Post on 01-Apr-2015
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.
Kekulé Formulation of BenzeneKekulé Formulation of BenzeneKekulé Formulation of BenzeneKekulé Formulation of Benzene
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.
Kekulé Formulation of BenzeneKekulé Formulation of BenzeneKekulé Formulation of BenzeneKekulé Formulation of Benzene
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
Kekulé Formulation of BenzeneKekulé Formulation of BenzeneKekulé Formulation of BenzeneKekulé Formulation of Benzene
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
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)
11-19Dr. Wolf's CHM 201 & 202
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
11-20Dr. Wolf's CHM 201 & 202
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
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)!)!
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)
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
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
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
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
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
11-27Dr. Wolf's CHM 201 & 202
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
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
Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives
CHCH
OO
11-41Dr. Wolf's CHM 201 & 202
Benzoic acidBenzoic acid
Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives
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
Benzene DerivativesBenzene DerivativesBenzene DerivativesBenzene Derivatives
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
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
––
••
••••
OCHOCH33
HH
••••••••
11-62Dr. Wolf's CHM 201 & 202
Step 2: Proton transfer from methanolStep 2: Proton transfer from methanol
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
––
••
••••
OCHOCH33
HH
••••••••
HH
HH
HH
HH
HH
HH
HH
••
OCHOCH33••••••••••••––
11-63Dr. Wolf's CHM 201 & 202
Step 3: Electron transfer from sodiumStep 3: Electron transfer from sodium
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
Step 3: Electron transfer from sodiumStep 3: Electron transfer from sodium
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••
HH
HH
HH
HH
HH
HH
HH
••••++ NaNa++
––
11-65Dr. Wolf's CHM 201 & 202
Step 4: Proton transfer from methanolStep 4: Proton transfer from methanol
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
••••––
•• OCHOCH33
HH
••••••
11-66Dr. Wolf's CHM 201 & 202
Step 4: Proton transfer from methanolStep 4: Proton transfer from methanol
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
••••––
•• 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
368 kJ/mol368 kJ/mol
356 kJ/mol356 kJ/mol
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
••
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
11-75Dr. 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
11-76Dr. 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
11-77Dr. 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
ExampleExampleExampleExample
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
ExampleExampleExampleExample
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++
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
11-90Dr. 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
++
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
11-91Dr. Wolf's CHM 201 & 202
Resonance in Benzyl CationResonance in Benzyl CationResonance in Benzyl CationResonance in Benzyl Cation
CCHH
HH
HHHH
HH
HH
HH
++
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
11-92Dr. Wolf's CHM 201 & 202
Resonance in Benzyl CationResonance in Benzyl CationResonance in Benzyl CationResonance in Benzyl Cation
CCHH
HH
HHHH
HH
HH
HH
++
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
11-93Dr. 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
Acid-Catalyzed Dehydration of Benzylic Alcohols
KHSOKHSO44
heatheat
(80-82%)(80-82%)
CHCH22CHCHCHCHCHCH33
OHOH
ClCl ClCl ++ HH22OO
11-99Dr. Wolf's CHM 201 & 202
Acid-Catalyzed Dehydration of Benzylic Alcohols
Acid-Catalyzed Dehydration of Benzylic Alcohols
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
Hückel's RuleHückel's RuleHückel's RuleHückel's Rule
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
Hückel's RuleHückel's RuleHückel's RuleHückel's Rule
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
AntibondingAntibonding
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
AntibondingAntibonding
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)
-MOs of Cyclobutadiene-MOs of Cyclobutadiene(square planar)(square planar)
11-135Dr. Wolf's CHM 201 & 202
Cyclo-Cyclo-butadienebutadiene
AntibondingAntibonding
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
-MOs of Cyclobutadiene-MOs of Cyclobutadiene(square planar)(square planar)
-MOs of Cyclobutadiene-MOs of Cyclobutadiene(square planar)(square planar)
11-136Dr. Wolf's CHM 201 & 202
AntibondingAntibonding
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)
-MOs of Cyclooctatetraene-MOs of Cyclooctatetraene(square planar)(square planar)
Cyclo-Cyclo-octatetraeneoctatetraene
11-137Dr. Wolf's CHM 201 & 202
AntibondingAntibonding
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
-MOs of Cyclooctatetraene-MOs of Cyclooctatetraene(square planar)(square planar)
-MOs of Cyclooctatetraene-MOs of Cyclooctatetraene(square planar)(square planar)
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
[10]Annulene[10]Annulene[10]Annulene[10]Annulene
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
[10]Annulene[10]Annulene[10]Annulene[10]Annulene
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
[14]Annulene[14]Annulene[14]Annulene[14]Annulene
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
[16]Annulene[16]Annulene[16]Annulene[16]Annulene
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
[18]Annulene[18]Annulene[18]Annulene[18]Annulene
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
Electron Delocalization in Cyclopentadienide AnionElectron Delocalization in Cyclopentadienide AnionElectron Delocalization in Cyclopentadienide AnionElectron Delocalization in Cyclopentadienide Anion
HH HH
HH HH
HH
••••––
HH HH
HH HH
HH
••••––
11-157Dr. 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
••••––
HH HH
HH HH
HH
••••––
HH HH
HH HH
HH
•••• ––
11-158Dr. 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
••••––
HH HH
HH HH
HH
••••––
HH HH
HH HH
HH
•••• ––
HH HH
HH HH
HH
–– ••••
11-159Dr. 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
••••––
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
••••––
Compare Acidities ofCompare Acidities ofCyclopentadiene and CycloheptatrieneCyclopentadiene and Cycloheptatriene
Compare Acidities ofCompare Acidities ofCyclopentadiene and CycloheptatrieneCyclopentadiene and Cycloheptatriene
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