Hückel’s Rule

• Hückel’s rule is a criterion used to determine if a molecule is aromatic.
• Aromatic compounds exhibit unusual stability and unique properties due to the delocalization of π electrons in a conjugated ring system.

Here are the key points:

1. Cyclic Structure: The molecule must have a ring structure.
2. Planarity: The molecule must be planar, which allows for effective overlap of p orbitals.
3. Conjugated System: The molecule must have a conjugated system with alternating single and double bonds.
4. π Electrons: The molecule must have a specific number of π electrons, calculated using the formula 4n+2, where n is a non-negative integer (0, 1, 2, …).

For example, benzene (C₆H₆):

• Cyclic Structure: Benzene has a ring structure.
• Planarity: Benzene is planar.
• Conjugated System: Benzene has alternating single and double bonds, forming a conjugated system.
• π Electrons: Benzene has 6 π electrons (one from each carbon).

  • Applying Hückel’s rule:

    • 4n + 2 = 6
    • Solving for n:

• • • Since n = 1 is a non-negative integer, benzene satisfies Hückel’s rule and is aromatic.
    • This explains benzene’s stability and unique properties.

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