Aromaticity

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Chapter 15 Aromaticity Reactions of Benzene
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Criteria for Aromaticity
1. A compound must have uninterrupted cyclic
cloud of p electrons above and below the
plane of the molecule
2. The p cloud must contain an odd number of
pairs of p electrons
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Hückels Rule
For a planar, cyclic compound to be aromatic, its
uninterrupted p cloud must contain (4n 2) p
electrons, where n is any whole number
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Monocyclic hydrocarbons with alternating single
and double bonds are called annulenes
Cyclobutadiene is not aromatic because it has an
even number of electron pairs
Cyclooctatetraene is not aromatic because it is
nonplanar
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Neither cyclopropene nor the cyclopropenyl anion
is aromatic
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sp3
sp3
cyclopentadiene
cycloheptatriene
2 pairs of p electrons not aromatic
interrupted p cloud not aromatic
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These compounds are aromatic
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Aromatic Heterocyclic Compounds
A heterocyclic compound is a cyclic compound in
which one or more of the ring atoms is an atom
other than carbon
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Pyridine Is Aromatic
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Pyrrole Is Aromatic
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Furan Is Aromatic
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Examples of Heterocyclic Aromatic Compounds
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The Effect of Aromaticity on the pKa Values of
Some Compounds
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Aromaticity influences chemical reactivity
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A compound is antiaromatic if it is a planar,
cyclic compound with an uninterrupted ring of p
cloud, but it contains even number of pairs of p
electrons
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A Molecular Orbital Description of Aromaticity
and Antiaromaticity
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All CC bond distances 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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Huckel Rule or 4n 2 Rule
An oromatic compound must have an odd number of
pairs of p electrons
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The Chemistry
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• Five common electrophilic aromatic substitution
  reactions
• Halogenation a Br, Cl, I substitutes for
  hydrogen
• Nitration a NO2 group substitutes for hydrogen
• Sulfonation a SO3H group substitutes for
  hydrogen
• Friedel Crafts acylation a RCO group
  substitutes for
• hydrogen
• 5. Friedel Crafts alkylation a alkyl group (R)

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Chemistry of Aromatic Compounds
Remember
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Benzene is a nucleophile that reacts with an
electrophile
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Reaction Coordinate Diagrams for the Two Benzene
Reactions
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General Mechanism for Electrophilic Aromatic
Substitution of Benzene
slow
fast
B
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Halogenation of Benzene
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Nitration of Benzene
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Sulfonation of Benzene
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Principle of Microscopic Reversibility
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Principle of Microscopic Reversibility
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Principle of Microscopic Reversibility
The forward and reverse reactions must have the
same intermediates and the rate determing is the
the same in both directions
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FriedelCrafts acylation must be carried out with
more than one equivalent of AlCl3
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Acylation-Reduction
permits primary alkyl groups to be attachedto an
aromatic ring
RCCl
AlCl3

• Reduction of aldehyde and ketonecarbonyl groups
  using Zn(Hg) and HCl is called the Clemmensen
  reduction.

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A problem with Acylation of Benzyne
A solution use only 1 equivalent of AlCl3
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Alkylation of Benzyne
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The carbocation will rearrange to a more stable
species
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alkylation of benzene by an alkene
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It is not possible to obtain a good yield of an
alkylbenzene containing a straight-chain group
via FriedelCrafts alkylation
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However, a FriedelCrafts acylationreduction
works well
This method avoids using a large excess of
benzene in the reaction
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Methodologies Used for the Reduction Step
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One needs to consider an alternative if there is
another functional group present in the compound