Class notes
CHM 121 Chapter 13 Notes
- Course
- Chemistry 121
- Institution
- Harper College
This is a comprehensive and detailed note on chapter 13; Arrangement of Arenes; Electrophilic Aromatic Substitution. *Essential!!
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CH. 13Chapter 13
Reactions of Arenes
Electrophilic Aromatic Substitution
Electrophiles add to aromatic rings in a fashion somewhat similar to the addition of
electrophiles to alkenes.
Recall:
R3 R4 E Y E
Y
C C + E Y R1 C C R4 R1 C C R4
R2 R1 δ+ δ−
R2 R3 R2 R3
In aromatic rings, however, we see substitution of one of the benzene ring hydrogens for
an electrophile.
H E
+ E Y + Y H
δ+ δ−
The mechanism is the same regardless of the electrophile. It involves two steps: (1)
Addition of the electrophile to form a high-energy carbocation. (2) Elimination of the
proton to restore the aromatic ring system.
H H
H H
E Y slow
step 1 + + Y
E
δ+ δ−
high energy arenium ion
H
step 2 H E
fast
+ Y + Y H
E
The first step is the slow step since the aromaticity of the benzene ring system is destroyed
on formation of the arenium ion intermediate. This is a high energy species but it is
stabilized by resonance with the remaining two double bonds. The second step is very fast
since it restores the aromatic stabilization.
1
, CH. 13
H H H
H H H
E E E
There are five electrophilic aromatic substitution reactions that we will study.
(1) Nitration
H NO2
H2SO4
+ HNO3
(2) Sulfonation
H SO3H
+ H2SO4
(3) Halogenation with bromine or chlorine
H X
FeX3 X = Br, Cl
+ X2
(4) Friedel-Crafts Alkylation
H R
+ AlX3
RX
(5) Friedel-Crafts Acylation
O
H O C
AlX3 R
+ Cl C R
2
, CH. 13
Nitration of Benzene
We generate the highly electrophilic nitronium ion in situ using sulfuric acid to dehydrate
nitric acid. The nitronium ion is then attacked by the π−electrons to give the arenium ion,
which then loses a proton to regain its aromaticity.
O O O 2
H
HO N + HO S OH O N O N O O N O
O H O
O nitronium ion
very electrophilic nitrogen deficient nitrogen
H H
H H O
H OH2
+ O N O O N
N O
O
Sulfonation of Benzene
H H O
H O
O H O S OH
H O S OH
S S O
+ O O O
O
O
The sulfonation reaction is reversible. Heating the benzene sulfonyl compound in aqueous
acid removes the sulfonyl group and replaces it with a proton.
O
S OH H3O+ H
O + H2SO4
H2O
Halogenation of Benzene
Generally a Lewis acid catalyst is needed to activate the bromine and make it into a better
electrophile. Bromine itself does not react with benzene.
3
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