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Is cyclohepta-1,3,5-triene aromatic?

Cyclohepta-1,3,5-triene

I think it should not be, because even though the π-system possesses six π-electrons, the conjugation is not complete around the ring.

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You are correct. The conjugation does not extend all the way around the ring, so the molecule is not aromatic. In addition, the single methylene group ($\ce{CH2}$) sticks out of the plane of the rest of the molecule.

(On the other hand, removing a hydride ion $\ce{H-}$ from the $\ce{CH2}$ group leaves the tropylium ion $\ce{C7H7+}$, which is planar and aromatic, with resonance structures putting the positive charge on any of the seven carbons.)

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To define an aromatic compound, there are four basic rules:

  • P(lanarity)
  • D(elocalization)
  • H(ückel's rule): $4n + 2$ π-electrons
  • C(onjugation)

In the above compound, if any one of the carbon is $\mathrm{sp^3}$ hybridised, then it is not planar. In the above compound, there is an $\mathrm{sp^3}$ carbon, so it is not aromatic.

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