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Dr. Wolf's CHM 201 & 202 11-1
Chapter 11Chapter 11Arenes and AromaticityArenes and Aromaticity
Dr. Wolf's CHM 201 & 202 11-2
BenzeneBenzene TolueneToluene
NaphthaleneNaphthalene
Examples of Aromatic HydrocarbonsExamples of Aromatic HydrocarbonsExamples of Aromatic HydrocarbonsExamples of Aromatic Hydrocarbons
HH
HH HH
HHHH
HH
CHCH33
HH
HH
HHHH
HH HHHH
HH
HHHH
HH
HH HH
Dr. Wolf's CHM 201 & 202 11-3
11.1Benzene
Dr. Wolf's CHM 201 & 202 11-4
Some historySome historySome historySome history
18341834 Eilhardt Mitscherlich isolates a new Eilhardt Mitscherlich isolates a new hydrocarbon and determines its hydrocarbon and determines its
empirical empirical formula to be Cformula to be CnnHHnn. Compound . Compound
comes comes to be called to be called benzenebenzene..
18451845 August W. von Hofmann isolates August W. von Hofmann isolates benzene benzene from coal tar.from coal tar.
18661866 August Kekulé proposes structure of August Kekulé proposes structure of benzene.benzene.
Dr. Wolf's CHM 201 & 202 11-5
11.2Kekulé and the
Structure of Benzene
Dr. Wolf's CHM 201 & 202 11-6
Kekulé proposed a cyclic structure for CKekulé proposed a cyclic structure for C66HH66
with alternating single and double bonds.with alternating single and double bonds.
Kekulé Formulation of BenzeneKekulé Formulation of BenzeneKekulé Formulation of BenzeneKekulé Formulation of Benzene
HH
HH HH
HHHH
HH
Dr. Wolf's CHM 201 & 202 11-7
Later, Kekulé revised his proposal by suggestingLater, Kekulé revised his proposal by suggestinga rapid equilibrium between two equivalenta rapid equilibrium between two equivalentstructures.structures.
Kekulé Formulation of BenzeneKekulé Formulation of BenzeneKekulé Formulation of BenzeneKekulé Formulation of Benzene
HH
HH HH
HHHH
HH
HH
HH
HH
HHHH
HH
Dr. Wolf's CHM 201 & 202 11-8
However, this proposal suggested isomers of theHowever, this proposal suggested isomers of thekind shown were possible. Yet, none were everkind shown were possible. Yet, none were everfound.found.
Kekulé Formulation of BenzeneKekulé Formulation of BenzeneKekulé Formulation of BenzeneKekulé Formulation of Benzene
HH
XX XX
HHHH
HH
HH
XX
XX
HHHH
HH
Dr. Wolf's CHM 201 & 202 11-9
Structural studies of benzene do not support theStructural studies of benzene do not support theKekulé formulation. Instead of alternating singleKekulé formulation. Instead of alternating singleand double bonds, all of the C—C bonds are theand double bonds, all of the C—C bonds are thesame length.same length.
Structure of BenzeneStructure of BenzeneStructure of BenzeneStructure of Benzene
Benzene has the shape of a regular hexagon.Benzene has the shape of a regular hexagon.
Dr. Wolf's CHM 201 & 202 11-10
140 pm140 pm 140 pm140 pm
140 pm140 pm 140 pm140 pm
140 pm140 pm140 pm140 pm
All C—C bond distances = 140 pmAll C—C bond distances = 140 pmAll C—C bond distances = 140 pmAll C—C bond distances = 140 pm
Dr. Wolf's CHM 201 & 202 11-11
140 pm140 pm 140 pm140 pm
140 pm140 pm 140 pm140 pm
140 pm140 pm140 pm140 pm
146 pm146 pm
134 pm134 pm
All C—C bond distances = 140 pmAll C—C bond distances = 140 pmAll C—C bond distances = 140 pmAll C—C bond distances = 140 pm
140 pm is the average between the C—C 140 pm is the average between the C—C single bond distance and the double bond single bond distance and the double bond distance in 1,3-butadiene.distance in 1,3-butadiene.
Dr. Wolf's CHM 201 & 202 11-12
11.3A Resonance Picture of
Bonding in Benzene
Dr. Wolf's CHM 201 & 202 11-13
Instead of Kekulé's suggestion of a rapidInstead of Kekulé's suggestion of a rapidequilibrium between two structures:equilibrium between two structures:
HH
HH HH
HHHH
HH
HH
HH
HH
HHHH
HH
Kekulé Formulation of BenzeneKekulé Formulation of BenzeneKekulé Formulation of BenzeneKekulé Formulation of Benzene
Dr. Wolf's CHM 201 & 202 11-14
express the structure of benzene as a express the structure of benzene as a resonanceresonancehybridhybrid of the two Lewis structures. Electrons are of the two Lewis structures. Electrons arenot localized in alternating single and double bonds,not localized in alternating single and double bonds,but are delocalized over all six ring carbons.but are delocalized over all six ring carbons.
Resonance Formulation of BenzeneResonance Formulation of BenzeneResonance Formulation of BenzeneResonance Formulation of Benzene
HH
HH HH
HHHH
HH
HH
HH
HH
HHHH
HH
Dr. Wolf's CHM 201 & 202 11-15
Circle-in-a-ring notation stands for resonance Circle-in-a-ring notation stands for resonance description of benzene (hybrid of two Kekulé description of benzene (hybrid of two Kekulé structures)structures)
Resonance Formulation of BenzeneResonance Formulation of BenzeneResonance Formulation of BenzeneResonance Formulation of Benzene
Dr. Wolf's CHM 201 & 202 11-16
11.4The Stability of Benzene
benzene is the best and most familiar example benzene is the best and most familiar example of a substance that possesses "special stability" of a substance that possesses "special stability" or "aromaticity"or "aromaticity"
aromaticity is a level of stability that is substantially aromaticity is a level of stability that is substantially greater for a molecule than would be expected on greater for a molecule than would be expected on the basis of any of the Lewis structures written for it the basis of any of the Lewis structures written for it
Dr. Wolf's CHM 201 & 202 11-17
heat of hydrogenation: heat of hydrogenation: compare experimentalcompare experimentalvalue with "expected" value for hypotheticalvalue with "expected" value for hypothetical"cyclohexatriene""cyclohexatriene"
H°= – 208 kJH°= – 208 kJ
Thermochemical Measures of StabilityThermochemical Measures of StabilityThermochemical Measures of StabilityThermochemical Measures of Stability
++ 3H3H22
PtPt
Dr. Wolf's CHM 201 & 202 11-18
120 kJ/mol120 kJ/mol120 kJ/mol120 kJ/mol
231 kJ/mol231 kJ/mol231 kJ/mol231 kJ/mol
208 kJ/mol208 kJ/mol
360 kJ/mol360 kJ/mol360 kJ/mol360 kJ/mol 3 x cyclohexene3 x cyclohexeneFigure 11.2 (p 404)Figure 11.2 (p 404)Figure 11.2 (p 404)Figure 11.2 (p 404)
Dr. Wolf's CHM 201 & 202 11-19
120 kJ/mol120 kJ/mol120 kJ/mol120 kJ/mol
360 kJ/mol360 kJ/mol360 kJ/mol360 kJ/mol 3 x cyclohexene3 x cyclohexeneFigure 11.2 (p 404)Figure 11.2 (p 404)Figure 11.2 (p 404)Figure 11.2 (p 404)
"expected" "expected" heat of heat of hydrogenation hydrogenation of benzene is of benzene is 3 x heat of 3 x heat of hydrogenation hydrogenation of cyclohexeneof cyclohexene
Dr. Wolf's CHM 201 & 202 11-20
208 kJ/mol208 kJ/mol
360 kJ/mol360 kJ/mol360 kJ/mol360 kJ/mol 3 x cyclohexene3 x cyclohexeneFigure 11.2 (p 404)Figure 11.2 (p 404)Figure 11.2 (p 404)Figure 11.2 (p 404)
observed heat of observed heat of hydrogenation is hydrogenation is 152 kJ/mol less 152 kJ/mol less than "expected"than "expected"
benzene is 152 benzene is 152 kJ/mol more kJ/mol more stable thanstable thanexpectedexpected
152 kJ/mol is the 152 kJ/mol is the resonance energy resonance energy of benzeneof benzene
Dr. Wolf's CHM 201 & 202 11-21
hydrogenation hydrogenation of 1,3-of 1,3-cyclohexadiene cyclohexadiene (2H(2H22) gives off ) gives off
more heat than more heat than hydrogenation hydrogenation of benzene of benzene (3H(3H22)!)!
231 kJ/mol231 kJ/mol231 kJ/mol231 kJ/mol
208 kJ/mol208 kJ/mol Figure 11.2 (p 404)Figure 11.2 (p 404)Figure 11.2 (p 404)Figure 11.2 (p 404)
Dr. Wolf's CHM 201 & 202 11-22
heat of hydrogenation = 208 kJ/molheat of hydrogenation = 208 kJ/mol
heat of hydrogenation = 337 kJ/molheat of hydrogenation = 337 kJ/mol
3H3H22
PtPt
3H3H22
PtPt
Cyclic conjugation versus noncyclic conjugationCyclic conjugation versus noncyclic conjugationCyclic conjugation versus noncyclic conjugationCyclic conjugation versus noncyclic conjugation
Dr. Wolf's CHM 201 & 202 11-23
compared to localized 1,3,5-cyclohexatrienecompared to localized 1,3,5-cyclohexatriene
152 kJ/mol152 kJ/mol
compared to 1,3,5-hexatrienecompared to 1,3,5-hexatriene
129 kJ/mol129 kJ/mol
exact value of resonance energy of benzene exact value of resonance energy of benzene depends on what it is compared to, but depends on what it is compared to, but regardless of model, benzene is more stable regardless of model, benzene is more stable than expected by a substantial amount than expected by a substantial amount
Resonance Energy of BenzeneResonance Energy of BenzeneResonance Energy of BenzeneResonance Energy of Benzene
Dr. Wolf's CHM 201 & 202 11-24
11.5An Orbital Hybridization View
of Bonding in Benzene
Dr. Wolf's CHM 201 & 202 11-25
Orbital Hybridization Model of Orbital Hybridization Model of Bonding in BenzeneBonding in Benzene
Orbital Hybridization Model of Orbital Hybridization Model of Bonding in BenzeneBonding in Benzene
Figure 11.3Figure 11.3Figure 11.3Figure 11.3
Planar ring of 6 Planar ring of 6 spsp22 hybridized carbons hybridized carbons
Dr. Wolf's CHM 201 & 202 11-26
Orbital Hybridization Model of Orbital Hybridization Model of Bonding in BenzeneBonding in Benzene
Orbital Hybridization Model of Orbital Hybridization Model of Bonding in BenzeneBonding in Benzene
Figure 11.3Figure 11.3Figure 11.3Figure 11.3
Each carbon contributes a Each carbon contributes a pp orbital orbital
Six Six pp orbitals overlap to give cyclic orbitals overlap to give cyclic system; system;six six electrons delocalized throughout electrons delocalized throughout system system
Dr. Wolf's CHM 201 & 202 11-27
Orbital Hybridization Model of Orbital Hybridization Model of Bonding in BenzeneBonding in Benzene
Orbital Hybridization Model of Orbital Hybridization Model of Bonding in BenzeneBonding in Benzene
Figure 11.3Figure 11.3Figure 11.3Figure 11.3
High electron density above and below plane High electron density above and below plane of ringof ring
Dr. Wolf's CHM 201 & 202 11-28
11.6The Molecular Orbitals
of Benzene
Dr. Wolf's CHM 201 & 202 11-29
EnergyEnergy
BondingBondingorbitalsorbitals
AntibondingAntibondingorbitalsorbitals
Benzene MOsBenzene MOsBenzene MOsBenzene MOs
6 6 p p AOs combine to give 6 AOs combine to give 6 MOs MOs3 MOs are bonding; 3 are antibonding3 MOs are bonding; 3 are antibonding
Dr. Wolf's CHM 201 & 202 11-30
EnergyEnergy
BondingBondingorbitalsorbitals
AntibondingAntibondingorbitalsorbitals
Benzene MOsBenzene MOsBenzene MOsBenzene MOs
All bonding MOs are filledAll bonding MOs are filledNo electrons in antibonding orbitalsNo electrons in antibonding orbitals
Dr. Wolf's CHM 201 & 202 11-31
The Three Bonding The Three Bonding MOs of Benzene MOs of BenzeneThe Three Bonding The Three Bonding MOs of Benzene MOs of Benzene
Dr. Wolf's CHM 201 & 202 11-32
11.711.7
Substituted Derivatives of Benzene Substituted Derivatives of Benzene
and Their Nomenclature and Their Nomenclature
Dr. Wolf's CHM 201 & 202 11-33
1) Benzene is considered as the parent and1) Benzene is considered as the parent andcomes last in the name.comes last in the name.
General PointsGeneral PointsGeneral PointsGeneral Points
Dr. Wolf's CHM 201 & 202 11-34
ExamplesExamplesExamplesExamples BromobenzeneBromobenzene terttert-Butylbenzene-Butylbenzene NitrobenzeneNitrobenzene
NONO22C(CHC(CH33))33BrBr
Dr. Wolf's CHM 201 & 202 11-35
1) Benzene is considered as the parent and1) Benzene is considered as the parent andcomes last in the name.comes last in the name.
2) List substituents in alphabetical order 2) List substituents in alphabetical order
3) Number ring in direction that gives lowest 3) Number ring in direction that gives lowest locant at first point of differencelocant at first point of difference
General PointsGeneral PointsGeneral PointsGeneral Points
Dr. Wolf's CHM 201 & 202 11-36
2-2-bromobromo-1--1-chlorochloro-4--4-fluorofluorobenzenebenzene
ExampleExampleExampleExample
BrBr
ClCl
FF
Dr. Wolf's CHM 201 & 202 11-37
Ortho, Meta, and ParaOrtho, Meta, and ParaOrtho, Meta, and ParaOrtho, Meta, and Para alternative locants for disubstitutedalternative locants for disubstitutedderivatives of benzenederivatives of benzene
1,2 = ortho1,2 = ortho(abbreviated (abbreviated oo-)-)
1,3 = meta1,3 = meta(abbreviated (abbreviated mm-)-)
1,4 = para1,4 = para(abbreviated (abbreviated pp-)-)
Dr. Wolf's CHM 201 & 202 11-38
ExamplesExamplesExamplesExamples oo-ethylnitrobenzene-ethylnitrobenzene
NONO22
CHCH22CHCH33
ClCl
ClCl
mm-dichlorobenzene-dichlorobenzene
(1-ethyl-2-nitrobenzene)(1-ethyl-2-nitrobenzene) (1,3-dichlorobenzene)(1,3-dichlorobenzene)
Dr. Wolf's CHM 201 & 202 11-39
Certain monosubstituted derivatives of benzene Certain monosubstituted derivatives of benzene have unique nameshave unique names
Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives
Dr. Wolf's CHM 201 & 202 11-40
BenzaldehydeBenzaldehyde
Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives
CHCH
OO
Dr. Wolf's CHM 201 & 202 11-41
Benzoic acidBenzoic acid
Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives
COHCOH
OO
Dr. Wolf's CHM 201 & 202 11-42
StyreneStyrene
Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives CHCH22CHCH
Dr. Wolf's CHM 201 & 202 11-43
TolueneToluene
Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives CHCH33
Dr. Wolf's CHM 201 & 202 11-44
AcetophenoneAcetophenone
Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives
CCHCCH33
OO
Dr. Wolf's CHM 201 & 202 11-45
PhenolPhenol
Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives OHOH
Dr. Wolf's CHM 201 & 202 11-46
AnisoleAnisole
Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives OCHOCH33
Dr. Wolf's CHM 201 & 202 11-47
AnilineAniline
Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives NHNH22
Dr. Wolf's CHM 201 & 202 11-48
Benzene derivative names can be used as parentBenzene derivative names can be used as parentBenzene derivative names can be used as parentBenzene derivative names can be used as parent OCHOCH33
NONO22
OCHOCH33
AnisoleAnisole pp-Nitroanisole-Nitroanisoleoror
4-Nitroanisole4-Nitroanisole
Dr. Wolf's CHM 201 & 202 11-49
Easily confused namesEasily confused namesEasily confused namesEasily confused names phenylphenyl phenolphenol benzylbenzyl
OHOH CHCH22——
Dr. Wolf's CHM 201 & 202 11-50
11.8Polycyclic Aromatic Hydrocarbons
Dr. Wolf's CHM 201 & 202 11-51
resonance energy = 255 kJ/molresonance energy = 255 kJ/mol
most stable Lewis structure;most stable Lewis structure;both rings correspond to both rings correspond to
Kekulé benzeneKekulé benzene
NaphthaleneNaphthaleneNaphthaleneNaphthalene
Dr. Wolf's CHM 201 & 202 11-52
AnthraceneAnthracene PhenanthrenePhenanthrene
resonance energy:resonance energy:
347 kJ/mol347 kJ/mol 381 kJ/mol381 kJ/mol
Anthracene and PhenanthreneAnthracene and PhenanthreneAnthracene and PhenanthreneAnthracene and Phenanthrene
Dr. Wolf's CHM 201 & 202 11-53
11.9Physical Properties of Arenes
Dr. Wolf's CHM 201 & 202 11-54
Resemble other hydrocarbonsResemble other hydrocarbons
nonpolarnonpolar
insoluble in waterinsoluble in water
less dense than waterless dense than water
Physical PropertiesPhysical PropertiesPhysical PropertiesPhysical Properties
Dr. Wolf's CHM 201 & 202 11-55
11.1011.10
Reactions of Arenes:Reactions of Arenes:
A PreviewA Preview
1. Some reactions involve the ring.1. Some reactions involve the ring.
2. In other reactions the ring is a substituent.2. In other reactions the ring is a substituent.
Dr. Wolf's CHM 201 & 202 11-56
a) Reductiona) Reduction
Catalytic hydrogenation (Section 11.4)Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) Birch reduction (Section 11.11)
b) Electrophilic aromatic substitutionb) Electrophilic aromatic substitution(Chapter 12)(Chapter 12)
c) Nucleophilic aromatic substitutionc) Nucleophilic aromatic substitution(Chapter 23)(Chapter 23)
1. Reactions involving the ring1. Reactions involving the ring
2. The ring as a substituent (Sections 11.12-11.17)2. The ring as a substituent (Sections 11.12-11.17)
Dr. Wolf's CHM 201 & 202 11-57
catalytic catalytic hydrogenationhydrogenation(Section 11.4)(Section 11.4)
Birch reductionBirch reduction(Section 11.11)(Section 11.11)
HH
HH
HH HH
HHHH
HH HH
HH
HH HH
HH
HH HH
Reduction of Benzene RingsReduction of Benzene RingsReduction of Benzene RingsReduction of Benzene Rings
HHHH
HHHH HH
HH
HH
HHHH
HH HH
HH