Chemistry

Introduction to Aromatic Reactivity

• We categorized substituents on an aromatic ring and discussed their values in terms of reactivity and stability.

• The need for ranking substituents to understand their effect on the aromatic reaction rate.

The Effects of Substituents on Aromatic Reactivity

• Electron Donating Groups (EDG):

  • Inject electron density into the aromatic ring, making the ring a stronger nucleophile and more reactive.

• Electron Withdrawing Groups (EWG):

  • Extract electron density from the aromatic ring, weakening the nucleophilic strength and reducing the rate of reactions.

Regioselectivity in Electrophilic Aromatic Substitution (EAS)

• Definition of Regioselectivity:

  • Preference for specific sites of substitution on the aromatic ring upon the addition of an electrophile.

  • The presence of substituents affects which hydrogens can be replaced by new electrophiles.

• Importance of site positions: ortho, meta, and para described in relation to substituents on the aromatic ring:

  • Ortho Position: Adjacent to the substituent.

  • Meta Position: Two carbons away.

  • Para Position: Opposite the substituent.

Electrophilic Addition to Aromatic Rings

• Illustrative case of an aromatic ring with an alcohol substituent reacting with an electrophile.

  • Possible sites of reaction are ortho, meta, and para.

  • Introduced carbocation intermediates and resonance forms depending on the position of the incoming electrophile.

• Practice drawing resonance structures based on electrophilic added positions and carbocation stability.

Resonance Structures and Stability

• When an electrophile adds at the ortho or para positions:

  • The carbocation ends up adjacent to the electron donating substituent, yielding a more stable resonance structure.

• When an electrophile adds at the meta position:

  • The relevant resonance forms do not place the positive charge near the electron donating substituent, reducing stability.

Summary of Directing Effects

• Activating Groups (EDG): Preferentially direct electrophile addition to ortho and para positions.

• Deactivating Groups (EWG): Direct electrophile addition to meta positions due to unstable resonance at ortho and para sites.

• Special Cases:

  • Alkyl Groups: EDG despite no resonance effect; direct ortho and para.

  • Halogens: Have lone pairs but are EWGs; they are ortho/para directors due to lone pairs yet destabilize due to inductive withdrawal.

Ranking of Substituents

• Ranking of substituent ability to activate:

  • EDGs elevate reactivity and favor ortho/para attachment due to resonance contributing to stability.

  • EWGs decrease reactivity and favor meta attachment as ortho and para additions are less stable.

• Prioritization matters when evaluating competing substituents on the aromatic ring

  • The substituent higher on the ranking will determine preferred sites for electrophile addition.

Application of Concepts Through Practice Problems

• Students tasked with determining effects of given groups (alkyls, nitro, sulfonic acid) on reactivity and regioselectivity.

• Evaluating possible sites for substitution and assessing sterics to conclude the favored substitutions.

Conclusion

• Recap on how substituents influence both reactivity and regioselectivity for electrophilic aromatic substitution, emphasizing the significance of lone pairs and inductive effects in determining the behavior of groups.