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Lecture No.38 Electrophilic Aromatic Substitution Reactions

• At the end of this lecture, student will be able to

– Discuss the concept of aromaticity and Huckel’s rule– Differentiate aromatic and nonaromatic compounds– Explain the general mechanism of electrophilic aromatic

substitution reaction– List out examples for EAS reactions

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Structure of Benzene

• It contains a six-membered ring and three additional degrees of unsaturation

• It is planar• All C – C bond lengths are equal• Resonance• Benzene is conjugated

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Structure of Benzene Contd…

• Benzene’s six π -electrons make it electron rich and so it readily reacts with electrophiles

• Overlap of six adjacent p orbitals creates two rings of electron density

• Delocalisation of π electrons • Clouds of π electrons above and below the plane of benzene ring

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Nomenclature of Benzene Derivatives

• Monosubstituted Benzenes

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Nomenclature of Benzene Derivatives

• Disubstituted benzenes

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Nomenclature of Benzene Derivatives

• Arrange the substituents in alphabetical order

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Nomenclature of Benzene Derivatives

• Polysubstituted benzenes

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Nomenclature of Benzene Derivatives

• Phenyl and benzyl group

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Criteria for Aromaticity• A molecule must have uninterrupted cyclic cloud of π electrons• For the π cloud to be cyclic, the molecule must be cyclic• For the π cloud to be uninterrupted, every atom in the ring must have a p-orbital• For the π cloud to form, each p orbital must overlap with the p orbitals on either side of

it• Therefore, the molecule must be planar• Odd number of pairs of π electrons• Huckel’s rule or 4n + 2 rule states that for a planar, cyclic compound to be aromatic its

uninterrupted π cloud must contain (4n+2) π electrons, where n is any whole number (0,1,2,…)

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Criteria for Aromaticity Contd…

• Example 1

Benzene – 3 pairs of π electrons – aromatic Cyclobutadiene & cyclooctatetraene – non aromatic

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Criteria for Aromaticity Contd…

• Example 2

Cyclopropene – non aromatic Cyclopropenyl cation – aromatic Cyclopropenyl anion – non aromatic

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Criteria for Aromaticity Contd…

• Example 3

Naphthalene – 5 pairs of π electrons – aromatic Phenanthrene – 7 pairs of π electrons – non aromatic

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Electrophilic Aromatic Substitution Reactions

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Electrophilic Aromatic Substitution Reactions Contd…

• Benzene - does not undergo addition reactions like unsaturated hydrocarbons

• Typical reactions of benzene - substitution

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Electrophilic Aromatic Substitution Reactions Contd…

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Electrophilic Aromatic Substitution Reactions Contd…

Mechanism•Step 1 Addition of electrophile to form a carbocation Rate determining step•Step 2 Loss of a proton to form the substituted product

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Electrophilic Aromatic Substitution Reactions Contd…

• How to draw resonance structures for carbocation?

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Summary

• Benzene – planar and resonance stabilized• Huckel’s rule states that for a planar, cyclic compound to be

aromatic its uninterrupted π cloud must contain (4n+2) π electrons, where n is any whole number (0,1,2,…)

• Benzene undergoes electrophilic aromatic substitution reactions• Sigma complex is formed which is resonance stabilized• Nitration, sulphonation, halogenation, Friedel Craft’s alkylation and

Friedel Craft’s acylation of benzene are electrophilic substitution reactions