Aromatic Compounds Contain Benzene; C6H6

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Aromatic Compounds Contain Benzene C6H6
Chapter 11Arenes and Aromaticity
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Chapter 11 Arenes and Aromaticity

• Nomenclature
• Physical Properties
• Boiling Point
• Resonance Stabilization of Benzene
• Aromaticity Requirements
• Rules
• Aromatic ions
• Aromatic heterocycles
• Aromatic allotropes Polycyclic Aromatic
  Hydrocarbons

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Chapter 11 Arenes and Aromaticity

• Reactions of Substituted Alkyl Benzenes
• Free Radical Halogenation
• Oxidation
• Nucleophilic Substitution

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Examples of Aromatic Hydrocarbons
Benzene
Toluene
Naphthalene
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Resonance Formulation of Benzene

• The structure of benzene is 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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Orbital Hybridization Model of Bonding in Benzene
Each carbon has a p orbital Six p orbitals
overlap to give cyclic ? system

• Planar ring of 6 sp2
• hybridized carbons

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Nomenclature

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

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Nomenclature

• If two groups come off benzene ring
• List substituents in alphabetical order and
  number
• ring in direction that gives lowest locant at
  first point
• of difference OR use ortho, para, meta notation.

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Ortho, Meta, and Para
alternative locants for disubstitutedderivatives
of benzene
1,2 ortho(abbreviated o-)
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Examples
NO2
CH2CH3
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Certain monosubstituted derivatives of benzene
have unique names
Benzoic acid
Benzaldehyde
NH2
OCH3
Aniline
Anisole
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Acetophenone
Anisole
Phenol
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Names in Table 11.1 can be used as parent
Anisole
p-Nitroanisoleor4-Nitroanisole
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Nomenclature

1. Aromatic compounds with more than two groups
   coming off the ring are named by giving groups
   the lowest possible combination of numbers (Dont
   use ortho, meta, para nomenclature)

2-bromo-1-chloro-4-fluorobenzene
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Easily confused names
R group an alkyl group contains only C H
and has only single bonds
Ar group an arene group contains a benzene ring
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Nomenclature

1. If an alkyl group off a benzene ring has more
   than 6 carbons the benzene ring is a substituent
   rather than the parent chain and the compound is
   named as a phenylalkane.

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Physical Properties

• Arenes (aromatic hydrocarbons) resembleother
  hydrocarbons. They are
• nonpolar
• insoluble in water
• less dense than water

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Boiling Point
11.9 Physical Properties of Arenes
C6H14 BP 69 C
C6H6 BP 80.1 C
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Boiling Point
170C
181C
173C
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Special Stability of Aromatic Compounds Shown by
DH

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Figure 11.2 (p 429)
3 x cyclohexene

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

360 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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Requirements for Aromaticity
Aromaticity Unusual Stability Associated with
Fully Conjugated Cyclic Systems

• Cyclic Compound
• Planar Compound
• Fully Conjugated System
• 4n 2 p electrons (Huckels Rule)
• n integer (1,2,3,)

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Aromatic IonsCycloheptatrienyl Cation
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Aromatic IonsCyclopentadienide Anion
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Electron Delocalization in Cyclopentadienide Anion


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Cyclooctatetraene Dianion
H
H
H
H

H
H
H
H

alsowritten as
2

H
H
H
H

H
H
H
H

• n 2
• 4n 2 10?? electrons

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11.22Heterocyclic Aromatic Compounds
Furan
Thiophene
Pyrrole
Pyridine
Isoquinoline
Quinoline
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Pyridine
Heterocyclic Aromatic Compounds

• 6 ? electrons in ring
• lone pair on nitrogen is in ansp2 hybridized
  orbitalnot part of ??system of ring

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Pyrrole

• lone pair on nitrogen must be
• part of ring ? system if ring
• is to have6 ? electrons
• lone pair must be in a p orbitalin order to
  overlap with ring ?system

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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 notpart of ring ? system

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Aromatic Allotropes of Carbon
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Polycyclic Aromatic Hydrocarbons (PAH)
n4
n 2
Naphthalene
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• 11.10 Reactions of Arenes A Preview
• Some reactions involve the ring.
• a) Electrophilic aromatic substitution (Chapter
  12)
• b) Nucleophilic aromatic substitution (Chapter
  23)
• In other reactions the ring is a substituent.
• (Sections 11.12-11.17)

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Benzylic Hydrogens are Reactive Sites

• Free Radical Halogenation of Alkyl Benzenes
• Oxidation of Alkyl Benzenes
• Nucleophilic Substitution of Benzylic Halides

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Free Radical Substitution Reactions
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Free-radical chlorination of toluene
Cl2
CH3
CH2Cl
lightorheat
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Bond-dissociation energies of propene and toluene
368 kJ/mol
H2C
CH
-H
356 kJ/mol
-H
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Resonance in Benzyl Radical

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

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Site of Oxidation is Benzylic Carbon
Oxidation of Alkylbenzenes
or
or
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A Benzylic Hydrogen is Required for Oxidation
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11.14 Nucleophilic Substitutionin Benzylic
Halides

• R-L Nu -gt R-Nu L

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Solvolysis
CH3CH2OH
(87)
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SN1 Substitution Reactions
Relative solvolysis rates in aqueous acetone
600
1

• tertiary benzylic carbocation is formedmore
  rapidly than tertiary carbocationtherefore,
  more stable

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SN1 MechanismStep 1 Leaving Group Leaves
Carbocation FormedStep 2 Product is formed
Relative rates of formation
CH3
CH3


CH3
C
C
CH3
CH3
more stable
less stable
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Resonance in Benzyl Cation
H
H
H
H
H
H
H
H


H
H
H
H
H
H

• positive charge is delocalized between benzylic
  carbon and the ring carbons that are ortho and
  para to it