AROMATIC SUBSTITUTION

1
AROMATIC SUBSTITUTION REACTIONS
2
NOMENCLATURE
3
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the Organic Nomeclature Software
1-bromo-3-nitrobenzene
methylbenzene
(toluene)
1-chloro-3-methylbenzene
1,4-dimethylbenzene
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SOME SPECIAL NAMES
toluene
aniline
anisole
phenol
o-xylene
m-xylene
benzoic acid
p-xylene
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ortho, meta and para Positions
m-nitrotoluene
3-nitrotoluene
ipso
1
ortho
o-
2
6
1-methyl-3-nitrobenzene
meta
m-
3
5
4
para
p-dichlorobenzene
p-
1,4-dichlorobenzene
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REVIEW OF BENZENE PROPERTIES
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BENZENE RESONANCE
Review Sections 6.8 - 6.14
KEKULE STRUCTURES
Resonance Energy 36 Kcal / mole
All bonds are equivalent
The ring is symmetric. Bond lengths are between a
single and a double bond.
Very Stable
Less reactive than other groupings of atoms.
8
.
.
H
H
.
.
H
.
.
H
H
H
All 2p orbitals overlap equally.
9
BENZENE
Isodensity surfaces - electron potential mapped
in color.
(van der Waals)
Color adjusted to enhance the pi system.
Highest electron density is red.
Note the symmetry.
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BENZENE - DETERMINATION OF RESONANCE ENERGY
cyclohexatriene
(hypothetical)
benzene
RESONANCE ENERGY 36 kcal/mol
cyclohexene
-49.8 kcal/mol
-85.8 kcal/mol
(calculated)
-28.6 kcal/mol
cyclohexane
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REACTIVITY
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The Double Bonds in a Benzene Ring Do Not React
Like Others
Alkene
Benzene
no reaction


no reaction


no reaction


no reaction


13
Benzene is a Weak Base and Poor Nucleophile

Stronger base
Readily donates electrons to an electrophile.
alkene

Donation of electrons would interrupt ring
resonance (36 kcal / mole).
Weaker base
A strong electrophile is required - and often
a catalyst.
benzene
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Benzene Reactivity
Benzene requires a strong electrophile and a
catalyst ..and then it undergoes substitution
reactions, not addition.


catalyst
substitution
compare

no catalyst
addition
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Some Substitution Reactions of Benzene
Halogenation

Friedel-Crafts Alkylation

Friedel-Crafts Acylation

-


Nitration

-
Sulfonation

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MECHANISM
All of the reactions follow the same pattern of
mechanism.
The reagents combine to form a strong
electrophile E ,and its partner (X ), which
react as follows
ELECTROPHILIC AROMATIC SUBSTITUTION
()

HX
()

slow
X
intermediate
restores ring resonance
benzenium ion
resonance structures are shown by the () symbols
also called a benzenonium ion
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ENERGY PROFILE FOR AROMATIC SUBSTITUTION
()
benzenium intermediate
Transition state 1

()
Transition state 2
intermediate
Ea
activation energy


H
slow
fast
STEP 1
STEP 2
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HALOGENATION
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Formation of the Chloronium Ion Complex
..
..




d
..
..
..
..
..
..
d-





..
..
..
..
..
..




..
..
sp2
..


..
..
..

-



..
..
..


..
chloronium ion complex
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Chlorination of Benzene
-
Cl2 AlCl3
-




benzenium ion
chloronium ion complex
HAlCl4
HCl AlCl3
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FRIEDEL-CRAFTS REACTIONS
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FRIEDEL-CRAFTS ALKYLATION
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Formation of a Carbocation Complex
..
..




d
..
..
..
..
d-



..
..
..
..




..
..
Other aliphatic R-Cl may be used
..


..
..
-



..
..


..
carbocation
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Friedel-Crafts Alkylation
-
CH3Cl AlCl3
-




HAlCl4
HCl AlCl3
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REARRANGEMENTS ARE COMMON IN FRIEDEL-CRAFTS
ALKYLATION
AlCl3
-

-

carbocation rearrangement
AlCl3

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FRIEDEL-CRAFTS ACYLATION
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Formation of an Acylonium Complex
..
..




..
..
..
..
d
d-



..
..
..
..




..
..
..
Other acid chlorides (RCOCl) may be used


..
..
-


..
..



..
acylium ion
(acylonium ion)
Rearrangements DO NOT occur
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Friedel-Crafts Acylation
-
AlCl3
-




HAlCl4
HCl AlCl3
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LINEAR CHAINS ARE MADE VIA ACYLATION (no
rearrangement) AND REMOVAL OF CO
Clemmensen
X
doesnt work - rearranges
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NITRATION
31
Formation of Nitronium Ion
nitronium ion
..

Powerful Electrophile

Reacts with benzene.

..
32
Nitration of Benzene

..
..
..

HNO3






H2SO4
..
-

..
..

..


..
-
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SULFONATION
34
Fuming Sulfuric Acid
.
H2SO4 SO3
..
..


..
..

..
..


..
..
sulfur trioxide
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Sulfonation of Benzene
-
.
H2SO4 SO3

H3O
D
can be reversed in boiling water or steam
(acidic)
H2SO4
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REMOVAL OF THE SULFONATE GROUP
-

heat or steam
excess H2O
-
-

37
Benzoic Acid Syntheses
38
Synthesis of Benzoic Acids and Benzoate Esters
CH3Cl
KMnO4
AlCl3
CH3OH
1 Li 2 CO2 3 H3O
Br2
AlBr3
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AROMATIC ?
WHERE DID THE TERM ORIGINALLY COME FROM ?
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A LOT OF NICE-SMELLING COMPOUNDS (SPICES
IN PARTICULAR) HAVE BENZENE RINGS
thymol
anisaldehyde
(thyme)
cinnamaldehyde
(anise)
(cinnamon)
eugenol
cuminaldehyde
(cloves)
(cumin)
Hence, compounds having benzene rings eventually
came to be know as AROMATIC COMPOUNDS.
Today chemists have a different definition of
AROMATIC which we will discuss later in the
chapter.