Arenes and AromaticityArenes and Aromaticity

HydrocarbonsHydrocarbonsHydrocarbonsHydrocarbons

AromaticAromaticAromaticAromaticAliphaticAliphaticAliphaticAliphatic

AlkanesAlkanesAlkanesAlkanes AlkynesAlkynesAlkynesAlkynesAlkenesAlkenesAlkenesAlkenes

BenzeneBenzene TolueneToluene

NaphthaleneNaphthalene

Examples of Aromatic HydrocarbonsExamples of Aromatic Hydrocarbons Examples of Aromatic HydrocarbonsExamples of Aromatic Hydrocarbons

HH

HH HH

HHHH

HH

CHCH33

HH

HH

HHHH

HH HHHH

HH

HHHH

HH

HH HH

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.

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

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 Benzene Structure of BenzeneStructure of Benzene

Benzene has the shape of a regular hexagon.Benzene has the shape of a regular hexagon.

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 FormulationResonance Formulation of Benzene of Benzene Resonance FormulationResonance Formulation of Benzene of Benzene

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

High electron density above and below plane High electron density above and below plane of ringof ring

among planar, monocyclic, completely among planar, monocyclic, completely conjugated polyenes, only those with 4conjugated polyenes, only those with 4nn + 2 + 2 electrons possess special stability (are electrons possess special stability (are aromatic)aromatic)

nn 44nn+2+2

0 0 22

1 1 66

22 1010

33 1414

44 1818

Hückel's RuleHückel's RuleHückel's RuleHückel's Rule

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

2-2-bromobromo-1--1-chlorochloro-4--4-fluorofluorobenzenebenzene

ExampleExampleExampleExample BrBr

ClCl

FF

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-)-)

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

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

Reactions of Arenes:Reactions of Arenes:Electrophilic Aromatic SubstitutionElectrophilic Aromatic Substitution

HH

EE

++ EE YY ++ HH YY++ ––

Representative Electrophilic Aromatic Substitution Reactions of Benzene HH

EE

++ EE YY ++ HH YY++ ––

HH

EE

++ EE YY ++ HH YY++ ––

Electrophilic aromatic substitutions include:Electrophilic aromatic substitutions include:Electrophilic aromatic substitutions include:Electrophilic aromatic substitutions include:

NitrationNitration

SulfonationSulfonation

HalogenationHalogenation

Friedel-Crafts AlkylationFriedel-Crafts Alkylation

Friedel-Crafts AcylationFriedel-Crafts Acylation

Nitration of BenzeneNitration of BenzeneNitration of BenzeneNitration of Benzene HH

++ ++ HH22OO

HH22SOSO44

HOHONONO22

NONO22

NitrobenzeneNitrobenzene(95%)(95%)

Sulfonation of BenzeneSulfonation of BenzeneSulfonation of BenzeneSulfonation of Benzene HH

++ ++ HH22OO

heatheatHOHOSOSO22OHOH

SOSO22OHOH

Benzenesulfonic acidBenzenesulfonic acid(100%)(100%)

Halogenation of BenzeneHalogenation of BenzeneHalogenation of BenzeneHalogenation of Benzene HH

++ ++ HHBrBr

FeBrFeBr33

BrBr22

BrBr

BromobenzeneBromobenzene(65-75%)(65-75%)

Friedel-Crafts Alkylation of BenzeneFriedel-Crafts Alkylation of BenzeneFriedel-Crafts Alkylation of BenzeneFriedel-Crafts Alkylation of Benzene

terttert-Butylbenzene-Butylbenzene(60%)(60%)

HH

++ ++ HHClCl

AlClAlCl33

C(CHC(CH33))33

(CH(CH33))33CCClCl

Friedel-Crafts Acylation of BenzeneFriedel-Crafts Acylation of BenzeneFriedel-Crafts Acylation of BenzeneFriedel-Crafts Acylation of Benzene

1-Phenyl-1-propanone1-Phenyl-1-propanone(88%)(88%)

HH++ ++ HHClCl

AlClAlCl33OO

CHCH33CHCH22CCClCl

CCHCCH22CHCH33

OO