Download AROMATIC COMPOUNDS

AROMATIC COMPOUNDS
By PUAN AZDUWIN BINTI KHASRI
Criteria for Aromaticity
1. A compound must have an uninterrupted cyclic cloud
of 𝜋 electrons above and below the plane of the molecule
2. The p cloud must contain an odd number of pairs
of p electrons.
BENZENE
Benzene is an aromatic compound because it is cyclic and planar,
every carbon in the ring has a p orbital, and the 𝜋 cloud contains
three pairs of 𝜋 electrons.
Hückel’s 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
Monocyclic hydrocarbons with alternating single and
double bonds are called annulenes:
Cyclobutadiene and cyclooctatetraene are NOT AROMATIC,
because they have an even number of p electron pairs
not
aromatic
not
aromatic
aromatic
Cyclopentadiene does not have an uninterrupted ring of
p orbital-bearing atoms
Cyclopentadienyl cation has an even number of p
electron pairs
Cyclopentadienyl anion has an uninterrupted ring of p
orbital-bearing atoms and an odd number of p electron
pairs
5
The resonance hybrid shows that all the carbons in the
cyclopentadienyl anion are equivalent
These compounds consist of fused benzene rings and are
aromatic:
Any compound consisting of fused benzene rings is
aromatic
7
Aromatic Heterocyclic Compounds
A compound does not have to be a hydrocarbon to be
aromatic.
A HETEROCYCLIC compound has ring atoms other than
carbon
Example:Heterocyclic Compounds
Antiaromaticity
A compound is antiaromatic if it is a planar, cyclic, continuous
loop of p orbitals with an even number of
pairs of p electrons
Antiaromatic compounds are highly unstable, but the
nonplanar versions are stable
A compound is classified as being antiaromatic if it fulfills
the first criterion for aromaticity but does not fulfill the
second criterion.
EXAMPLE: Antiaromaticity
Nomenclature of Monosubstituted Benzenes
Some are named by attaching “benzene” after the name
of the substituent:
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Some have to be memorized:
A benzene substituent is called phenyl.
A benzene substituent with a methylene
group is called benzyl.
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Electrophilic Aromatic Substitution Reactions
1. Halogenation
2. Nitration
3. Sulfonation
4.Friedel–Crafts acylation
5.Friedel–Crafts alkylation
General Mechanism for Electrophilic
Aromatic Substitution of Benzene
Carbocation
intermediate
1. Halogenation of Benzene
LEWIS ACID CATALYST
LEWIS ACID CATALYST
16
Lewis acid weakens the Br–Br (or Cl–Cl) bond, which
makes the halogen a better electrophile:
17
Mechanism for bromination
B: Bromide or Benzene
The catalyst is regenerated:
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2. Nitration of Benzene
Nitration of benzene with nitric acid requires
sulfuric acid as a catalyst.
20
Nitronium ion formation:
Mechanism for Nitration;
21
3.Sulfonation of Benzene
Fuming sulfuric acid (a solution of in sulfuric acid) or
concentrated sulfuric acid is used to sulfonate aromatic
rings
Mechanism for sulfonation
Sulfonation of benzene is a reversible reaction.
Mechanism for desulfonation
Reaction coordinate diagram for the sulfonation
the desulfonation
sulfonation of
B
DESULFONATION
B-A
RATE DETERMINING
STEP
A
C-B has a smaller rate
constant than B-A
(because once B is formed,
its easier for B to get to C
Desulfonation
of
and
proceed to A)
SULFONATION
benzene A-B
RATE DETERMINING STEP
Has a smaller rate constant
(Higher energy hill, thus
slower reaction) than B-C
C
benzenesulfonic acid
Friedel–Crafts Acylation vs Friedel–
Crafts Alkylation
4.Friedel–Crafts Acylation Reactions
Either an acyl halide or an acid anhydride can be used
for Friedel–Crafts acylation.
Mechanism for Friedel–Crafts acylation:
Must be carried out with more than one equivalent of AlCl3:
27
5.Friedel–Crafts Alkylation of Benzene
The Friedel–Crafts alkylation reaction substitutes an alkyl
group for a hydrogen.
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Mechanism for Friedel–Crafts alkylation:
29
The carbocation will rearrange to a more stable species:
However, 100% of the 2-methyl-2-phenylbutane
product can be obtained if a bulky alkyl halide is used:
31
Friedel–Crafts alkylation will not produce a good yield of an
alkylbenzene containing a straight-chain group, because the
carbocation will rearrange:
Acylium ions, however, do not rearrange:
32
Methodologies Used for
the Reduction Step
There are more general methods available to reduce a
ketone carbonyl group to a methylene group
33
Using Coupling Reactions to Alkylate Benzene
The Gilman reagent:
The Stille reaction:
The Suzuki reaction:
The resulting halide product can undergo a nucleophilic
substitution reaction:
Oxidation of an alkyl group bonded to a benzene ring
Provided that a hydrogen is bonded to the benzylic
carbon,
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The same reagent that oxidizes alkyl substituents will
oxidize benzylic alcohols:
37
However, aldehydes or ketones can be generated if a
milder oxidizing agent is used:
38
Reducing a Nitro Substituent
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It is possible to selectively reduce just one of the two nitro
groups:
40
Summary of Electrophilic Aromatic Substitution
Reactions
41
Summary of Friedel–Crafts Reactions
42
THE END