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.

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