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Chapter 11Chapter 11Arenes and AromaticityArenes and Aromaticity
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Benzene Toluene
Naphthalene
Examples of Aromatic Hydrocarbons
H
H H
HH
H
CH3
H
H
HH
H HH
H
HH
H
H H
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Some history of Benzene
1825 Michael Faraday isolated a new hydrocarbon from illuminating gas.
1834 Eilhardt Mitscherlich isolated same substance and determines its empirical
formula to be CnHn. Compound comes to be called benzene.
1845 August W. von Hofmann isolated benzene from coal tar.
1866 August Kekulé proposed structure of benzene.
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Kekulé (1866) proposed a cyclic structure for C6H6 with alternating single and double bonds.
The structure of Benzene: Kekulé
H
H H
HH
H
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Later, Kekulé revised his proposal by suggestinga rapid equilibrium between two equivalentstructures.
Kekulé Formulation of Benzene
H
H H
HH
H
H
H
H
HH
HProposed
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However, this proposal suggested isomers of thekind shown were possible. Yet, none were everfound.
Kekulé Formulation of Benzene
H
X X
HH
H
H
XX
HH
H
NeverObserved
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Structural studies of benzene do not support theKekulé formulation. Instead of alternating singleand double bonds, all of the C—C bonds in benzene are the same length, 140 pm.
Structure of Benzene
Benzene has the shape of a regular hexagon.
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140 pm 140 pm
140 pm 140 pm
140 pm140 pm
146 pm146 pm
134 pm134 pm
All C—C bond distances = 140 pm
140 pm is the average between the C—C single bond distance and the double bond distance in 1,3-butadiene.
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So, instead of Kekulé's suggestion of a rapidequilibrium between two structures:
H
H H
HH
H
H
H
H
HH
H
Kekulé Formulation of Benzene
Equilibriumnot valid
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The structure of benzene is expressed as a resonance hybrid of the two Lewis structures. Electrons are not localized in alternating single and double bonds,but are delocalized over all six ring carbons.
Resonance Formulation of Benzene
H
H H
HH
H
H
H
H
HH
HResonance model
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The "circle-in-a-ring" notation is an acceptable resonance description of benzene (a hybrid of two Kekulé structures). But it does not show the Lewis structure electrons.
Resonance Formulation of Benzene
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11.3The 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.
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Heat of hydrogenation: compare experimentalvalue with "expected" value for hypothetical"cyclohexatriene".
H°= – 208 kJ
Thermochemical Measures of Stability
+ 3H2
Pt
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208 kJ/mol
231 kJ/mol
120 kJ/mol
360 kJ/mol
3 x cyclohexeneFigure 11.2 (p 433)
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120 kJ/mol
360 kJ/mol 3 x cyclohexeneFigure 11.2 (p 433)
"Expected" heat of hydrogenation of benzene is 3x the heat of hydrogenation of cyclohexene.
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208 kJ/mol
360 kJ/mol 3 x cyclohexeneFigure 11.2 (p 433)
The observed heat of hydrogenation is 152 kJ/mol (36 kcal/mol) less than "expected", so benzene is 152 kJ/mol more stable than expected.
This 152 kJ/mol is the resonance energy of benzene.
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Hydrogenation of 1,3- cyclohexadiene (using + 2 H2) gives off more heat than hydrogenation of benzene (using + 3 H2) !
231 kJ/mol
208 kJ/mol Figure 11.2 (p 433)
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heat of hydrogenation = 208 kJ/mol
heat of hydrogenation = 337 kJ/mol
3H2
Pt
3H2
Pt
Cyclic conjugation versus noncyclic conjugation
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Compared to localized 1,3,5-cyclohexatriene
benzene is 152 kJ/mol more stable.
Compared to 1,3,5-hexatriene
benzene is 129 kJ/mol more stable.
Exact value of resonance energy of benzene depends on what it is compared to, but regardless of model, benzene is more stable than expected by a substantial amount.
Resonance Energy of Benzene
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11.4An Orbital Hybridization View
of Bonding in Benzene
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Orbital Hybridization Model of Bonding in Benzene
Figure 11.3
Planar ring of 6 sp2 hybridized carbons.
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Orbital Hybridization Model of Bonding in Benzene
Figure 11.3
Each carbon contributes a p orbital.Six p orbitals overlap to give cyclic system;six electrons are delocalized throughout the system.
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Orbital Hybridization Model of Bonding in Benzene
Figure 11.3
High electron density above and below plane of ring.
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Requirements for Aromaticity
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1. The compound must be cyclic.
2. Each ring atom must have a p-orbital.
3. The ring must be planar. ( electrons are completely conjugated)
4. An additional factor that is required for a molecule to be aromatic is Huckel’s Rule. (4n + 2 electrons).
Requirements for Aromaticity
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Aromatic compounds meet all of these four conditions.
A nonaromatic compound does not obey one or more of conditions 1-3.
An antiaromatic obeys conditions 1-3 but does not meet condition 4.
Annulenes: cyclic compounds that have completely alternating double bonds and may or may not be aromatic.
Definitions
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Heat of hydrogenation of benzene is 152 kJ/mol, less than 3 times heat of hydrogenation of cyclohexene (resonance energy).
To give cyclohexane (kJ/mol):
Heats of Hydrogenation 120 231 208
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Heats of Hydrogenation
To give cyclooctane (kJ/mol):
Heat of hydrogenation of cyclooctatetraene is roughly four times the heat of hydrogenation of cyclooctene (no stabilization).
97 205 303 410
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Structure of Cyclooctatetraene
Cyclooctatetraene is not planar. It has alternating long (146 pm)
and short (133 pm) bonds.
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Structure of Cyclobutadiene
H H
H H
135 pm
156 pm
MO calculations give alternating short and longbonds for cyclobutadiene (no resonance or bonds would all be equal).
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The actual structure of a stabilized derivative is characterized by alternating short bonds and long bonds.
Structure of Cyclobutadiene
C(CH3)3(CH3)3C
CO2CH3(CH3)3C
138 pm
151 pm
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Cyclobutadiene is observed to be highly reactive, and too unstable to be isolated and stored in the customary way.
Not only is cyclobutadiene not aromatic, it is antiaromatic when meeting rules 1-3.
An antiaromatic substance is one that is destabilized by cyclic conjugation.
Stability of Cyclobutadiene
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Requirements for Aromaticity not
aromatic aromaticnot
aromatic
Antiaromatic when square.
Antiaromatic when planar.
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11.19Huckel’s Rule
The additional factor that influences The additional factor that influences aromaticity is the number of aromaticity is the number of electrons. electrons.
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Among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2 electrons possess special stability (are aromatic).
n 4n+2
0 2
1 6 Benzene!
2 10
3 14
4 18
Hückel's Rule
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Hückel restricted his analysis to planar,completely conjugated, monocyclic polyenes.
He found that the molecular orbitals ofthese compounds had a distinctive pattern.
One orbital was lowest in energy, another was highest in energy, and the others were arranged in pairs between the highestand the lowest.
Hückel's Rule
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-Electron Requirement for Aromaticity not
aromaticaromatic
notaromatic
4 electrons 6 electrons 8 electrons
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Completely Conjugated Polyenes aromatic
6 electrons;completely conjugated.
notaromatic
6 electrons; not completely conjugated, one C lacks a p orbital.
H H
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11.20Annulenes
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Annulenes are planar, monocyclic, completely conjugated polyenes (alternating double and single bonds) kind of hydrocarbons treated by Hückel's rule.
They may or may not be aromatic.
Annulenes
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Predicted to be aromatic by Hückel's rule,but too much angle strain when planar and all double bonds are cis.
10-sided regular polygon has angles of 144°.
[10]Annulene
Not planar
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Incorporating two trans double bonds intothe ring relieves angle strain but introducesvan der Waals strain into the structure andcauses the ring to be distorted from planarity.
[10]Annulene
van der Waalsstrain between
these two hydrogens
Not planar
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14 electrons satisfies Hückel's rule.
van der Waals strain between hydrogens inside the ring.
[14]Annulene
H H
H HAromatic
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16 electrons does not satisfy Hückel's rule.
Alternating short (134 pm) and long (146 pm) bonds.
Is an antiaromatic 4n -electron system.
[16]Annulene Antiaromatic
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18 electrons satisfies Hückel's rule.
Resonance energy = 418 kJ/mol.
Bond distances range between 137-143 pm.
[18]Annulene
H HH
HH
HAromatic
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11.21Aromatic Ions
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It has 6 electrons (4n + 2) delocalized over 7 carbons.The positive charge is dispersed over 7 carbons.Is a very stable carbocation (aromatic) also called a tropylium cation.
Cycloheptatrienyl Cation(the tropylium ion) H H
HH
H H
H
+
+
H H
HH
H H
H
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The tropylium cation is so stable that tropyliumbromide is ionic rather than covalent. mp 203°C; soluble in water; insoluble in diethyl ether
Cycloheptatrienyl Cation H Br
+ Br–
Ionic Covalent
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Cyclopentadienide Anion
It has 6 electrons delocalized over 5 carbons.The negative charge is dispersed over 5 carbons.It is a stabilized anion (aromatic).
H H
H H
H
••
–
H H
H H
H
–
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Acidity of Cyclopentadiene H H
H H
H H
H H
H H
H
••
–
H+ +
pKa = 16, Ka = 10-16
Cyclopentadiene is unusually acidic for a hydrocarbon.
Increased acidity is due to stability of cyclopentadienyl anion (becomes aromatic).
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Electron Delocalization in Cyclopentadienide Anion
••–
H H
H H
H
H H
H H
H
••– H H
H H
H
–
H H
H H
H
–
H H
H H
H
••–
••••
Resonance structures
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Compare Acidities ofCyclopentadiene and Cycloheptatriene H H
H H
H H
pKa = 16
Ka = 10-16pKa = 36
Ka = 10-36
H H
HHH
H H
H H
By losing H+ itbecomes aromatic.
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H H
H H
H
••
–
Compare Acidities ofCyclopentadiene and Cycloheptatriene
An aromatic anion6 electrons
An antiaromatic anion8 electrons
H H
HH
H H
H
••–
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n = 0
4n +2 = 2 electrons
Cyclopropenyl Cation +
H H
H
H H
H
+also written as:
An aromatic cation.
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n = 24n +2 = 10 electrons
Cyclooctatetraene Dianion H H H H
HHHH
H H H H
HHHH
2–••
••
–
–
also written as
An aromatic dianion.
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11.5The Molecular Orbitals
of Benzene
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Energy
Bondingorbitals
Antibondingorbitals
Benzene MOs
6 p AOs combine to give 6 MOs3 MOs are bonding; 3 are antibonding
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Energy
Bondingorbitals
Antibondingorbitals
Benzene MOs
All bonding MOs are filled.No electrons in antibonding orbitals.
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Benzene MOs
Figure 11.4
p 435
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Frost's Circle (Polygon Rule)
MOs of Benzene
Bonding
Antibonding
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Frost's Circle (Polygon Rule)
+
+
··
6 es 6 es 8 es
2 es 4 es 6 es
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-MOs of Benzene
Benzene
Antibonding
Bonding6 p orbitals give 6 orbitals.
3 orbitals are bonding; 3 are antibonding.
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Benzene
Antibonding
Bonding6 electrons fill all of the bonding orbitals.
All antibonding orbitals are empty.
-MOs of Benzene
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Cyclo-butadiene
Antibonding
Bonding
4 p orbitals give 4 orbitals.
1 orbital is bonding, one is antibonding, and 2 are nonbonding.
-MOs of Cyclobutadiene(Square Planar)
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Cyclo-butadiene
Antibonding
Bonding
4 electrons; bonding orbital is filled; other 2 electrons singly occupy two nonbonding orbitals.
-MOs of Cyclobutadiene(Square Planar)
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Antibonding
Bonding
8 p orbitals give 8 orbitals.
3 orbitals are bonding, 3 are antibonding, and 2 are nonbonding.
-MOs of Cyclooctatetraene(Planar)
Cyclo-octatetraene
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Antibonding
Bonding
8 electrons; 3 bonding orbitals are filled; 2nonbonding orbitals are each half-filled.
-MOs of Cyclooctatetraene(Planar)
Cyclo-octatetraene
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11.6Substituted Derivatives of Benzene
and their Nomenclature
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1. Benzene is considered as the parent andcomes last in the name.
2. Substituents are listed in alphabetical order.
3. Number ring in direction that gives lowest numbering.
General Points on Naming
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Benzene is considered as the parent and comes last in the name.
General Points on Naming Bromobenzene tert-Butylbenzene Nitrobenzene
NO2C(CH3)3Br
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2-bromo-1-chloro-4-fluorobenzene
Example
Br
Cl
F
Substituents are listed in alphabetical order.
Number ring in direction that gives lowest numbering.
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Ortho, Meta, and Para Can use ortho, meta and para for disubstituted derivatives of benzene.
1,2 = ortho(abbreviated o-)
1,3 = meta
(abbreviated m-)
1,4 = para
(abbreviated p-)
o-xylene m-xylene p-xylene
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Examples NO2
CH2CH3
Cl
Cl
o-ethylnitrobenzene m-dichlorobenzene(1-ethyl-2-nitrobenzene) (1,3-dichlorobenzene)
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Benzaldehyde
Table 11.1
CH
O
Benzoic acid
COH
O
Certain monosubstituted derivatives of benzene have unique names.
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Phenol
Table 11.1 OH
Acetophenone
CCH3
O
Styrene
CH2CH
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Anisole
Table 11.1 OCH3
Aniline
NH2
OCH3
NO2
OCH3
p-Nitroanisoleor
4-Nitroanisole
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Easily Confused Names
phenyl phenol benzyl OH
CH2—
a group(substituent)
a group(substituent)
a compound
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11.7Polycyclic Aromatic Hydrocarbons
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Napthtalene resonance energy = 255 kJ/mol (~127 kJ/mol per ring). The resonance energy per ring decreases as the number of rings increases or with fewer possible Kekule structures (benzene = 152 kJ/m).
Most stable Lewis structure;here both rings correspond to Kekulé benzene.
Naphthalene
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Anthracene Phenanthrene
resonance energy:347 kJ/mol
(~116 kJ/mol per ring)381 kJ/mol
(~127 kJ/mol per ring)
Anthracene and Phenanthrene
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11.8Physical Properties of Arenes
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Arenes (aromatic hydrocarbons) resembleother hydrocarbons. They are:
nonpolar,
insoluble in water and
less dense than water.
Physical Properties
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11.9Reactions of Arenes:
A Preview
1. Some reactions involve the ring.1. Some reactions involve the ring.
2. Some reactions involve groups attached to 2. Some reactions involve groups attached to the ring.the ring.
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a) Reduction:
Catalytic hydrogenation (Section 11.3)Birch reduction (Section 11.10)
b) Electrophilic aromatic substitution:(Chapter 12)
c) Nucleophilic aromatic substitution:(Chapter 12)
1. Reactions involving the ring:
2. Reactions involving a substituent: (Sections 11.11-11.16)
Reactions of Aromatics
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catalytic hydrogenation(Section 11.3)
H
H
H H
HH
Birch reduction(Section 11.10)
H H
H
H H
H
H H
Reduction of Benzene Rings
HH
HH H
H
H
HH
H H
H
High T High P Dissolving Metal
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11.10The Birch Reduction
A reaction involving the ring.A reaction involving the ring.
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(80%)
H
H
H H
HH
H H
H
H H
H
H H
Na, NH3
CH3OH
Birch Reduction of Benzene
Product is non-conjugated diene.Reaction stops here. There is no further reduction.
Reaction is not hydrogenation (H2 is not involved),
it is a dissolving metal reduction.
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H
H
H H
HH
Step 1: Electron transfer from sodium
+ Na•
Mechanism of the Birch Reduction
+ Na+
H
H
H
H
H
H•
••–
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Step 2: Proton transfer from methanol
Mechanism of the Birch Reduction
H
H
H
H
H
H
–
•
••
••
OCH3
H
••
H
H
H
H
H
H
H
•
OCH3••••••–
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Step 3: Electron transfer from sodium
Mechanism of the Birch Reduction H
H
H
H
H
H
H
•+ Na•
H
H
H
H
H
H
H
••+ Na+
–
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Step 4: Proton transfer from methanol
Mechanism of the Birch Reduction H
H
H
H
H
H
H
••–
H H
H
H H
H
H H
–• OCH3••
•••
• OCH3
H
•••
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(86%)
H
H
H C(CH3)3
HH
H H
H
H C(CH3)3
H
H H
Na, NH3
CH3OH
Birch Reduction of an Alkylbenzene
If an alkyl group is present on the ring, it ends up asa substituent on the double bond.
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11.11Free-Radical Halogenation
of Alkylbenzenes
A reaction involving an alkyl substituent.A reaction involving an alkyl substituent.
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allylic radical
A Substituent on the Benzene Ring
•CC
C •C
benzylic radical
Benzylic carbon is analogous to allylic carbon.
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The more stable the free radical R•, the weaker the bond, and the smaller the bond-dissociation energy (allylic is more stable than 3o, Chapter 4).
Recall:
R—H R• •H+
Bond-dissociation energy for C—H bond is equal to H° for:
and is about 400 kJ/mol for alkanes.
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Bond-dissociation Energies of Propene and Toluene
368 kJ/mol
356 kJ/mol
H
H2C CH C H
H H
C H
H
H2C CH-H•
-H•
H
C
H
•
H
C
H
•
Low BDEs indicate allyl and benzyl radical are more stable than simple alkyl radicals.
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Resonance in Benzyl Radical
CH
H
HH
H
H
H
•
Unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it.
CH
H
HH
H
H
H•
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Resonance in Benzyl Radical
CH
H
HH
H
H
H
•
Unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it.
CH
H
HH
H
H
H
•
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An industrial process that is highly regioselective for benzylic position. Bromine is commonly used in lab.
Free-radical Chlorination of Toluene CH3
Cl2
lightor
heat
CH2Cl
Toluene Benzyl chloride
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Similarly, dichlorination and trichlorination areselective for the benzylic carbon. Furtherchlorination gives:
Free-radical Chlorination of Toluene CCl3
(Dichloromethyl) benzene
CHCl2
(Dichloromethyl) benzene
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Br2 is used in the laboratory to introduce a halogen at the benzylic position.
Benzylic Bromination CH3
NO2
+ Br2
CCl4, 80°C
light
p-Nitrotoluene
+ HBr
NO2
CH2Br
p-Nitrobenzylbromide (71%)
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As was the case in allylic bromination, NBS is a convenient reagent for benzylic bromination.
N-Bromosuccinimide (NBS) NBr
O
O
CCl4
benzoylperoxide,
heat
CH2CH3 +
CHCH3 NH
O
O
+
Br
(87%)
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11.12Oxidation of Alkylbenzenes
A reaction involving an alkyl substituent.A reaction involving an alkyl substituent.
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Site of Oxidation is Benzylic Carbon CH3 CH2R CHR2
or
or
COH
ONa2Cr2O7
H2SO4
H2O
heat
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Example
Na2Cr2O7
H2SO4
H2O
heat
COH
O CH3
NO2
p-Nitrotoluene
NO2
p-Nitrobenzoicacid (82-86%)
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Example
1. conc. KMnO4
NaOH, heat
2. H+ , HOH
CH(CH3)2
CH3
(45%)
COH
O COH
OBasic KMnO4 is also an effective oxidant for this reaction.
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11.13SN1 Reactions of Benzylic Halides
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Tertiary benzylic carbocation is formedmore rapidly than tertiary carbocation;therefore, more stable.
Benzylic SN1 Reactions C
CH3
CH3
Cl
620 1
C
CH3
CH3
ClCH3
Relative solvolysis rates in aqueous acetone:
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Benzylic SN1 Reactions C
more stable less stable
CCH3
Relative rates of formation:
CH3
CH3
+
CH3
CH3
+
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allylic carbocation
Compare
+CC
C +C
benzylic carbocation
Benzylic carbon is analogous to allylic carbon.
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Resonance in Benzyl Cation CH
H
HH
H
H
H+
Positive charge is delocalized between benzylic carbon and the ring carbons that are ortho and para to it as shown by resonance.
CH
H
HH
H
H
H
+
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Resonance in Benzyl Cation
CH
H
HH
H
H
H
+
Additional resonance structures give this benzylic carbocation greater stability.
CH
H
HH
H
H
H
+
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Solvolysis C
CH3
CH3
Cl
CH3CH2OH
C
CH3
CH3
OCH2CH3
(87%)
Mechanism is SN1
Solvolysis of a 3o benzylic halide is faster than a normal tertiary halide.
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11.14SN2 Reactions of Benzylic Halides
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Primary Benzylic Halides
acetic acid
CH2Cl
O2N
NaOCCH3
O
CH2OCCH3
O2N
O
Mechanism is SN2
Like allylic halide, substitution is faster than a normal primary halide.
(78-82%)
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11.15Preparation of Alkenyl Benzenes
•dehydrogenationdehydrogenation
•dehydrationdehydration
•dehydrohalogenationdehydrohalogenation
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•Industrial preparation of styrene
•Almost 12 billion lbs. produced annually in U.S.
Dehydrogenation CH2CH3
630°C
ZnO
CH2CH
+ H2
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Acid-Catalyzed Dehydration of Benzylic Alcohols
KHSO4
heat (80-82%)
CH2CHCHCH3
OH
Cl Cl CHCH3
Cl +
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Dehydrohalogenation
H3C CH2CHCH3
Br
NaOCH2CH3 ethanol, 50°C
(99%)
CHH3C
CHCH3
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11.16Addition Reactions of Alkenyl Benzenes
•hydrogenationhydrogenation
•halogenationhalogenation
•addition of hydrogen halidesaddition of hydrogen halides
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Hydrogenation
H2
Pt
(92%)
Br
C
CH3
CHCH3
Br
CHCH2CH3
CH3
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Halogenation CH2CH
Br2
CH2CH
BrBr
(82%)
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HCl
Addition of Hydrogen Halides (75-84%)
Cl via benzylic carbocation
+
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Free-Radical Addition of HBr CH2CH
CH2CH2Br
HBr
peroxides
via benzylic radical
CH2BrCH
•
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11.22Heterocyclic Aromatic Compounds
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Pyridine
N••
Examples
••
O••
••
S••
N
H
••
Pyrrole Furan Thiophene N••
N ••
Quinoline Isoquinoline
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11.23Heterocyclic Aromatic Compounds
and Huckel’s Rule
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Furan
Two lone pairs on oxygen.One pair is in a p orbital and is part of ring system; other is in an sp2 hybridized orbital and is not part of ring system.
••
O••
Pyridine N••
6 electrons in ring.Lone pair on nitrogen is in an sp2
hybridized orbital; not part of system of ring.
Pyrrole N
H
••
Lone pair on nitrogen must be part of ring system if ring is to have 6 electrons.Lone pair must be in a p orbital in order to overlap with ring system.
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End of Chapter 11End of Chapter 11Arenes and AromaticityArenes and Aromaticity