Chapter 11 Arenes and Aromaticity

1
Chapter 11Arenes and Aromaticity
2
Examples of Aromatic Hydrocarbons
Benzene
Toluene
Naphthalene
3
Another Example of an Aromatic Hydrocarbon
Common source gum benzoin tree
Methyl paraben
Used as a preservative in food and cosmetics.
American queen bee pheromone
4
11.1Benzene
5
Some History

• 1825 Michael Faraday isolates a new hydrocarbon
  from illuminating gas.
• 1834 Eilhardt Mitscherlich isolates same
  substance and determines its empirical formula
  to be CnHn. Compound comes to be called benzene.
• 1845 August W. von Hofmann isolates benzene from
  coal tar.
• 1865 August Kekulé proposes cyclic structure of
  benzene.
• 1929 Kathleen Lonsdale confirms cyclic structure
  of benzene by X-ray crystallography.

6
11.2Kekulé and theStructure of Benzene
7
Kekulé Formulation of Benzene

• Kekulé proposed a cyclic structure for C6H6with
  alternating single and double bonds.

8
Kekulé Formulation of Benzene

• However, this proposal suggested isomers of
  thekind shown were possible. Yet, none were
  everfound.

9
Kekulé Formulation of Benzene

• Kekulé revised his proposal by suggestinga rapid
  equilibrium between two equivalentstructures.

10
Structure of Benzene

• Structural studies of benzene demonstrate that it
  does not have alternating single and double
  bonds. All of the CC bonds are the same length.

Benzene has the shape of a regular hexagon.
11
All CC bond distances 140 pm
140 pm
140 pm
140 pm
140 pm
140 pm
140 pm

• 140 pm is the average between the CC single
  bond distance and the double bond distance in
  1,3-butadiene.

12
11.3A Resonance Picture of Bonding in Benzene
13
Kekulé Formulation of Benzene

• Instead of Kekulé's suggestion of a
  rapidequilibrium between two structures

14
Resonance Formulation of Benzene

• Express the structure of benzene as a
  resonancehybrid of the two Lewis structures.
  Electrons arenot localized in alternating single
  and double bonds,but are delocalized over all
  six ring carbons.

15
Resonance Formulation of Benzene

• Circle-in-a-ring notation stands for resonance
  description of benzene (hybrid of two Kekulé
  structures).

16
11.4The 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.

17
Thermochemical Measures of Stability

• Heat of hydrogenation compare experimentalvalue
  with "expected" value for hypothetical"cyclohexat
  riene."

Pt

3H2
?H 208 kJ/mol
18

19
3 x cyclohexene

• "Expected" heat of hydrogenation of benzene is 3
  x heat of hydrogenation of cyclohexene.

360 kJ/mol
120 kJ/mol
20
3 x cyclohexene

• Observed heat of hydrogenation is 152 kJ/mol
  less than "expected."
• Benzene is 152 kJ/mol more stable thanexpected.
• 152 kJ/mol is the resonance energy of benzene.

360 kJ/mol
208 kJ/mol
21

• Hydrogenation of 1,3-cyclohexadiene (2H2) gives
  off more heat than hydrogenation of benzene (3H2)!

231 kJ/mol
208 kJ/mol
22
Cyclic Conjugation versus Noncyclic Conjugation
3H2
Pt
Heat of hydrogenation 208 kJ/mol
3H2
Pt

Heat of hydrogenation 337 kJ/mol
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.

24
11.5An Orbital Hybridization Viewof Bonding in
Benzene
25
Orbital Hybridization Model of Bonding in Benzene

• Planar ring of 6 sp2 hybridized carbons

26
Orbital Hybridization Model of Bonding in Benzene

• Each carbon contributes a p orbital.
• Six p orbitals overlap to give cyclic ?
  system.Six ? electrons delocalized throughout ?
  system.

27
Orbital Hybridization Model of Bonding in Benzene

• High electron density above and below plane of
  ring.

28
11.6The ? Molecular Orbitalsof Benzene
29
Benzene MOs
Antibondingorbitals
Bondingorbitals

• 6 p AOs combine to give 6 ? MOs.
• 3 MOs are bonding 3 are antibonding.

30
Benzene MOs
Antibondingorbitals
Bondingorbitals

• All bonding MOs are filled.
• No electrons in antibonding orbitals.

31
Benzene MOs
32
11.7Substituted Derivatives of Benzene and
Their Nomenclature
33
General Points

• 1) Benzene is considered as the parent andcomes
  last in the name.

34
General Points

• 1. Benzene is considered as the parent andcomes
  last in the name.
• 2. Number ring in direction that gives lowest
  locant at first point of difference.
• 3. List substituents in alphabetical order.

35
Example
Cl
Br
F
2-Bromo-1-chloro-4-fluorobenzene
36
Ortho, Meta and Para
Alternative locants for disubstitutedderivatives
of benzene.
1,2 ortho(abbreviated o-)
37
Examples
NO2
CH2CH3
38
Certain Monosubstituted Derivatives of Benzene
Have Unique Names
39
Benzaldehyde
40
Benzoic acid
41
Styrene
42
Acetophenone
43
Phenol
44
Anisole
45
Aniline
46
Names can be used as parent
Anisole
p-Nitroanisoleor4-Nitroanisole
47
Names can be used as parent
Benzoic acid
o-Acetoxybenzoic acidor 2-Acetoxybenzoic acid
(Acetylsalicylic acid Aspirin)
48
Easily Confused Names
49
11.8Polycyclic Aromatic Hydrocarbons
50
Naphthalene

• Resonance energy 255 kJ/mol

Most stable Lewis structureboth rings
correspond to Kekulé benzene.
51
Anthracene and Phenanthrene
Phenanthrene
Anthracene
52
11.9Physical Properties of Arenes
53
Physical Properties

• Arenes (aromatic hydrocarbons) resembleother
  hydrocarbons. They are
• Nonpolar
• Insoluble in water
• Less dense than water

54
11.10Reactions of ArenesA Preview

• 1. Some reactions involve the ring.
• 2. In other reactions the ring is a substituent.

55
1. Reactions involving the ring

• a) Reduction
• Catalytic hydrogenation (Section 11.4) Birch
  reduction (Section 11.11)
• b) Electrophilic aromatic substitution (Chapter
  12)

2. The ring as a substituent (Sections
11.12-11.17)
56
Reduction of Benzene Rings
Catalytic hydrogenation
57
11.11The Birch Reduction
58
Birch Reduction of Benzene
Na, NH3
CH3OH
Benzene
1,4-Cyclohexadiene (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.

59
Mechanism of the Birch Reduction

• Step 1 Electron transfer from sodium

Benzene
Benzene anion radical
60
Mechanism of the Birch Reduction

• Step 2 Proton transfer from methanol

H
H
H


H
H

H

61
Mechanism of the Birch Reduction

• Step 3 Electron transfer from sodium

62
Mechanism of the Birch Reduction

• Step 4 Proton transfer from methanol

H

H
H

H
H
H
H
63
Birch Reduction of an Alkylbenzene
Na, NH3
CH3OH
tert-Butyl-1,4-cyclohexadiene (86)
tert-Butylbenzene

• If an alkyl group is present on the ring, it ends
  up asa substituent on the double bond.

64
11.12Free-Radical Halogenationof Alkylbenzenes
65
The Benzene Ring as a Substituent
allylic radical
benzylic radical

• Benzylic carbon is analogous to allylic carbon.

66
Recall
Bond-dissociation energy for CH bond is equal
to ?H for

RH
R
H
and is about 400 kJ/mol for alkanes (380- 410
kJ/mol, depending on degree of substitution).

• The more stable the free radical R, the weaker
  the bond, and the smaller the bond-dissociation
  energy.

67
Bond-Dissociation Energies of Propene and Toluene
368 kJ/mol
H2C
CH
-H
356 kJ/mol
-H

• Low BDEs indicate allyl and benzyl radical are
  more stable than simple alkyl radicals.

68
Resonance in Benzyl Radical
H
H

H
H
H
H
H

• Unpaired electron is delocalized between
  benzylic carbon and the ring carbons that are
  ortho and para to it.

69
Resonance in Benzyl Radical

• Unpaired electron is delocalized between
  benzylic carbon and the ring carbons that are
  ortho and para to it.

70
Resonance in Benzyl Radical

• Unpaired electron is delocalized between
  benzylic carbon and the ring carbons that are
  ortho and para to it.

71
Resonance in Benzyl Radical
H
H
H
H

H
H
H

• Unpaired electron is delocalized between
  benzylic carbon and the ring carbons that are
  ortho and para to it.

72
Spin Density in Benzyl Radical

• Unpaired electron is delocalized between
  benzylic carbon and the ring carbons that are
  ortho and para to it.

73
Free-Radical Chlorination of Toluene

• Industrial process.
• Highly regioselective for benzylic position.

74
Free-Radical Chlorination of Toluene

• Similarly, dichlorination and trichlorination
  areselective for the benzylic carbon.
  Furtherchlorination gives

(Dichloromethyl)benzene
(Trichloromethyl)benzene
75
Benzylic Bromination

• Is used in the laboratory to introduce a halogen
  at the benzylic position.

Br2
p-Nitrotoluene
76
N-Bromosuccinimide (NBS)

• A convenient reagent for benzylic bromination.

77
11.13Oxidation of Alkylbenzenes
78
Site of Oxidation is Benzylic Carbon
or
or
Potassium permanganate (KMnO4) will also oxidize
the benzylic carbon.
79
Example

• KMnO4, H2O,
• heat

NO2
p-Nitrotoluene
p-Nitrobenzoicacid (82-86)
80
Example
(45)
81
11.14 11.15Nucleophilic Substitutionin
Benzylic Halides
82
Primary Benzylic Halides

• Mechanism is SN2.

acetic acid
(78-82)
83
What about SN1?
Relative solvolysis rates in aqueous acetone.
620
1

• Both are tertiary chlorides, but benzylic
  carbocation is formed more rapidly than
  tert-butyl carbocation it is more stable.

84
What about SN1?
Relative rates of reaction determined by
stabilities of the carbocation intermediates
CH3
C
C
More stable
Less stable
85
Compare...
allylic carbocation
benzylic carbocation

• Benzylic carbon is analogous to allylic carbon.

86
Resonance in Benzyl Cation
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.

87
Resonance in Benzyl Cation
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.
88
Resonance in Benzyl Cation
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.
89
Resonance in Benzyl Cation
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.
90
Solvolysis
CH3CH2OH
(87)
91
11.16Preparation of Alkenylbenzenes

• Dehydrogenation
• Dehydration
• Dehydrohalogenation

92
Dehydrogenation

• Industrial preparation of styrene.
• About 13 billion lbs. produced annually in U.S.

630C
ZnO
H2
93
Acid-Catalyzed Dehydration of Benzylic Alcohols
KHSO4
heat
(80-82)
94
Dehydrohalogenation
95
11.17Addition Reactions of Alkenylbenzenes

• Hydrogenation
• Halogenation
• Addition of hydrogen halides

96
Hydrogenation
(92)
97
Halogenation
Br2
CH
CH2
Br
Br
(82)
98
Addition of Hydrogen Halides
(75-84)
99
Free-Radical Addition of HBr
HBr
peroxides
Note In the absence of peroxides, the benzylic
position would be brominated instead.
100
11.18Polymerization of Styrene
101
Polymerization of Styrene
polystyrene
102
Mechanism

RO
103
Mechanism

RO
H2C
CHC6H5

104
Mechanism

RO
H2C
CHC6H5
105
Mechanism

RO
H2C
CHC6H5
H2C
CHC6H5

106
Mechanism

RO
H2C
CHC6H5
H2C
CHC6H5
107
Mechanism

108
11.19Cyclobutadiene and Cyclooctatetraene
109
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.

110
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.

111
Structure of Cyclooctatetraene

• Cyclooctatetraene is not planar.
• It has alternating long (146 pm)and short (133
  pm) bonds.

112
Structure of Cyclobutadiene

• MO calculations give alternating short and
  longbonds for cyclobutadiene.

H
H
135 pm
156 pm
H
H
113
Structure of Cyclobutadiene

• Structure of a stabilized derivative is
  characterizedby alternating short bonds and long
  bonds.

114
Stability of Cyclobutadiene

• Cyclobutadiene is observed to be highly reactive,
  and too unstable to be isolated and stored in
  thecustomary way.
• Not only is cyclobutadiene not aromatic, it is
  antiaromatic.
• An antiaromatic substance is one that is
  destabilizedby cyclic conjugation.

115
Requirements for Aromaticity

• Cyclic conjugation is necessary, but not
  sufficient.

116
Conclusion

• There must be some factor in additionto cyclic
  conjugation that determines whether a molecule
  is aromatic or not.

117
11.20Hückel's Rule

• The additional factor that influences aromaticity
  is the number of ? electrons.

118
Hückel's Rule

• Among planar, monocyclic, completely conjugated
  polyenes, only those with 4n 2 ? electrons
  possess special stability (are aromatic)
• n 4n2
• 0 2
• 1 6 Benzene!
• 2 10
• 3 14
• 4 18

119
Hückel's Rule

• Hückel restricted his analysis to
  planar,completely conjugated, monocyclic
  polyenes.
• He found that the ? molecular orbitals ofthese
  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 highestand the lowest.

120
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 planar monocyclic conjugated
  system.

1. Draw a circle.
2. Inscribe a regular polygon inside the circleso
   that one of its corners is at the bottom.
3. Every point where a corner of the polygontouches
   the circle corresponds to a p electronenergy
   level.
4. The middle of the circle separates bondingand
   antibonding orbitals.

121
Frost's Circle

122
?-MOs of Benzene
Antibonding
Benzene
Bonding

• 6 p orbitals give 6 ? orbitals.
• 3 orbitals are bonding 3 are antibonding.

123
?-MOs of Benzene
Antibonding
Benzene
Bonding

• 6 ? electrons fill all of the bonding orbitals.
• All ? antibonding orbitals are empty.

124
?-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.

125
?-MOs of Cyclobutadiene(square planar)
Antibonding
Cyclo-butadiene
Bonding

• 4 ? electrons bonding orbital is filled other
  2? electrons singly occupy two nonbonding
  orbitals.

126
?-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.

127
?-MOs of Cyclooctatetraene(planar)
Antibonding
Cyclo-octatetraene
Bonding

• 8 ? electrons 3 bonding orbitals are filled
  2nonbonding orbitals are each half-filled.

128
?-Electron Requirement for Aromaticity
4 ? electrons
6 ? electrons
8 ? electrons
Antiaromatic
Antiaromatic
Aromatic
129
Completely Conjugated Polyenes
6 ? electronsnot completelyconjugated
6 ? electronscompletely conjugated
Notaromatic
Aromatic
130
11.21Annulenes
131
Annulenes

• Annulenes are planar, monocyclic, completely
  conjugated polyenes. That is, they are the kind
  of hydrocarbons treated by Hückel's rule.

132
10Annulene

• 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.

133
10Annulene

• Incorporating two trans double bonds intothe
  ring relieves angle strain but introducesvan der
  Waals strain into the structure andcauses the
  ring to be distorted from planarity.

134
14Annulene
H
H
H
H

• 14 ? electrons satisfies Hückel's rule.
• van der Waals strain between hydrogens insidethe
  ring.

135
16Annulene

• 16 ? electrons does not satisfy Hückel's rule.
• Alternating short (134 pm) and long (146 pm)
  bonds.
• Is an antiaromatic 4n p-electron system.

136
18Annulene

• 18 ? electrons satisfies Hückel's rule.
• Resonance energy 418 kJ/mol.
• Bond distances range between 137-143 pm.

137
11.22Aromatic Ions
138
Cycloheptatrienyl Cation

• 6 ? electrons delocalizedover 7 carbons.
• Positive charge dispersedover 7 carbons.
• Very stable carbocation.
• Also called tropylium cation.

139
Cycloheptatrienyl Cation
140
Cycloheptatrienyl Cation
Br
Ionic
Covalent

• Tropylium cation is so stable that
  tropyliumbromide is ionic rather than covalent.
• mp 203 C soluble in water insoluble indiethyl
  ether.

141
Cyclopentadienyl Anion

• 6 ? electrons delocalizedover 5 carbons.
• Negative charge dispersedover 5 carbons.
• Stabilized anion.

142
Cyclopentadienide Anion
143
Acidity of Cyclopentadiene

• Cyclopentadiene is unusually acidic for a
  hydrocarbon.
• Increased acidity is due to stability of
  cyclopentadienyl anion.

pKa 16 Ka 10-16
144
Electron Delocalization in Cyclopentadienyl Anion


145
Compare Acidities ofCyclopentadiene and
Cycloheptatriene
pKa 16 Ka 10-16
pKa 36 Ka 10-36
146
Compare Acidities ofCyclopentadiene and
Cycloheptatriene

Aromatic anion 6 ? electrons
Antiaromatic anion8 ? electrons
147
Cyclopropenyl Cation
also written as

• n 0
• 4n 2 2 ? electrons

148
Cyclooctatetraene Dianion
H
H
H
H

H
H
H
H

alsowritten as
2

H
H
H
H

H
H
H
H

• n 2
• 4n 2 10?? electrons

149
11.23Heterocyclic Aromatic Compounds
150
Examples
Pyridine
Pyrrole
Furan
Thiophene
151
Examples
Adenine
Quinoline
Isoquinoline
152
11.24Heterocyclic Aromatic CompoundsandHückel's
Rule
153
Pyridine

• 6 ? electrons in ring.
• Lone pair on nitrogen is in ansp2 hybridized
  orbitalnot part of ??system of ring.

154
Pyrrole

• Lone pair on nitrogen must be part of ring ?
  system if ring is to have6 ? electrons.
• Lone pair must be in a p orbitalin order to
  overlap with ring ?system.

155
Furan

• Two lone pairs on oxygen.
• One pair is in a p orbital and is partof ring ?
  system other is in an sp2 hybridized orbital
  and is notpart of ring ? system.