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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

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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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