(••) Despite having only sp²-hybridized carbons and having 10 electrons (4n + 2) , the annulene shown is not aromatic. Why?


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Verified step by step guidance

1. Identify the criteria for aromaticity: a molecule must be cyclic, planar, fully conjugated, and follow Huckel's rule (4n + 2 π electrons).
2. Verify that the annulene is cyclic and contains only sp²-hybridized carbons, which suggests it is fully conjugated.
3. Check the electron count: the molecule has 10 π electrons, which fits Huckel's rule for n=2 (4n + 2 = 10).
4. Examine the planarity of the molecule: for a molecule to be aromatic, it must be planar to allow for delocalization of π electrons.
5. Consider steric hindrance or structural constraints that might prevent the molecule from being planar, thus disrupting conjugation and preventing aromaticity.

Key Concepts

Aromaticity

Aromaticity is a property of cyclic compounds that exhibit enhanced stability due to delocalized π electrons. For a compound to be aromatic, it must meet Huckel's rule, which states that it should have a planar structure, be cyclic, and contain 4n + 2 π electrons, where n is a non-negative integer. This delocalization allows for resonance, contributing to the compound's stability.

Hybridization

Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals that can accommodate bonding in molecules. In the case of sp² hybridization, one s orbital and two p orbitals combine to form three equivalent sp² hybrid orbitals, which are arranged in a trigonal planar geometry. This hybridization is crucial for understanding the bonding and geometry of the carbons in the annulene structure.

Planarity and Conjugation

For a compound to be aromatic, it must be planar, allowing for effective overlap of p orbitals and continuous conjugation of π electrons. Planarity ensures that the π electrons can delocalize over the entire ring structure, which is essential for achieving the stability associated with aromatic compounds. If a compound is not fully planar, it may disrupt this delocalization, preventing aromaticity despite having the correct number of π electrons.