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Non-planar 3D structures.
Not fully conjugated.
Contain sp3 carbons.
Not cyclic (e.g., linear molecules).
Conclusion: Aromatic.
Properties: Cyclic, conjugated, planar, and 14 pi electrons (Huckel's rule).
Structure: Three fused rings.
Conclusion: Aromatic.
Properties: Cyclic, conjugated, planar, and 10 pi electrons (Huckel's rule).
Structure: Two fused rings.
Cyclopentadiene: Contains an sp3 carbon, thus non-aromatic.
1,3-Butadiene: Conjugated but not cyclic, thus non-aromatic.
If pi electrons follow odd numbers (e.g., 3, 5, 7, 9).
Examples include molecules that are:
Fail to meet at least one of the conditions for aromaticity or anti-aromaticity.
Cyclobutadiene: Cyclic, conjugated, planar, and has 4 pi electrons following the 4n rule. Hence, cyclobutadiene is anti-aromatic.
Electron count follows 4n rule: The number of pi electrons must be a multiple of 4 (e.g., 4, 8, 12).
Planar structure: The molecule must be flat.
Conjugated system: Pi electrons free to move around the ring.
No sp3 carbons: Only sp2 or sp carbons allowed.
Cyclic: The molecule must form a ring.
Similar to aromatic compounds but must follow a different electron rule.
Benzene: Benzene is cyclic, conjugated, planar, and follows Huckel's rule with 6 pi electrons (4n + 2 = 6). Hence, benzene is aromatic.
Huckel's Rule: The number of pi electrons must follow the formula for (4n + 2), where n is an integer (e.g., 2, 6, 10, 14).
Planar structure: The molecule must be flat (2D structure).
Conjugated system: Pi electrons must be free to move around the ring, allowing overlap of adjacent p orbitals.
Each atom must have a p orbital: No sp3 carbons allowed, only sp2 or sp.
Cyclic: The molecule must be in a ring structure.
Non-aromatic compounds do not meet the criteria for aromaticity or anti-aromaticity and have normal stability.
Anti-aromatic compounds are similar but have 4n pi electrons, leading to instability.
Aromatic compounds are cyclic, conjugated, planar molecules with (4n + 2) pi electrons.
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Phenanthrene
Naphthalene
Anti-aromatic compounds: Highly unstable.
Non-aromatic compounds: Normal stability.
Aromatic compounds: Highly stable.
Example:
Conditions for Non-Aromaticity:
Example:
Conditions for Anti-Aromaticity:
Example:
Conditions for Aromaticity:
Summary:
Additional Resources:
Additional Examples:
Stability Comparison:
Non-Aromatic Compounds
Anti-Aromatic Compounds
Aromatic Compounds
Aromatic, Anti-Aromatic, and Non-Aromatic Compounds