PDA104T_38 Electrophilic Aromatic Substitution Reactions

Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences

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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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• Disubstituted benzenes


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• Arrange the substituents in alphabetical order


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• Polysubstituted benzenes


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• 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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• Example 2

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


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• 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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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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• 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

PDA104T_38 Electrophilic Aromatic Substitution Reactions

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