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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
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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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
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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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
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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Nomenclature of Benzene Derivatives
• Monosubstituted Benzenes
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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Nomenclature of Benzene Derivatives
• Disubstituted benzenes
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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Nomenclature of Benzene Derivatives
• Arrange the substituents in alphabetical order
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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Nomenclature of Benzene Derivatives
• Polysubstituted benzenes
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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Nomenclature of Benzene Derivatives
• Phenyl and benzyl group
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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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,…)
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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Criteria for Aromaticity Contd…
• Example 1
Benzene – 3 pairs of π electrons – aromatic Cyclobutadiene & cyclooctatetraene – non aromatic
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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Criteria for Aromaticity Contd…
• Example 2
Cyclopropene – non aromatic Cyclopropenyl cation – aromatic Cyclopropenyl anion – non aromatic
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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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
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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Electrophilic Aromatic Substitution Reactions
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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Electrophilic Aromatic Substitution Reactions Contd…
• Benzene - does not undergo addition reactions like unsaturated hydrocarbons
• Typical reactions of benzene - substitution
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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Electrophilic Aromatic Substitution Reactions Contd…
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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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
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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Electrophilic Aromatic Substitution Reactions Contd…
• How to draw resonance structures for carbocation?
Faculty of Pharmacy ©M. S. Ramaiah University of Applied Sciences
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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