9-Electrophilic Aromatic Reactions
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Transcript of 9-Electrophilic Aromatic Reactions
8/3/2019 9-Electrophilic Aromatic Reactions
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KHONGO waNGEGHA’S
Reactions ofAromatic
Compounds
M.W. KamiyangoPremedical SciencesCollege of Medicine
Malawi
Electrophilic Aromatic Substitution
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Study Objectives
By the end of this lesson students should be able to:
• Describe the mechanism of electrophilic substitution inarenes using halogenation and nitration of benzene as
examples.
• Predict whether halogenation will occur in the side-chain inarenes depending on reaction conditions.
• Predict products for the oxidation of the side-chain inarenes.
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Reactions of Benzene• Even though benzene is highly unsaturated it does not
undergo any of the regular reactions of alkenes such as
addition or oxidation
•
Benzene can be induced to react with bromine if a Lewisacid catalyst is present however the reaction is asubstitution and not an addition
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Electrophilic Aromatic Substitution• Aromatic compounds undergo electrophilic aromatic
substitution (EAS) reactionsThe electrophile involved has a full or partial positive charge.
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A General Mechanism for Electrophilic AromaticSubstitution
In step 1 of the mechanism, the electrophile reacts with two
pi electrons from the aromatic ring to form an arenium ion(stabilized by resonance)
In step 2 , a proton is removed and the aromatic system isregenerated
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Halogenation of BenzeneHalogenation of benzene requires the presence of a Lewisacid
Fluorination occurs so rapidly it is hard to stop at
monofluorination of the ring
Iodine is so unreactive that an alternative method must beused
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Mechanism of Benzene Bromination
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In step 1 of the mechanism, bromine reacts with ferricbromide to generate an electrophilic bromine species
In step 2 , the highly electrophilic bromine reacts with pielectrons of the benzene ring, forming an arenium ion
In step 3 , a proton is removed from the arenium ion andaromaticity is regenerated
The FeBr 3 catalyst is regenerated
Mechanism of BenzeneBromination
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Nitration of Benzene
• Nitration of benzene is a reaction that results in theintroduction of a nitrogen dioxide (-NO 2) group on abenzene ring.
• Similar to halogenation, nitration is a substitution reactionwhere a proton is substituted by an -NO 2 group.
• The electrophile involved is the nitronium ion (NO 2+)
which is generated from a reaction between nitric acidand sulphuric acid.
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Mechanism for the Nitration ofBenzene
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• Sulfuric acid protonates the hydroxyl group of nitricacid, allowing it to leave as water, forming anitronium ion.
• Nitric acid is not the electrophile, it is a nitroniumion formed by protonation and dehydration ofHNO 3 by H 2SO 4.
• It is the nitronium ion the species that is attacked
by the benzene.• Loss of a proton from the sigma complex produces
nitrobenzene.
Mechanism for the Nitration ofBenzene
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• Alkyl benzene compounds are derivatives of benzene in
which at least one proton has been substituted by an alkylgroup, e.g. Toluene:
• Alkyl benzenes can undergo radical reactions in which thecarbon atom carrying the unpaired electron is on the sidechain.
• Such radicals are easier to form because they arestabilized by resonance.
Reactions of Alkyl Benzenes
CH 3
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Reactions of Alkyl Benzenes:Side Chain Halogenation
Alkyl benzenes undergo free-radical halogenation muchmore easily than alkanes because abstraction of a hydrogenatom at a benzylic position gives a resonance-stabilizedbenzylic radical.
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Oxidation of the Side Chain• Alkyl and unsaturated side chains of aromatic rings can
be oxidized to the carboxylic acid using hot KMnO4
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