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CHM 121 Chapter 13 Notes

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This is a comprehensive and detailed note on chapter 13; Arrangement of Arenes; Electrophilic Aromatic Substitution. *Essential!!

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  • October 20, 2024
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  • 2022/2023
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CH. 13



Chapter 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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