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

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HHHH

HH

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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

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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

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HHHH

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HH

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HH

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Reduction of Benzene RingsReduction of Benzene RingsReduction of Benzene RingsReduction of Benzene Rings

HHHH

HHHH HH

HH

HH

HHHH

HH HH

HH