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:

  1. Analyze the attached substituent.

  2. 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.