lecture recording on 09 January 2025 at 11.26.31 AM

Aromaticity and Extended Conjugation

• Aromaticity is a property associated with extended conjugation in cyclic compounds.

  • Involves circulating double bonds (example: cyclohexatriene).

  • Each atom in the ring shares the same hybridization, allowing for delocalization of electrons.

• Conjugation is necessary:

  • Requires alternating double and single bonds.

  • Key factor is the hybridization of carbon atoms involved.

Hybridization and Electron Delocalization

• Hybridization States:

  • Atoms in the cyclic structure must be sp² hybridized to allow for extended resonance.

  • Radicals or carbocations can facilitate delocalization in shorter carbon chains.

Structure of Benzene

• Benzene has a planar structure due to sp² hybrid orbitals (trigonal planar) arrangement.

  • Unhybridized p orbitals are perpendicular to the ring plane.

  • Results in a donut-shaped electron cloud above and below the ring due to electron circulation.

  • Uncertainty in electron position is governed by Heisenberg’s uncertainty principle.

Energy Levels and Molecular Orbitals

• Molecular Orbital Theory:

  • Focus on **

• Only pi electrons are delocalized:

  • Count of total pi electrons:

    • Benzene has 6 pi electrons (circularly delocalized).

    • Naphthalene has 10 pi electrons (5 pi bonds).

    • Anthracene has 14 pi electrons (7 pi bonds).

• Energy levels for benzene: 3 bonding molecular orbitals and pi star levels as nonbonding orbitals.

• Biochemical importance:

  • Detection occurs via UV or infrared radiation interactions with pi electrons.

Conditions for Aromaticity

• Criteria for Aromatic Compounds (Hückel's Rule):

  • Must have cyclic structure.

  • All carbons must be in sp² hybridization.

  • Must have 4n + 2 pi electrons (where n = whole number).

• Examples include benzene, naphthalene, phenanthrene.

Non-Aromatic and Anti-Aromatic Compounds

• Non-aromatic compounds:

  • Break in conjugation (e.g., presence of sp³ hybridized carbons).

  • Examples are cyclical systems with sp³ carbons.

• Anti-aromatic compounds:

  • Have 4n pi electrons (less stable) with conjugation present.

  • Examples include cyclobutadiene (4 pi electrons) and cyclooctatetraene (8 pi electrons).

Extended Aromatic Systems

• Polycyclic aromatic hydrocarbons like naphthalene and anthracene maintain aromaticity through extended conjugated systems.

• Nomenclature of polycyclic structures:

  • Use specific naming and numbering patterns due to structural overlaps.

Electrophilic Aromatic Substitution (EAS)

• Common reactions: Halogenation, Nitration, and Sulfonation.

• Mechanism Overview:

  • Attack of electrophile leads to formation of resonance-stabilized carbocation.

  • Restoring aromaticity is key to completing the reaction.

• Halogenation Example:

  • Requires catalyst (e.g., FeBr3, AlCl3) to form electrophilic bromine or chlorine species.

Key Mechanisms of EAS

Halogenation

• Reactants: Benzene + halogen (Cl₂ or Br₂) + Lewis acid catalyst.

• Formation of electrophile through activation by metals.

• Subsequent attachment producing resonance-stabilized carbocation and regeneration of aromatic structure.

Nitration

• Require concentrated HNO₃ and H₂SO₄.

• Produce nitronium ion (NO₂⁺) as the electrophile through loss of water.

• Mechanism is similar to halogenation with resonance structures.

Reaction Summary

• EAS reactions modify the aromatic stability based on the nature and position of substituents.

• The focus of further study will involve detailed mechanisms and potentially more complex substitution reactions.