Arenes and Aromaticity

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Chapter 11

• Arenes and Aromaticity

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Aromatic Compounds

• Arenes hydrocarbons based on the benzene ring
  as a structural unit.
• Aromatic Compounds Compounds possessing more
  stability than would be predicted due to having
  conjugated double bonds.

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Benzene

• Benzene has been known since 1825. It was
  discovered by Michael Faraday. The empirical
  formula was determined by Eirlhardt Mitscherlich.
  Structure worked out by August Kekule in 1866.

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Resonance Picture of Benzene

• The two equivalent Kekule structures represent a
  resonance hybrid shown by the right hand
  structure.

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Stability of Benzene
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The Orbital Hybridization View of Benzene
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Molecular Orbitals of Benzene
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Nomenclature of Benzene Derivatives

• Monosubstituted Benzene Compounds are often named
  by attaching the name of the substitutent as a
  prefix of benzene.
• Some Monosubstituted Benzene Derivatives have
  long standing common names.

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Some benzene derivatives have long standing
common names.
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Disubstituted Benzene Derivatives

• ortho- 1,2 disubstituted benzene
• meta- 1,3 disubstituted benzene
• para- 1,4 disubstituted benzene

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Trisubstituted Benzene Derivatives

• o,m,p are not used when three or more
  substituents are on the benzene ring. Instead
  the ring is numbered and the base name is that of
  the ranking substituent. The substituents are
  named in alphabetical order. The numbering is
  such that the next substituted position is given
  the lowest number.
• Ex Try pick one p. 467 27 and p. 468 28 b.

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Benzene as a Substituent

• Phenyl- C6H5-
• Benzyl- C6H5CH2-
• Biphenyl

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Polycyclic Aromatic Compounds

• These compounds are composed of benzene rings
  fused together. They have substantial resonance
  energy. (See 11.30)
• Napthalene
• Anthracene
• Phenanthrene

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Physical Properties of Arenes

• Arenes are similar to hydrocarbons in physical
  properties.
• Nonpolar
• Insoluble in Water
• Less Dense than Water

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Reactions of Arenes

• Two main Classes of Reaction
• Reactions of the Ring itself
• Reduction
• Electrophilic Aromatic Substitution
• 2. Reactions in which the ring influences
  reactivity of a functional unit to which it is
  attached.

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Birch Reduction

• Reduction of the benzene ring by treatment with
  sodium and methanol in liquid ammonia. It is
  used to synthesize dienes.

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Birch Reduction Mechanism
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Free Radical Halogenation of Alkylbenzenes

• Benzylic Radicals are especially stable.
  Therefore free radical attack favors the benzylic
  position. The benzylic radical is analogous to
  the allylic radical.

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Free Radical Halogenation
Br2
p-Nitrotoluene
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Spin Density Representation of Benzyl Radical
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lowM.O. View of Benzyl Radical (lowest energy
M.O.)
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NBS Bromination (lab prep)
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Oxidation of Alkyl Benzenes
or
or
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Nucleophilic Substitution of Benzylic Halides

• Benzylic halides make ideal substrates for SN2
  reactions because they react readily toward good
  nucleophiles and cannot undergo competing
  elimination.

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Primary Benzylic Halides

• Mechanism is SN2

acetic acid
(78-82)
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Preparation of Alkenylbenzenes

• Industrial Prep Dehydrogenation of Ethylbenzene

630C
ZnO
H2
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Laboratory Preparation of Alkenylbenzenes

• Acid catalyzed dehydration or base promoted
  elimination reactions of appropriate leaving
  groups will produce alkneylbenzenes

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Dehydration of Benzyl Alcohols
KHSO4
heat
(80-82)
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Dehydrohalogenation
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Addition Reactions of Alkenylbenzenes

• Hydrogenation

(92)
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Addition Reactions of Alkenylbenzenes

• Halogenation

Br2
CH
CH2
Br
Br
(82)
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Addition Reactions of Alkenylbenzenes

• Hydrohalogenation

(75-84)
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Polymerization of Styrene
polystyrene
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Polymerization of Styrene (Mechanism)
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Huckels Rule

• Planar, monocyclic, fully conjugated polyenes,
  only with (4n2) p electrons will have a special
  stability (be aromatic).
• Planar, monocyclic, fully conjugated polyenes
  with (4n) p electrons are especially unstable and
  are therefore antiaromatic.

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Annulenes

• Annulenes are planar, monocyclic, completely
  conjugated polyenes. That is, they are the
  kind of hydrocarbons treated by Hückel's rule.
  
• Annulenes are named in this manner
• x annulene where x of carbons.
• Many have been prepared to test Huckels rule.

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10Annulene
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
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10Annulene
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
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14Annulene
H
H
H
H
14 ? electrons satisfies Hückel's rule van der
Waals strain between hydrogens insidethe ring
Therefore it is nonplanar.
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16Annulene
16 ? electrons does not satisfy Hückel's
rule alternating short (134 pm) and long (146 pm)
bonds is an antiaromatic 4n p-electron system
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18Annulene
18 ? electrons satisfies Hückel's rule resonance
energy 418 kJ/mol bond distances range between
137-143 pm
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Aromatic Ions

• Some nonaromatic compounds will form stabilized
  aromatic ions.

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Aromatic Ions
Cyclopentadiene is unusually acidic for a
hydrocarbon. Increased acidity is due to
stability of cyclopentadienide anion.
pKa 16 Ka 10-16
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Heterocyclic Aromatic Compounds
Examples
Pyridine
Pyrrole
Furan
Thiophene
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Pyridine
6 ? electrons in ring lone pair on nitrogen is in
ansp2 hybridized orbitalnot part of ??system
of ring