Ochem lecture 13

Chemistry 2420 - Lecture 18

Lecture Details

• Date: February 14th, 2025

• Instructor: Dr. Amani A. Abdelghani

• Contact: aabdelghani@upei.ca

Chapter Overview

• Chapter 8 Topics:

  • 8.16 Benzene is an Aromatic Compound

  • 8.17 The Two Criteria for Aromaticity

  • 8.18 Applying the Criteria for Aromaticity

  • 8.20 Aromatic Heterocyclic Compounds

  • 8.21 How Benzene Reacts

8.16 Benzene is an Aromatic Compound

• Definition of Aromatic Compounds:

  • Special cyclic compounds with unusual stability.

  • Named for the “aroma” of benzene.

• Reactivity:

  • Benzene does not typically undergo electrophilic addition reactions unless under extreme conditions.

Resonance and Stability

• Stability Factors:

  • Benzene's stability is attributed to large delocalization energy.

  • Resonance energy quantified as 152 kJ/mol, contributing to unusual stability.

• Comparison with Other Molecules:

  • Other stable resonance structures do not necessarily exhibit similar stability as benzene.

  • Resonance explains stability but is not the sole reason.

8.17 The Two Criteria for Aromaticity

Criteria for Aromaticity:

1. Uninterrupted p Orbital Ring:

   • The compound must have a cyclic structure with a p orbital on each atom.

   • Essential for creating a cloud of electrons above and below the ring (visualized as a donut).

   • Requirements:

     • Cyclic structure

     • Planar geometry for orbital overlap

     • P orbitals on each atom in the ring

2. Odd Number of P Electron Pairs:

   • Apply Huckel’s Rule:

   • Formula: 4n + 2 = π electrons (n = 0, 1, 2…)

   • Examples:

     • n = 0 → 2 π electrons (1 pair)

     • n = 1 → 6 π electrons (3 pairs)

Antiaromatic Compounds

• Definition:

  • Compounds satisfying criterion #1 but failing criterion #2 (not 4n+2).

• Properties:

  • Cyclic and flat, but with instability due to resonance lack.

  • Extremely unstable compared to non-resonant compounds.

8.18 Applying the Criteria for Aromaticity

• Evaluate various compounds to determine their aromatic status based on the criteria.

Comparison of Aromatic, Anti-Aromatic, and Non-Aromatic Compounds

• Aromatic:

  • Very stable, follows 4n+2 rule for π electrons, cyclic, conjugated sp² hybridization, and planar.

• Anti-Aromatic:

  • Very unstable, 4n rule, cyclic, pi bonds with even number of electrons.

• Non-Aromatic:

  • Cyclic or noncyclic, not completely conjugated and can have sp³ hybridization.

Compounds with Fused Rings

• Naphthalene:

  • Contains 5 pairs of π electrons (aromatic).

• Anthracene:

  • Contains 7 pairs of π electrons (aromatic).

Cyclic Ions

• Definitions and Characteristics:

  • Ions can exhibit aromatic characteristics if they have uninterrupted rings with p orbitals.

  • Certain structures may be classified as aromatic, anti-aromatic, or non-aromatic based on sp hybridization and p orbital availability.

8.20 Aromatic Heterocyclic Compounds

• Definition:

  • Cyclic compounds with one or more ring atoms being non-carbon (e.g., nitrogen).

• Example:

  • In heteroatoms already part of double bonds, their lone pairs may influence aromaticity depending on the hybridization state.

Conclusion

• Final Remarks:

• The discussion of resonant structures and their roles in the stability of aromatic compounds highlights key principles in understanding chemical behavior.

• Next Lecture: Thank you and see you next lecture!