11-1 dr. wolf's chm 201 & 202 chapter 11 arenes and aromaticity

11-1Dr. Wolf's CHM 201 & 202

Chapter 11Chapter 11Arenes and AromaticityArenes and Aromaticity

11-2Dr. Wolf's CHM 201 & 202

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

11-3Dr. Wolf's CHM 201 & 202

Benzene

11-4Dr. Wolf's CHM 201 & 202

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.

11-5Dr. Wolf's CHM 201 & 202

Kekulé and theStructure of Benzene

11-6Dr. Wolf's CHM 201 & 202

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

11-7Dr. Wolf's CHM 201 & 202

Later, Kekulé revised his proposal by suggestingLater, Kekulé revised his proposal by suggestinga rapid equilibrium between two equivalenta rapid equilibrium between two equivalentstructures.structures.


HH

HH HH

HHHH

HH

HH

HH

HH

HHHH

HH

11-8Dr. Wolf's CHM 201 & 202

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.


HH

XX XX

HHHH

HH

HH

XX

XX

HHHH

HH

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

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.

11-10Dr. Wolf's CHM 201 & 202

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

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

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.

11-12Dr. Wolf's CHM 201 & 202

A Resonance Picture of Bonding in Benzene

11-13Dr. Wolf's CHM 201 & 202

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


11-14Dr. Wolf's CHM 201 & 202

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

11-15Dr. Wolf's CHM 201 & 202

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

11-16Dr. Wolf's CHM 201 & 202

The 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

11-17Dr. Wolf's CHM 201 & 202

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

11-18Dr. Wolf's CHM 201 & 202

120 kJ/mol120 kJ/mol120 kJ/mol120 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)

11-19Dr. Wolf's CHM 201 & 202



"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

11-20Dr. Wolf's CHM 201 & 202



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

11-21Dr. Wolf's CHM 201 & 202

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)!)!


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)

11-22Dr. Wolf's CHM 201 & 202

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

11-23Dr. Wolf's CHM 201 & 202

compared to localized 1,3,5-cyclohexatrienecompared to localized 1,3,5-cyclohexatriene


compared to 1,3,5-hexatrienecompared to 1,3,5-hexatriene


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

11-24Dr. Wolf's CHM 201 & 202

An Orbital Hybridization Viewof Bonding in Benzene

11-25Dr. Wolf's CHM 201 & 202

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

11-26Dr. Wolf's CHM 201 & 202




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

11-27Dr. Wolf's CHM 201 & 202




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

11-28Dr. Wolf's CHM 201 & 202

The Molecular Orbitalsof Benzene

11-29Dr. Wolf's CHM 201 & 202

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

11-30Dr. Wolf's CHM 201 & 202

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

11-31Dr. Wolf's CHM 201 & 202

The Three Bonding The Three Bonding MOs of Benzene MOs of BenzeneThe Three Bonding The Three Bonding MOs of Benzene MOs of Benzene

11-32Dr. Wolf's CHM 201 & 202

Substituted Derivatives of Benzene Substituted Derivatives of Benzene

and Their Nomenclature and Their Nomenclature

11-33Dr. Wolf's CHM 201 & 202

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

11-34Dr. Wolf's CHM 201 & 202

ExamplesExamplesExamplesExamples BromobenzeneBromobenzene terttert-Butylbenzene-Butylbenzene NitrobenzeneNitrobenzene

NONO22C(CHC(CH33))33BrBr

11-35Dr. Wolf's CHM 201 & 202

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

11-36Dr. Wolf's CHM 201 & 202

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

ExampleExampleExampleExample

BrBr

ClCl

FF

11-37Dr. Wolf's CHM 201 & 202

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-)-)

11-38Dr. Wolf's CHM 201 & 202

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)

11-39Dr. Wolf's CHM 201 & 202

Certain monosubstituted derivatives of benzene Certain monosubstituted derivatives of benzene have unique nameshave unique names

Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives

11-40Dr. Wolf's CHM 201 & 202

BenzaldehydeBenzaldehyde


CHCH

OO

11-41Dr. Wolf's CHM 201 & 202

Benzoic acidBenzoic acid


COHCOH

OO

11-42Dr. Wolf's CHM 201 & 202

StyreneStyrene

Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives CHCH22CHCH

11-43Dr. Wolf's CHM 201 & 202

TolueneToluene

Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives CHCH33

11-44Dr. Wolf's CHM 201 & 202

AcetophenoneAcetophenone


CCHCCH33

OO

11-45Dr. Wolf's CHM 201 & 202

PhenolPhenol

Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives OHOH

11-46Dr. Wolf's CHM 201 & 202

AnisoleAnisole

Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives OCHOCH33

11-47Dr. Wolf's CHM 201 & 202

AnilineAniline

Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives NHNH22

11-48Dr. Wolf's CHM 201 & 202

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

11-49Dr. Wolf's CHM 201 & 202

Easily confused namesEasily confused namesEasily confused namesEasily confused names phenylphenyl phenolphenol benzylbenzyl

OHOH CHCH22——

11-50Dr. Wolf's CHM 201 & 202

Polycyclic Aromatic Hydrocarbons

11-51Dr. Wolf's CHM 201 & 202

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

11-52Dr. Wolf's CHM 201 & 202

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

11-53Dr. Wolf's CHM 201 & 202

Physical Properties of Arenes

11-54Dr. Wolf's CHM 201 & 202

Resemble other hydrocarbonsResemble other hydrocarbons

nonpolarnonpolar

insoluble in waterinsoluble in water

less dense than waterless dense than water

Physical PropertiesPhysical PropertiesPhysical PropertiesPhysical Properties

11-55Dr. Wolf's CHM 201 & 202

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.

11-56Dr. Wolf's CHM 201 & 202

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 12b)(Chapter 12b)

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)

11-57Dr. Wolf's CHM 201 & 202

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

11-58Dr. Wolf's CHM 201 & 202

The Birch Reduction

11-59Dr. Wolf's CHM 201 & 202

(80%)(80%)

HH

HH

HH HH

HHHH

HH HH

HH

HH HH

HH

HH HH

Na, NHNa, NH33

CHCH33OHOH

Birch Reduction of BenzeneBirch Reduction of BenzeneBirch Reduction of BenzeneBirch Reduction of Benzene

Product is non-conjugated diene.Product is non-conjugated diene.

Reaction stops here. There is no further reduction.Reaction stops here. There is no further reduction.

Reaction is not hydrogenation. HReaction is not hydrogenation. H22 is not involved in any way. is not involved in any way.

11-60Dr. Wolf's CHM 201 & 202

HH

HH

HH HH

HHHH

Step 1: Electron transfer from sodiumStep 1: Electron transfer from sodium

++ NaNa•• ++ NaNa++

Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)

HH

HH

HH

HH

HH

HH••

••••––

11-61Dr. Wolf's CHM 201 & 202

Step 2: Proton transfer from methanolStep 2: Proton transfer from methanol


HH

HH

HH

HH

HH

HH

––

••

••••

OCHOCH33

HH

••••••••

11-62Dr. Wolf's CHM 201 & 202



HH

HH

HH

HH

HH

HH

––

••

••••

OCHOCH33

HH

••••••••

HH

HH

HH

HH

HH

HH

HH

••

OCHOCH33••••••••••••––

11-63Dr. Wolf's CHM 201 & 202


Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8) HH

HH

HH

HH

HH

HH

HH

••++ NaNa••

11-64Dr. Wolf's CHM 201 & 202



HH

HH

HH

HH

HH

HH

••++ NaNa••

HH

HH

HH

HH

HH

HH

HH

••••++ NaNa++

––

11-65Dr. Wolf's CHM 201 & 202



HH

HH

HH

HH

HH

HH

••••––

•• OCHOCH33

HH

••••••

11-66Dr. Wolf's CHM 201 & 202



HH

HH

HH

HH

HH

HH

••••––

•• OCHOCH33

HH

••••••

HH HH

HH

HH HH

HH

HH HH

––•• OCHOCH33••••

••••••

11-67Dr. Wolf's CHM 201 & 202

(86%)(86%)

HH

HH

HH C(CHC(CH33))33

HHHH

HH HH

HH

HH C(CHC(CH33))33

HH

HH HH

Na, NHNa, NH33

CHCH33OOHH

Birch Reduction of an AlkylbenzeneBirch Reduction of an AlkylbenzeneBirch Reduction of an AlkylbenzeneBirch Reduction of an Alkylbenzene

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

11-68Dr. Wolf's CHM 201 & 202

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)

11-69Dr. Wolf's CHM 201 & 202

Free-Radical HalogenationFree-Radical Halogenation

of Alkylbenzenesof Alkylbenzenes

11-70Dr. Wolf's CHM 201 & 202

allylic radicalallylic radical

The Benzene Ring as a SubstituentThe Benzene Ring as a SubstituentThe Benzene Ring as a SubstituentThe Benzene Ring as a Substituent

••CCCC

CC ••CC

benzylic radicalbenzylic radical

benzylic carbon is analogous to allylic carbonbenzylic carbon is analogous to allylic carbon

11-71Dr. Wolf's CHM 201 & 202

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

Recall:Recall:Recall:Recall:

R—HR—H R•R• ••HH++

Bond-dissociation energy for C—H bond Bond-dissociation energy for C—H bond is equal to is equal to HH° for:° for:

and is about 400 kJ/mol for alkanes.and is about 400 kJ/mol for alkanes.

11-72Dr. Wolf's CHM 201 & 202

Bond-dissociation energies of propene and tolueneBond-dissociation energies of propene and tolueneBond-dissociation energies of propene and tolueneBond-dissociation energies of propene and toluene



HH

HH22CC CHCH CC HH

HH HH

CC HH

HH

HH22CC CHCH-H•-H•

-H•-H•

HH

CC

HH

••

HH

CC

HH

••

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

11-73Dr. Wolf's CHM 201 & 202

Resonance in Benzyl RadicalResonance in Benzyl RadicalResonance in Benzyl RadicalResonance in Benzyl Radical CCHH

HH

HHHH

HH

HH

HH••

unpaired electron is delocalized between unpaired electron is delocalized between benzylic carbon and the ring carbons that are benzylic carbon and the ring carbons that are ortho and para to itortho and para to it

11-74Dr. Wolf's CHM 201 & 202

Resonance in Benzyl RadicalResonance in Benzyl RadicalResonance in Benzyl RadicalResonance in Benzyl Radical

CCHH

HH

HHHH

HH

HH

HH

••


11-75Dr. Wolf's CHM 201 & 202


CCHH

HH

HHHH

HH

HH

HH

••


11-76Dr. Wolf's CHM 201 & 202


CCHH

HH

HHHH

HH

HH

HH

••


11-77Dr. Wolf's CHM 201 & 202

industrial processindustrial process

highly regioselective for benzylic positionhighly regioselective for benzylic position CHCH33

Free-radical chlorination of tolueneFree-radical chlorination of tolueneFree-radical chlorination of tolueneFree-radical chlorination of toluene

ClCl22

lightlightoror

heatheat

CHCH22ClCl

TolueneToluene Benzyl chlorideBenzyl chloride

11-78Dr. Wolf's CHM 201 & 202

Similarly, dichlorination and trichlorination areSimilarly, dichlorination and trichlorination areselective for the benzylic carbon. Furtherselective for the benzylic carbon. Furtherchlorination gives:chlorination gives:

Free-radical chlorination of tolueneFree-radical chlorination of tolueneFree-radical chlorination of tolueneFree-radical chlorination of toluene CClCCl33

(Dichloromethyl)benzene(Dichloromethyl)benzene

CHClCHCl22

(Trichloromethyl)benzene(Trichloromethyl)benzene

11-79Dr. Wolf's CHM 201 & 202

is used in the laboratory to introduce a is used in the laboratory to introduce a halogen at the benzylic positionhalogen at the benzylic position

Benzylic BrominationBenzylic BrominationBenzylic BrominationBenzylic Bromination CHCH33

NONO22

+ Br+ Br22

CClCCl44, 80°C, 80°C

lightlight+ HBr+ HBr

NONO22

CHCH22BrBr

pp-Nitrotoluene-Nitrotoluene pp-Nitrobenzyl-Nitrobenzylbromide (71%)bromide (71%)

11-80Dr. Wolf's CHM 201 & 202

N-Bromosuccinimide (NBS)N-Bromosuccinimide (NBS)N-Bromosuccinimide (NBS)N-Bromosuccinimide (NBS)

CClCCl44

benzoylbenzoylperoxide,peroxide,

heatheat

CHCH22CHCH33 ++

NNBrBr

OO

OO

CHCHCHCH33 NHNH

OO

OO

++

BrBr

(87%)(87%)

11-81Dr. Wolf's CHM 201 & 202

Oxidation of AlkylbenzenesOxidation of Alkylbenzenes

11-82Dr. Wolf's CHM 201 & 202

Site of Oxidation is Benzylic CarbonSite of Oxidation is Benzylic CarbonSite of Oxidation is Benzylic CarbonSite of Oxidation is Benzylic Carbon CHCH33 CHCH22RR CHRCHR22

oror

oror

COHCOH

OONaNa22CrCr22OO77

HH22SOSO44

HH22OO

heatheat

11-83Dr. Wolf's CHM 201 & 202


NaNa22CrCr22OO77

HH22SOSO44

HH22OO

heatheat

COHCOH

OO CHCH33

NONO22

pp-Nitrotoluene-Nitrotoluene

NONO22

pp-Nitrobenzoic-Nitrobenzoicacid (82-86%)acid (82-86%)

11-84Dr. Wolf's CHM 201 & 202


NaNa22CrCr22OO77

HH22SOSO44

HH22OO

heatheat

CH(CHCH(CH33))22

CHCH33

(45%)(45%)

COHCOH

OO COHCOH

OO

11-85Dr. Wolf's CHM 201 & 202

SSNN1 Reactions of1 Reactions of

Benzylic Halides Benzylic Halides

11-86Dr. Wolf's CHM 201 & 202

tertiary benzylic carbocation is formedtertiary benzylic carbocation is formedmore rapidly than tertiary carbocation;more rapidly than tertiary carbocation;therefore, more stabletherefore, more stable

What about SWhat about SNN1?1?What about SWhat about SNN1?1? CC

CHCH33

CHCH33

ClCl

600600 11

CC

CHCH33

CHCH33

ClClCHCH33

Relative solvolysis rates in aqueous acetoneRelative solvolysis rates in aqueous acetone

11-87Dr. Wolf's CHM 201 & 202

What about SWhat about SNN1?1?What about SWhat about SNN1?1? CC

more stablemore stable less stableless stable

CCCHCH33

Relative rates of formation:Relative rates of formation:

CHCH33

CHCH33

++

CHCH33

CHCH33

++

11-88Dr. Wolf's CHM 201 & 202

allylic carbocationallylic carbocation

Compare.Compare.Compare.Compare.

++CCCC

CC ++CC

benzylic carbocationbenzylic carbocation

benzylic carbon is analogous to allylic carbonbenzylic carbon is analogous to allylic carbon

11-89Dr. Wolf's CHM 201 & 202

Resonance in Benzyl CationResonance in Benzyl CationResonance in Benzyl CationResonance in Benzyl Cation CCHH

HH

HHHH

HH

HH

HH++


11-90Dr. Wolf's CHM 201 & 202

Resonance in Benzyl Cation Resonance in Benzyl Cation Resonance in Benzyl Cation Resonance in Benzyl Cation

CCHH

HH

HHHH

HH

HH

HH

++


11-91Dr. Wolf's CHM 201 & 202

Resonance in Benzyl CationResonance in Benzyl CationResonance in Benzyl CationResonance in Benzyl Cation

CCHH

HH

HHHH

HH

HH

HH

++


11-92Dr. Wolf's CHM 201 & 202

Resonance in Benzyl CationResonance in Benzyl CationResonance in Benzyl CationResonance in Benzyl Cation

CCHH

HH

HHHH

HH

HH

HH

++


11-93Dr. Wolf's CHM 201 & 202

SolvolysisSolvolysisSolvolysisSolvolysis CC

CHCH33

CHCH33

ClCl

CHCH33CHCH22OHOH

CC

CHCH33

CHCH33

OCHOCH22CHCH33

(87%)(87%)

11-94Dr. Wolf's CHM 201 & 202

SSNN2 Reactions of2 Reactions of

Benzylic Halides Benzylic Halides

11-95Dr. Wolf's CHM 201 & 202

Primary Benzylic HalidesPrimary Benzylic HalidesPrimary Benzylic HalidesPrimary Benzylic Halides

acetic acidacetic acid

CHCH22ClCl

OO22NN

NaOCCHNaOCCH33

OO

CHCH22OCCHOCCH33

OO22NN

OO

Mechanism is SMechanism is SNN22

(78-82%)(78-82%)

11-96Dr. Wolf's CHM 201 & 202

Preparation of Alkenylbenzenes

•dehydrogenation

•dehydration

•dehydrohalogenation

11-97Dr. Wolf's CHM 201 & 202

•industrial preparation of styrene

DehydrogenationDehydrogenation CHCH22CHCH33

630°C630°C

ZnOZnO

CHCH22CHCH

+ H+ H22

11-98Dr. Wolf's CHM 201 & 202

Acid-Catalyzed Dehydration of Benzylic Alcohols


KHSOKHSO44

heatheat

(80-82%)(80-82%)

CHCH22CHCHCHCHCHCH33

OHOH

ClCl ClCl ++ HH22OO

11-99Dr. Wolf's CHM 201 & 202



KHSOKHSO44

heatheat(80-82%)(80-82%)

CHCH22CHCHCHCHCHCH33

OHOH

ClCl ClCl CHCHCHCH33

ClCl ++

11-100Dr. Wolf's CHM 201 & 202

DehydrohalogenationDehydrohalogenation

NaOCHNaOCH22CHCH33 ethanol, 50°Cethanol, 50°C

(99%)(99%)

HH33CC CHCH22CHCHCHCH33

BrBr

CHCHHH33CC

CHCHCHCH33

11-101Dr. Wolf's CHM 201 & 202

•hydrogenation

•halogenation

•addition of hydrogen halides

Addition Reactions of Alkenylbenzenes

11-102Dr. Wolf's CHM 201 & 202

HydrogenationHydrogenation

HH22

PtPt

(92%)(92%)

BrBr

CC

CHCH33

CHCHCHCH33

BrBr

CHCHCHCH22CHCH33

CHCH33

11-103Dr. Wolf's CHM 201 & 202

HalogenationHalogenation CHCH22CHCH

BrBr22

CHCH22CHCH

BrBrBrBr

(82%)(82%)

11-104Dr. Wolf's CHM 201 & 202

Addition of Hydrogen HalidesAddition of Hydrogen Halides

HClHCl

(75-84%)(75-84%)

ClCl

11-105Dr. Wolf's CHM 201 & 202

Addition of Hydrogen HalidesAddition of Hydrogen Halides

HClHCl

via benzylic carbocationvia benzylic carbocation

ClCl ++

11-106Dr. Wolf's CHM 201 & 202

Free-Radical Addition of HBrFree-Radical Addition of HBr CHCH22CHCH

CHCH22CHCH22BrBr

HBrHBr

peroxidesperoxides

11-107Dr. Wolf's CHM 201 & 202

Free-Radical Addition of HBrFree-Radical Addition of HBr CHCH22CHCH

CHCH22CHCH22BrBr

HBrHBr

peroxidesperoxides

via benzylic radicalvia benzylic radical

CHCH22BrBrCHCH

••

11-108Dr. Wolf's CHM 201 & 202

Polymerization of Styrene

11-109Dr. Wolf's CHM 201 & 202

Polymerization of StyrenePolymerization of Styrene

HH22CC CHCHCC66HH55

CHCH22 CHCH

CC66HH55

CHCH22 CHCH

CC66HH55

CHCH22 CHCH

CC66HH55

polystyrenepolystyrene

11-110Dr. Wolf's CHM 201 & 202

MechanismMechanismRORO

•••• ••••

HH22CC CHCHCC66HH55

••

11-111Dr. Wolf's CHM 201 & 202

MechanismMechanism

HH22CC CHCHCC66HH55

RORO•••• ••••

••

11-112Dr. Wolf's CHM 201 & 202

MechanismMechanism

HH22CC CHCHCC66HH55

RORO•••• ••••

HH22CC CHCHCC66HH55

••

11-113Dr. Wolf's CHM 201 & 202

MechanismMechanism

HH22CC CHCHCC66HH55

RORO•••• ••••

HH22CC CHCHCC66HH55

••

11-114Dr. Wolf's CHM 201 & 202

MechanismMechanism

HH22CC CHCHCC66HH55

HH22CC CHCHCC66HH55

RORO•••• ••••

••

11-115Dr. Wolf's CHM 201 & 202

MechanismMechanism

HH22CC CHCHCC66HH55

HH22CC CHCHCC66HH55

RORO•••• ••••

HH22CC CHCHCC66HH55

••

11-116Dr. Wolf's CHM 201 & 202

MechanismMechanism

HH22CC CHCHCC66HH55

HH22CC CHCHCC66HH55

RORO•••• ••••

HH22CC CHCHCC66HH55

••

11-117Dr. Wolf's CHM 201 & 202

MechanismMechanism

HH22CC CHCHCC66HH55

HH22CC CHCHCC66HH55

HH22CC CHCHCC66HH55

RORO•••• ••••

••

11-118Dr. Wolf's CHM 201 & 202

MechanismMechanism

HH22CC CHCHCC66HH55

HH22CC CHCHCC66HH55

HH22CC CHCHCC66HH55

HH22CC CHCHCC66HH55

RORO•••• ••••

••

11-119Dr. Wolf's CHM 201 & 202

MechanismMechanism

HH22CC CHCHCC66HH55

HH22CC CHCHCC66HH55

HH22CC CHCHCC66HH55

HH22CC CHCHCC66HH55

RORO•••• •••• ••

11-120Dr. Wolf's CHM 201 & 202

Cyclobutadiene and CyclooctatetraeneCyclobutadiene and Cyclooctatetraene

11-121

heat of hydrogenation of benzene is 152 kJ/mol heat of hydrogenation of benzene is 152 kJ/mol less than 3 times heat of hydrogenation of less than 3 times heat of hydrogenation of cyclohexenecyclohexene

to give cyclohexane (kJ/mol)to give cyclohexane (kJ/mol)

Heats of HydrogenationHeats of Hydrogenation 120120 231231 208208

11-122Dr. Wolf's CHM 201 & 202

Heats of HydrogenationHeats of HydrogenationHeats of HydrogenationHeats of Hydrogenation

to give cyclooctane (kJ/mol)to give cyclooctane (kJ/mol)

heat of hydrogenation of cyclooctatetraene is heat of hydrogenation of cyclooctatetraene is more than 4 times heat of hydrogenation of more than 4 times heat of hydrogenation of cyclooctenecyclooctene

9797 205205 303303 410410

11-123Dr. Wolf's CHM 201 & 202

Structure of CyclooctatetraeneStructure of CyclooctatetraeneStructure of CyclooctatetraeneStructure of Cyclooctatetraene

cyclooctatetraene is not planarcyclooctatetraene is not planar

has alternating long (146 pm)has alternating long (146 pm)and short (133 pm) bondsand short (133 pm) bonds

11-124Dr. Wolf's CHM 201 & 202

structure of a stabilized derivative is characterizedstructure of a stabilized derivative is characterizedby alternating short bonds and long bondsby alternating short bonds and long bonds

Structure of CyclobutadieneStructure of CyclobutadieneStructure of CyclobutadieneStructure of Cyclobutadiene

C(CHC(CH33))33(CH(CH33))33CC

COCO22CHCH33(CH(CH33))33CC

138 pm138 pm

151 pm151 pm

11-125

Cyclobutadiene is observed to be highly reactive, Cyclobutadiene is observed to be highly reactive, and too unstable to be isolated and stored in theand too unstable to be isolated and stored in thecustomary way.customary way.

Not only is cyclobutadiene not aromatic, it is Not only is cyclobutadiene not aromatic, it is antiaromaticantiaromatic..

An antiaromatic substance is one that is An antiaromatic substance is one that is destabilizeddestabilizedby cyclic conjugation.by cyclic conjugation.

Stability of CyclobutadieneStability of Cyclobutadiene

11-126

Requirements for AromaticityRequirements for Aromaticity notnot

aromaticaromaticaromaticaromatic

notnotaromaticaromatic

Antiaromatic Antiaromatic when squarewhen square

Antiaromatic Antiaromatic when planarwhen planar

11-127Dr. Wolf's CHM 201 & 202

ConclusionConclusionConclusionConclusion

there must be some factor in additionthere must be some factor in additionto cyclic conjugation that determines to cyclic conjugation that determines whether a molecule is aromatic or notwhether a molecule is aromatic or not

11-128Dr. Wolf's CHM 201 & 202

Hückel's Rule:Hückel's Rule:

AnnulenesAnnulenes

the additional factor that influences the additional factor that influences aromaticity is the number of aromaticity is the number of electrons electrons

11-129Dr. Wolf's CHM 201 & 202

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

11-130Dr. Wolf's CHM 201 & 202

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 benzene!benzene!

22 1010

33 1414

44 1818


11-131Dr. Wolf's CHM 201 & 202

Hückel restricted his analysis to planar,Hückel restricted his analysis to planar,completely conjugated, monocyclic polyenescompletely conjugated, monocyclic polyenes

he found that the he found that the molecular orbitals of molecular orbitals ofthese compounds had a distinctive patternthese compounds had a distinctive pattern

one one orbital was lowest in energy, another orbital was lowest in energy, another was highest in energy, and the others was highest in energy, and the others were arranged in pairs between the highestwere arranged in pairs between the highestand the lowestand the lowest


11-132Dr. Wolf's CHM 201 & 202

-MOs of Benzene-MOs of Benzene-MOs of Benzene-MOs of Benzene

BenzeneBenzene

AntibondingAntibonding

BondingBonding6 6 pp orbitals give 6 orbitals give 6 orbitals orbitals

3 orbitals are bonding; 3 are antibonding3 orbitals are bonding; 3 are antibonding

11-133Dr. Wolf's CHM 201 & 202

BenzeneBenzene


BondingBonding6 6 electrons fill all of the bonding orbitals electrons fill all of the bonding orbitals

all all antibonding orbitals are empty antibonding orbitals are empty

-MOs of Benzene-MOs of Benzene-MOs of Benzene-MOs of Benzene

11-134Dr. Wolf's CHM 201 & 202

Cyclo-Cyclo-butadienebutadiene


BondingBonding

4 4 pp orbitals give 4 orbitals give 4 orbitals orbitals

1 orbital is bonding, one is antibonding, and 2 1 orbital is bonding, one is antibonding, and 2 are nonbondingare nonbonding

-MOs of Cyclobutadiene-MOs of Cyclobutadiene(square planar)(square planar)


11-135Dr. Wolf's CHM 201 & 202

Cyclo-Cyclo-butadienebutadiene


BondingBonding

4 4 electrons; bonding orbital is filled; other 2 electrons; bonding orbital is filled; other 2 electrons singly occupy two nonbonding orbitals electrons singly occupy two nonbonding orbitals



11-136Dr. Wolf's CHM 201 & 202


BondingBonding

8 8 pp orbitals give 8 orbitals give 8 orbitals orbitals

3 orbitals are bonding, 3 are antibonding, and 2 3 orbitals are bonding, 3 are antibonding, and 2 are nonbondingare nonbonding

-MOs of Cyclooctatetraene-MOs of Cyclooctatetraene(square planar)(square planar)


Cyclo-Cyclo-octatetraeneoctatetraene

11-137Dr. Wolf's CHM 201 & 202


BondingBonding

8 8 electrons; 3 bonding orbitals are filled; 2 electrons; 3 bonding orbitals are filled; 2nonbonding orbitals are each half-fillednonbonding orbitals are each half-filled



Cyclo-Cyclo-octatetraeneoctatetraene

11-138Dr. Wolf's CHM 201 & 202

-Electron Requirement for Aromaticity-Electron Requirement for Aromaticity-Electron Requirement for Aromaticity-Electron Requirement for Aromaticity notnot

aromaticaromatic aromaticaromatic notnotaromaticaromatic

4 4 electrons electrons 6 electrons6 electrons 8 8 electrons electrons

11-139Dr. Wolf's CHM 201 & 202

Completely Conjugated PolyenesCompletely Conjugated PolyenesCompletely Conjugated PolyenesCompletely Conjugated Polyenes aromaticaromatic

6 electrons;completely conjugated

6 electrons;completely conjugated

notnotaromaticaromatic

6 6 electrons; electrons;not completelynot completely

conjugatedconjugated HH HH

11-140

AnnulenesAnnulenes

11-141Dr. Wolf's CHM 201 & 202

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

AnnulenesAnnulenesAnnulenesAnnulenes

11-142Dr. Wolf's CHM 201 & 202

predicted to be aromatic by Hückel's rule,predicted to be aromatic by Hückel's rule,but too much angle strain when planar and but too much angle strain when planar and all double bonds are cisall double bonds are cis

10-sided regular polygon has angles of 144°10-sided regular polygon has angles of 144°

[10]Annulene[10]Annulene[10]Annulene[10]Annulene

11-143Dr. Wolf's CHM 201 & 202

incorporating two trans double bonds intoincorporating two trans double bonds intothe ring relieves angle strain but introducesthe ring relieves angle strain but introducesvan der Waals strain into the structure andvan der Waals strain into the structure andcauses the ring to be distorted from planaritycauses the ring to be distorted from planarity


11-144Dr. Wolf's CHM 201 & 202

incorporating two trans double bonds intoincorporating two trans double bonds intothe ring relieves angle strain but introducesthe ring relieves angle strain but introducesvan der Waals strain into the structure andvan der Waals strain into the structure andcauses the ring to be distorted from planaritycauses the ring to be distorted from planarity


van der Waalsvan der Waals

strain betweenstrain between

these two hydrogensthese two hydrogens

11-145Dr. Wolf's CHM 201 & 202

14 14 electrons satisfies Hückel's rule electrons satisfies Hückel's rule

van der Waals strain between hydrogens insidevan der Waals strain between hydrogens insidethe ringthe ring


HH HH

HH HH

11-146Dr. Wolf's CHM 201 & 202

16 16 electrons does not satisfy Hückel's rule electrons does not satisfy Hückel's rule

alternating short (134 pm) and long (146 pm) bondsalternating short (134 pm) and long (146 pm) bonds

not aromaticnot aromatic


11-147Dr. Wolf's CHM 201 & 202

18 18 electrons satisfies Hückel's rule electrons satisfies Hückel's rule

resonance energy = 418 kJ/molresonance energy = 418 kJ/mol

bond distances range between 137-143 pmbond distances range between 137-143 pm


HH HHHH

HHHH

HH

11-148Dr. Wolf's CHM 201 & 202

Aromatic IonsAromatic Ions

11-149Dr. Wolf's CHM 201 & 202

6 6 electrons delocalized electrons delocalizedover 7 carbonsover 7 carbonspositive charge dispersedpositive charge dispersedover 7 carbonsover 7 carbonsvery stable carbocationvery stable carbocationalso called tropylium cationalso called tropylium cation

Cycloheptatrienyl CationCycloheptatrienyl CationCycloheptatrienyl CationCycloheptatrienyl Cation HH HH

HHHH

HH HH

HH

++

11-150Dr. Wolf's CHM 201 & 202

Cycloheptatrienyl CationCycloheptatrienyl CationCycloheptatrienyl CationCycloheptatrienyl Cation HH HH

HHHH

HH HH

HH

++

++

HH HH

HHHH

HH HH

HH

11-151Dr. Wolf's CHM 201 & 202

Tropylium cation is so stable that tropyliumTropylium cation is so stable that tropyliumbromide is ionic rather than covalent.bromide is ionic rather than covalent.

mp 203 °C; soluble in water; insoluble inmp 203 °C; soluble in water; insoluble indiethyl etherdiethyl ether

Cycloheptatrienyl CationCycloheptatrienyl CationCycloheptatrienyl CationCycloheptatrienyl Cation HH BrBr

++ BrBr––

IonicIonic CovalentCovalent

11-152Dr. Wolf's CHM 201 & 202

Cyclopentadienide AnionCyclopentadienide AnionCyclopentadienide AnionCyclopentadienide Anion

HH HH

HH HH

HH

••••––

6 6 electrons delocalized electrons delocalizedover 5 carbonsover 5 carbonsnegative charge dispersednegative charge dispersedover 5 carbonsover 5 carbonsstabilized anionstabilized anion

11-153Dr. Wolf's CHM 201 & 202

Cyclopentadienide AnionCyclopentadienide AnionCyclopentadienide AnionCyclopentadienide Anion

HH HH

HH HH

HH

••••––

HH HH

HH HH

HH

––

11-154Dr. Wolf's CHM 201 & 202

Acidity of CyclopentadieneAcidity of CyclopentadieneAcidity of CyclopentadieneAcidity of Cyclopentadiene HH HH

HH HH

HH HH

HH HH

HH HH

HH

••••––

HH++ ++

ppKKaa = 16 = 16

KKaa = 10 = 10-16-16

Cyclopentadiene is Cyclopentadiene is unusually acidic for a unusually acidic for a hydrocarbon.hydrocarbon.

Increased acidity is due to Increased acidity is due to stability of cyclopentadienide stability of cyclopentadienide anion.anion.

11-155Dr. Wolf's CHM 201 & 202

Electron Delocalization in Cyclopentadienide AnionElectron Delocalization in Cyclopentadienide AnionElectron Delocalization in Cyclopentadienide AnionElectron Delocalization in Cyclopentadienide Anion

HH HH

HH HH

HH

••••––

11-156Dr. Wolf's CHM 201 & 202


HH HH

HH HH

HH

••••––

HH HH

HH HH

HH

••••––

11-157Dr. Wolf's CHM 201 & 202


HH HH

HH HH

HH

••••––

HH HH

HH HH

HH

••••––

HH HH

HH HH

HH

•••• ––

11-158Dr. Wolf's CHM 201 & 202


HH HH

HH HH

HH

••••––

HH HH

HH HH

HH

••••––

HH HH

HH HH

HH

•••• ––

HH HH

HH HH

HH

–– ••••

11-159Dr. Wolf's CHM 201 & 202


HH HH

HH HH

HH

••••––

HH HH

HH HH

HH

••••––

HH HH

HH HH

HH

HH HH

HH HH

HH

•••• ––

HH HH

HH HH

HH

–– ••••

••••

11-160Dr. Wolf's CHM 201 & 202

Compare Acidities ofCompare Acidities ofCyclopentadiene and CycloheptatrieneCyclopentadiene and Cycloheptatriene

Compare Acidities ofCompare Acidities ofCyclopentadiene and CycloheptatrieneCyclopentadiene and Cycloheptatriene HH HH

HH HH

HH HH

ppKKaa = 16 = 16

KKaa = 10 = 10-16-16

ppKKaa = 36 = 36

KKaa = 10 = 10-36-36

HH HH

HHHHHH

HH HH

HH HH

11-161Dr. Wolf's CHM 201 & 202

HH HH

HH HH

HH

••••––



Aromatic anionAromatic anion6 6 electrons electrons

Anion not aromaticAnion not aromatic8 8 electrons electrons

HH HH

HHHH

HH HH

HH

••••––

11-162Dr. Wolf's CHM 201 & 202

nn = 0 = 0

44n n +2 = 2 +2 = 2 electrons electrons

Cyclopropenyl CationCyclopropenyl CationCyclopropenyl CationCyclopropenyl Cation ++

HH HH

HH

HH HH

HH

++also written asalso written as

11-163Dr. Wolf's CHM 201 & 202

nn = 2 = 244nn +2 = 10 +2 = 10 electrons electrons

Cyclooctatetraene DianionCyclooctatetraene DianionCyclooctatetraene DianionCyclooctatetraene Dianion HH HH HH HH

HHHHHHHH

HH HH HH HH

HHHHHHHH

2–2–••••

••••

––

––

alsoalsowritten aswritten as

11-164Dr. Wolf's CHM 201 & 202

Heterocyclic Aromatic CompoundsHeterocyclic Aromatic Compounds

11-165Dr. Wolf's CHM 201 & 202

PyridinePyridine

NN••••

ExamplesExamplesExamplesExamples OO••••

••••

SS••••

••••

NN

HH

••••

PyrrolePyrrole FuranFuran ThiopheneThiophene

11-166Dr. Wolf's CHM 201 & 202

QuinolineQuinoline

ExamplesExamplesExamplesExamples NN••••

NN ••••

IsoquinolineIsoquinoline

11-167Dr. Wolf's CHM 201 & 202

Heterocyclic Aromatic CompoundsHeterocyclic Aromatic Compounds

andand

Hückel's RuleHückel's Rule

11-168Dr. Wolf's CHM 201 & 202

NN••••

PyridinePyridinePyridinePyridine

6 6 electrons in ring electrons in ring

lone pair on nitrogen is in anlone pair on nitrogen is in an

spsp2 2 hybridized orbital;hybridized orbital;

not part of not part of system of ringsystem of ring

11-169Dr. Wolf's CHM 201 & 202

PyrrolePyrrolePyrrolePyrrole

lone pair on nitrogen must be part lone pair on nitrogen must be part

of ring of ring system if ring is to have system if ring is to have

6 6 electrons electrons

lone pair must be in a lone pair must be in a pp orbital orbital

in order to overlap with ring in order to overlap with ring systemsystem

NN

HH

••••

11-170Dr. Wolf's CHM 201 & 202

FuranFuranFuranFuran

two lone pairs on oxygentwo lone pairs on oxygen

one pair is in a one pair is in a pp orbital and is part orbital and is part

of ring of ring system; other is in an system; other is in an

spsp22 hybridized orbital and is not hybridized orbital and is not

part of ring part of ring system system

OO••••

••••

11-171Dr. Wolf's CHM 201 & 202

End of Chapter 11End of Chapter 11

11-1 dr. wolf's chm 201 & 202 chapter 11 arenes and aromaticity

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