Failing Hückel
Cyclodecapentaene or [10]annulene is a compound which is unstable, despite its resemblance to benzene, an aromatic compound. What criterion of Hückel's rule does cyclodecapentaene fail to meet?
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It is not cyclic.
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It is not planar.
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It does not have electrons in its delocalised, conjugated -orbital cloud.
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Not all its atoms participate in delocalising electrons.
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1 solution
Let's observe the possible structures of cyclodecapentaene.
The first structure has bond angles of 1 4 4 ∘ , making it unfavourable as compared to the ideal bond angle of 1 2 0 ∘ predicted by VSEPR.
The second structure relieves the bond strain from the first structure but is still unstable due to the all-cis conformation.
The third structure would be stable if not for the steric repulsion from the internal hydrogen atoms.
The fourth structure, then is the most stable; however, it is clearly not planar and as such is not aromatic.