Ochem lecture 15
CHEMISTRY 2420 - LECTURE 15
Date: February 26th, 2025
Instructor: Amani A. Abdelghani
Email: aabdelghani@upei.ca
Chapter 18: Reactions of Benzene and Substituted Benzenes
Topics Covered
18.12 The Effect of Substituents on Reactivity (Pages 886-890)
18.13 The Effect of Substituents on Orientation (Pages 890-893)
18.14 The Ortho–Para Ratio (Page 894)
18.12 The Effect of Substituents on Reactivity
Presence of Groups on the Benzene Ring
Consideration of existing substituents on the benzene ring affects reactivity.
Why Less Ortho and More Para?
Steric Hindrance: Larger R groups increase para product prevalence due to steric hindrance in the ortho positions.
Why No Meta Substituents?
Resonance Effects: Alkyl (R) or aryl (Ar) groups direct electrophilic attacks (E+) to ortho and para positions due to stabilization of the carbocation (C+).
Key Steps:
Analyze the attached substituent.
Examine three resonance forms for the C+ intermediate.
18.12 Types of Substituents
Electron Withdrawing Groups
Deactivating Substituents: Withdraw electrons via resonance and inductively, reducing reactivity towards electrophilic aromatic substitution.
Electron Donating Groups
Activating Substituents: Donate electrons by resonance and hyperconjugation, enhancing reactivity towards electrophilic aromatic substitution.
Classification of Substituents
Strongly Activating: Donate electrons via lone pairs.
Moderately Activating: Donate by resonance but withdraw inductively.
Weakly Activating: Donates by hyperconjugation.
Weakly Deactivating: Donates by resonance but withdraws inductively.
Moderately Deactivating: Withdraw electrons inductively.
Strongly Deactivating: Withdraw electrons strongly via resonance and inductively.
18.13 The Effect of Substituents on Orientation
Stability of Carbocation Intermediates
In electrophilic aromatic substitution reactions, three carbocation types can form: ortho, meta, and para.
Stability of carbocations correlates with the rate of formation and reaction completion.
Directing Effects of Substituents
Ortho-Para Directors: All activating and weakly deactivating substituents (e.g., halogens).
Meta Directors: All moderately and strongly deactivating substituents.
Key Examples
Activation through Electron Donating Groups (e.g., -OH from alcohols) enhances resonance stability in carbocations.
Br reacting with CH3 demonstrates the different outcomes based on substituent placement (ortho, meta, para).
Deactivating Groups
Types of Deactivating Groups
Moderate and Strongly Deactivating Groups: Withdraw electrons effectively, making carbocations less stable, leading to slower reactions.
Meta Directorship: A characteristic of strongly deactivating groups due to their structure, affecting reaction orientation.
Summary of Substituent Effects
Activating Substituents: Lead to ortho-para directionality.
Weakly Deactivating Halogens: Also direct ortho-para.
Deactivating Substituents: Direct mainly to meta.
Conclusion
Review of the substituent effects on reactivity and orientation in electrophilic aromatic substitution.
End of Lecture 15
Instructor expresses willingness for further practice on resonance structures.