Lecture Notes: Aromaticity and Aromatic Stabilization

Overview

• Aromaticity: Refers to the extra stability that some cyclic molecules possess.
• Benzene vs. Cyclohexane: Benzene is more stable due to aromaticity, not just because it is conjugated.

Aromatic Compounds

• Criteria for Aromaticity:
  1. Must have a ring of continuously overlapping p orbitals.
  2. Molecule must be planar to allow p orbital overlap.
  3. Must follow Huckel's Rule: 4n + 2 π electrons (where n is a non-negative integer).

Benzene Analysis

• Pi Electrons in Benzene:
  • Total of 6 π electrons.
  • Each carbon in benzene is sp2 hybridized, having a free p orbital.
• P Orbital Overlap:
  • Benzene's planar structure allows side-by-side overlap of p orbitals.

Molecular Orbital (MO) Theory

• Conversion of Atomic Orbitals:
  • 6 atomic orbitals in benzene convert to 6 molecular orbitals.
• Energy Levels:
  • Use a Frost circle to visualize energy levels.
  • Inscribing a polygon (hexagon for benzene) helps determine molecular orbital energy levels.
• Molecular Orbitals in Benzene:
  • 3 bonding MOs (lower energy) and 3 antibonding MOs (higher energy).
  • No non-bonding MOs in benzene.

Filling Molecular Orbitals

• Electron Configuration Analogy:
  • Fill the lowest energy MOs first.
  • Benzene’s 6 π electrons fill the bonding MOs.

Huckel's Rule in Practice

• Huckel’s Calculation for Benzene:
  • 4n + 2 = 6 π electrons (n=1 satisfies the rule).
• Visualization:
  • Lower bonding orbitals relate to "+2" and upper ones to "4n".

Conclusion

• Aromaticity in Benzene:
  • Benzene fulfills all aromatic criteria and is stabilized by aromaticity.

Future Topics

• Further exploration of other aromatic compounds and ions in upcoming sessions.