Ochem lecture 13

Chemistry 2420 - Lecture 18

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

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.

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.

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
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)

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.

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

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.

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.

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.

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!