Textbook Question
Explain why each compound or ion should be aromatic, antiaromatic, or nonaromatic.
(a)
(b)
(c)
18. Aromaticity
Annulene
3:58 minutesProblem 64
Textbook Question
Despite having only sp2-hybridized carbons and having 10 electrons (4n + 2) , the annulene shown is not aromatic. Why?
Verified step by step guidance1
Identify the structure shown in the image as a 10-membered annulene, which is a cyclic hydrocarbon with alternating single and double bonds.
Recall the criteria for aromaticity: a molecule must be cyclic, planar, fully conjugated, and follow Huckel's rule (4n + 2) π electrons, where n is a non-negative integer.
Verify that the molecule has 10 π electrons, which satisfies Huckel's rule for n=2 (4n + 2 = 10).
Assess the planarity of the molecule. For a molecule to be aromatic, it must be planar to allow for the delocalization of π electrons across the ring.
Consider the size and strain of the 10-membered ring. Larger rings like this one often have difficulty maintaining planarity due to steric strain and flexibility, which can prevent effective π electron delocalization, thus making the molecule non-aromatic.
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3mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
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 be cyclic, planar, and follow Hückel's rule, which states that it should have 4n + 2 π electrons, where n is a non-negative integer. This delocalization allows for resonance, contributing to the compound's stability.
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Hückel's Rule
Hückel's rule is a criterion used to determine the aromaticity of a compound based on its π electron count. According to this rule, a planar, cyclic molecule is aromatic if it contains 4n + 2 π electrons. This means that while a compound may have the correct number of π electrons, other structural factors must also be considered to confirm its aromatic nature.
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Planarity and Hybridization
Planarity refers to the arrangement of atoms in a molecule such that all atoms lie in the same geometric plane, which is crucial for effective overlap of p orbitals in aromatic compounds. In the case of sp²-hybridized carbons, each carbon atom contributes one unhybridized p orbital for π bonding. If the molecule is not fully planar, even with the correct number of π electrons, it may not exhibit aromaticity.
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