Chapter 11 Arenes and Aromaticity

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Chapter 11Arenes and Aromaticity
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Examples of Aromatic Hydrocarbons
Benzene
Toluene
Naphthalene
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11.1Benzene
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Some history

• 1834 Eilhardt Mitscherlich isolates a new
  hydrocarbon and determines its empirical
  formula to be CnHn. Compound comes to be
  called benzene.
• 1845 August W. von Hofmann isolates benzene
  from coal tar.
• 1866 August Kekulé proposes structure of
  benzene.

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11.2Kekulé and theStructure of Benzene
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Kekulé Formulation of Benzene

• Kekulé proposed a cyclic structure for C6H6with
  alternating single and double bonds.

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Kekulé Formulation of Benzene

• Later, Kekulé revised his proposal by
  suggestinga rapid equilibrium between two
  equivalentstructures.

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Kekulé Formulation of Benzene

• However, this proposal suggested isomers of
  thekind shown were possible. Yet, none were
  everfound.

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Structure of Benzene

• Structural studies of benzene do not support
  theKekulé formulation. Instead of alternating
  singleand double bonds, all of the CC bonds are
  thesame length.

Benzene has the shape of a regular hexagon.
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All CC bond distances 140 pm
140 pm
140 pm
140 pm
140 pm
140 pm
140 pm
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All CC bond distances 140 pm
146 pm
140 pm
140 pm
140 pm
140 pm
134 pm
140 pm
140 pm

• 140 pm is the average between the CC single
  bond distance and the double bond distance in
  1,3-butadiene.

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11.3A Resonance Picture of Bonding in Benzene
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Kekulé Formulation of Benzene

• Instead of Kekulé's suggestion of a
  rapidequilibrium between two structures

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Resonance Formulation of Benzene

• express the structure of benzene as a
  resonancehybrid of the two Lewis structures.
  Electrons arenot localized in alternating single
  and double bonds,but are delocalized over all
  six ring carbons.

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Resonance Formulation of Benzene

• Circle-in-a-ring notation stands for resonance
  description of benzene (hybrid of two Kekulé
  structures)

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11.4The Stability of Benzene

• benzene is the best and most familiar example of
  a substance that possesses "special stability"
  or "aromaticity"
• aromaticity is a level of stability that is
  substantially greater for a molecule than would
  be expected on the basis of any of the Lewis
  structures written for it

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Thermochemical Measures of Stability

• heat of hydrogenation compare
  experimentalvalue with "expected" value for
  hypothetical"cyclohexatriene"

Pt

3H2
DH 208 kJ
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Figure 11.2 (p 404)
3 x cyclohexene

360 kJ/mol
231 kJ/mol
208 kJ/mol
120 kJ/mol
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Figure 11.2 (p 404)
3 x cyclohexene

• "expected" heat of hydrogenation of benzene is 3
  x heat of hydrogenation of cyclohexene

360 kJ/mol
120 kJ/mol
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Figure 11.2 (p 404)
3 x cyclohexene

• observed heat of hydrogenation is 152 kJ/mol
  less than "expected"
• benzene is 152 kJ/mol more stable thanexpected
• 152 kJ/mol is the resonance energy of benzene

360 kJ/mol
208 kJ/mol
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Figure 11.2 (p 404)

• hydrogenation of 1,3-cyclohexadiene (2H2) gives
  off more heat than hydrogenation of benzene (3H2)!

231 kJ/mol
208 kJ/mol
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Cyclic conjugation versus noncyclic conjugation
3H2
Pt
heat of hydrogenation 208 kJ/mol
3H2
Pt

heat of hydrogenation 337 kJ/mol
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Resonance Energy of Benzene

• compared to localized 1,3,5-cyclohexatriene
• 152 kJ/mol
• compared to 1,3,5-hexatriene
• 129 kJ/mol
• 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

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11.5An Orbital Hybridization Viewof Bonding in
Benzene
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Orbital Hybridization Model of Bonding in Benzene

• Planar ring of 6 sp2 hybridized carbons

Figure 11.3
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Orbital Hybridization Model of Bonding in Benzene

• Each carbon contributes a p orbital
• Six p orbitals overlap to give cyclic p
  systemsix p electrons delocalized throughout p
  system

Figure 11.3
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Orbital Hybridization Model of Bonding in Benzene

• High electron density above and below plane of
  ring

Figure 11.3
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11.6The p Molecular Orbitalsof Benzene
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Benzene MOs
Antibondingorbitals
Bondingorbitals

• 6 p AOs combine to give 6 p MOs
• 3 MOs are bonding 3 are antibonding

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Benzene MOs
Antibondingorbitals
Bondingorbitals

• All bonding MOs are filled
• No electrons in antibonding orbitals

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The Three Bonding p MOs of Benzene
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11.7Substituted Derivatives of Benzene and
Their Nomenclature
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General Points

• 1) Benzene is considered as the parent andcomes
  last in the name.

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Examples
NO2
Br
C(CH3)3
Bromobenzene
tert-Butylbenzene
Nitrobenzene
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General Points

• 1) Benzene is considered as the parent andcomes
  last in the name.
• 2) List substituents in alphabetical order
• 3) Number ring in direction that gives lowest
  locant at first point of difference

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Example
Cl
Br
F
2-bromo-1-chloro-4-fluorobenzene
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Ortho, Meta, and Para
alternative locants for disubstitutedderivatives
of benzene
1,2 ortho(abbreviated o-)
1,3 meta(abbreviated m-)
1,4 para(abbreviated p-)
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Examples
NO2
CH2CH3
o-ethylnitrobenzene
m-dichlorobenzene
(1-ethyl-2-nitrobenzene)
(1,3-dichlorobenzene)
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Benzene Derivatives
Certain monosubstituted derivatives of benzene
have unique names
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Benzene Derivatives
Benzaldehyde
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Benzene Derivatives
Benzoic acid
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Benzene Derivatives
Styrene
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Benzene Derivatives
Toluene
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Benzene Derivatives
Acetophenone
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Benzene Derivatives
Phenol
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Benzene Derivatives
Anisole
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Benzene Derivatives
Aniline
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Benzene derivative names can be used as parent
Anisole
p-Nitroanisoleor4-Nitroanisole
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Easily confused names
CH2
OH
phenyl
phenol
benzyl
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11.8Polycyclic Aromatic Hydrocarbons
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Naphthalene

• resonance energy 255 kJ/mol

most stable Lewis structureboth rings
correspond to Kekulé benzene
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Anthracene and Phenanthrene
Phenanthrene
Anthracene
resonance energy
347 kJ/mol
381 kJ/mol
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11.9Physical Properties of Arenes
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Physical Properties

• Resemble other hydrocarbons
• nonpolar
• insoluble in water
• less dense than water

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11.10Reactions of ArenesA Preview

• 1. Some reactions involve the ring.
• 2. In other reactions the ring is a substituent.

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1. Reactions involving the ring

• a) Reduction
• Catalytic hydrogenation (Section 11.4) Birch
  reduction (Section 11.11)
• b) Electrophilic aromatic substitution (Chapter
  12)
• c) Nucleophilic aromatic substitution (Chapter
  23)

2. The ring as a substituent (Sections
11.12-11.17)
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Reduction of Benzene Rings
catalytic hydrogenation (Section 11.4)
Birch reduction (Section 11.11)
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