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 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.
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.
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.
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 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).
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.
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.
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.
Require concentrated HNO₃ and H₂SO₄.
Produce nitronium ion (NO₂⁺) as the electrophile through loss of water.
Mechanism is similar to halogenation with resonance structures.
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.