals
all p antibonding orbitals are empty
Dr. Wolf's CHM 201 & 202 8

134. p-MOs of Cyclobutadiene (square planar)
Antibonding
Cyclo- butadiene
Bonding
4 p orbitals give 4p orbitals
1 orbital is bonding, one is antibonding, and 2 are nonbonding
Dr. Wolf's CHM 201 & 202 8

135. p-MOs of Cyclobutadiene (square planar)
Antibonding
Cyclo- butadiene
Bonding
4 p electrons; bonding orbital is filled; other 2 p electrons singly occupy two nonbonding orbitals
Dr. Wolf's CHM 201 & 202 8

136. p-MOs of Cyclooctatetraene (square planar)
Antibonding
Cyclo- octatetraene
Bonding
8 p orbitals give 8 p orbitals
3 orbitals are bonding, 3 are antibonding, and 2 are nonbonding
Dr. Wolf's CHM 201 & 202 8

137. p-MOs of Cyclooctatetraene (square planar)
Antibonding
Cyclo- octatetraene
Bonding
8 p electrons; 3 bonding orbitals are filled; 2 nonbonding orbitals are each half-filled
Dr. Wolf's CHM 201 & 202 8

138. p-Electron Requirement for Aromaticity
4 p electrons
6 p electrons
8 p electrons
not aromatic
not aromatic
aromatic
Dr. Wolf's CHM 201 & 202 2

139. Completely Conjugated Polyenes
6 p electrons; not completely conjugated
6 p electrons; completely conjugated H
not aromatic
aromatic
Dr. Wolf's CHM 201 & 202 2

140. Annulenes 1

141. Annulenes
Annulenes are planar, monocyclic, completely conjugated polyenes. That is, they are the kind of hydrocarbons treated by Hückel's rule.
Dr. Wolf's CHM 201 & 202 15

142. 10-sided regular polygon has angles of 144°
[10]Annulene
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°
Dr. Wolf's CHM 201 & 202 16

143. [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
Dr. Wolf's CHM 201 & 202 16

144. 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 introduces van der Waals strain into the structure and causes the ring to be distorted from planarity
Dr. Wolf's CHM 201 & 202 16

145. 14 p electrons satisfies Hückel's rule
[14]Annulene H H H H
14 p electrons satisfies Hückel's rule
van der Waals strain between hydrogens inside the ring
Dr. Wolf's CHM 201 & 202 16

146. 16 p electrons does not satisfy Hückel's rule
[16]Annulene
16 p electrons does not satisfy Hückel's rule
alternating short (134 pm) and long (146 pm) bonds
not aromatic
Dr. Wolf's CHM 201 & 202 16

147. 18 p electrons satisfies Hückel's rule resonance energy = 418 kJ/mol
[18]Annulene H
18 p electrons satisfies Hückel's rule
resonance energy = 418 kJ/mol
bond distances range between pm
Dr. Wolf's CHM 201 & 202 16

148. Aromatic Ions
Dr. Wolf's CHM 201 & 202 21

149. Cycloheptatrienyl Cation+
6 p electrons delocalized over 7 carbons
positive charge dispersed over 7 carbons
very stable carbocation
also called tropylium cation
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150. Cycloheptatrienyl Cation+ + H
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151. Cycloheptatrienyl CationBr +
Br–
Ionic
Covalent
Tropylium cation is so stable that tropylium bromide is ionic rather than covalent.
mp 203 °C; soluble in water; insoluble in diethyl ether
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152. Cyclopentadienide AnionH •• –
6 p electrons delocalized over 5 carbons
negative charge dispersed over 5 carbons
stabilized anion
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153. Cyclopentadienide AnionH •• – H –
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154. Acidity of CyclopentadieneH H •• – H+ +
pKa = 16
Ka = 10-16
Cyclopentadiene is unusually acidic for a hydrocarbon.
Increased acidity is due to stability of cyclopentadienide anion.
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155. Electron Delocalization in Cyclopentadienide AnionH •• –
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156. Electron Delocalization in Cyclopentadienide AnionH H •• – •• –
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157. Electron Delocalization in Cyclopentadienide AnionH H •• – •• – H • –
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158. Electron Delocalization in Cyclopentadienide AnionH H •• – •• – H H – • • –
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159. Electron Delocalization in Cyclopentadienide AnionH H •• – •• – H H H H – • •• • – H H H
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160. Compare Acidities of Cyclopentadiene and Cycloheptatriene
pKa = 16
Ka = 10-16
pKa = 36
Ka = 10-36
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161. Compare Acidities of Cyclopentadiene and Cycloheptatriene•• H •• – –
Aromatic anion
6 p electrons
Anion not aromatic 8 p electrons
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162. Cyclopropenyl Cation + H H + also written as n = 0
4n +2 = 2 p electrons
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163. Cyclooctatetraene DianionH H H H – H •• H H H
also written as 2– •• H H H H – H H H H
n = 2
4n +2 = 10 p electrons
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164. Heterocyclic Aromatic Compounds
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165. Examples N N H O S Pyridine Pyrrole Furan Thiophene •• •• •• •• 31
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166. Examples N •• N •
Quinoline
Isoquinoline
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167. Heterocyclic Aromatic Compounds and Hückel's Rule
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168. Pyridine 6 p electrons in ring
lone pair on nitrogen is in an sp2 hybridized orbital; not part of p system of ring N ••
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169. Pyrrole
lone pair on nitrogen must be part of ring p system if ring is to have 6 p electrons
lone pair must be in a p orbital in order to overlap with ring p system N H ••
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170. two lone pairs on oxygen
Furan
two lone pairs on oxygen
one pair is in a p orbital and is part of ring p system; other is in an sp2 hybridized orbital and is not part of ring p system O ••
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171. End of Chapter 11
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