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Reactions of Aromatic Compounds

The document discusses resonance structures, delocalized electrons, and electrophilic aromatic substitution reactions. It explains that benzene undergoes substitution rather than addition reactions due to its stable closed shell of delocalized pi electrons. Common electrophilic aromatic substitutions include halogenation, nitration, sulfonation, and Friedel-Crafts alkylation and acylation reactions, which introduce groups onto the benzene ring. Lewis acids are often used to activate the electrophiles. Substituents can activate or deactivate the benzene ring toward further reactions.

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258 views38 pages

Reactions of Aromatic Compounds

The document discusses resonance structures, delocalized electrons, and electrophilic aromatic substitution reactions. It explains that benzene undergoes substitution rather than addition reactions due to its stable closed shell of delocalized pi electrons. Common electrophilic aromatic substitutions include halogenation, nitration, sulfonation, and Friedel-Crafts alkylation and acylation reactions, which introduce groups onto the benzene ring. Lewis acids are often used to activate the electrophiles. Substituents can activate or deactivate the benzene ring toward further reactions.

Uploaded by

maulidya
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Three different but equivalent structures, 13:

Localized electrons : Electrons that are restricted


to a particular region

Delocalized electrons : neither belong to a single


atom nor are confined to a bond between two
atoms, but are shared by three or more atoms
1. None of these structures, which we call
resonance structures or resonance
contributors, will be a realistic representation
for the molecule or ion
2. The actual molecule or ion will be better
represented by a hybrid (average) of these
structures
1. Resonance structures exist only on paper
2. We are only allowed to move electrons in writing
resonance structures

3. All of the structures must be proper Lewis structures

4. The energy of the resonance hybrid is lower than the


energy of any contributing structure
Write the contributing resonance structures and resonance
hybrid for each of the following:
Write the contributing resonance structures and resonance
hybrid for each of the following:
Molecule of benzene is planar
All of its carboncarbon bonds are of equal length: 1.39
Electrophilic Aromatic Substitutions (EAS):
an electrophile replaces one of the hydrogen atoms of
the ring
allow the direct introduction of groups onto aromatic
rings such as benzene
Types of electrophilic aromatic substitutions
Benzene has something in common with alkenes
Benzenes closed shell of six electrons give it special stability
Although benzene is susceptible to electrophilic attack, it
undergoes substitution reactions rather than addition reactions
Benzene reacts with bromine and chlorine in the presence of
Lewis acids
The Lewis acids typically used are aluminum chloride (AlCl3)
and iron chloride (FeCl3) for chlorination, and iron bromide
(FeBr3) for bromination.
The purpose of the Lewis acid : to make the halogen a stronger
electrophile.
Iodine is unreactive direct iodination has to be
carried out in the presence of an oxidizing agent such
as nitric acid
Benzene undergoes nitration on reaction with a
mixture of concentrated nitric acid and concentrated
sulfuric acid

Concentrated sulfuric acid increases the rate of the


reaction by increasing the concentration of the
electrophile, the nitronium ion (NO2+)
Benzene reacts with fuming sulfuric acid at room
temperature to produce benzenesulfonic acid
Fuming sulfuric acid : sulfuric acid that contains added
sulfur trioxide (SO3)
Sulfonation also takes place in concentrated sulfuric acid
alone, but more slowly
Charles Friedel & James M. Crafts:
Preparation of alkylbenzenes (ArR) and acylbenzenes (ArCOR) the
FriedelCrafts alkylation and acylation reactions.
Mechanism :
Starts with the formation of a carbocation.
The carbocation then acts as an electrophile and attacks the benzene ring
to form an arenium ion.
The arenium ion then loses a proton.
Acylation reaction : a reaction whereby an acyl group is introduced into a
compound
Two common acyl groups : acetyl group and the benzoyl group
The FriedelCrafts acylation reaction : treating the
aromatic compound with an acyl halide (often an acyl
chloride).
The reaction requires the addition of at least one
equivalent of a Lewis acid (such as AlCl3)
The product of the reaction is an aryl ketone
Carbocation rearrange to be more stable the major
products obtained from the reaction are usually those
from the more stable carbocations
FriedelCrafts reactions usually give poor yields
when powerful electron-withdrawing groups
Electron-withdrawing groups make the ring less
reactive by making it electron deficient
Aryl and vinylic halides cannot be used as the
halide component because they do not form
carbocations readily
Polyalkylations often occur.
Alkyl groups are electron-releasing groups activates
the ring toward further substitution
Polyacylations are not a problem in FriedelCrafts
acylations
A substituent can affect
reactivity
Orientation

Activating group : A substituent can make the ring


more reactive than benzene
Deactivating groups : A substituent can make the ring
less reactive than benzene

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