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Recently I came across a question asking which compounds are anti-aromatic. This was one of the options:

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I’m confused as this is not an answer. I think the lone pair electrons on the carbanion participate in resonance and so there are 8π electrons which, follows the 4nπ electrons rule for anti aromaticity.

So why isn't it anti-aromatic?

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The rules in determining the aromaticity of a compound are as follows:

  1. The system must have 4n+2 π electrons
  2. The system must be planar
  3. The system must be completely conjugated
  4. The molecule must be cyclic.

The cycloheptatriene anion does not follow two out of these four rules. Namely,

  1. The anion system has 4n electrons (whereas it should have had 4n+2 electrons in it π system)
  2. The anion system has an sp3 carbon and is hence non-planar (where it should have been planar)

Due to these two issues (second issue mainly), there is no ring current in the complete ring and so no conditions need to be considered whether the compound is antiaromatic or aromatic in nature.

Cyclopentadienyl anion also has an sp3 carbon as an anion. However the anion formed is aromatic if planar and so this overcomes the ring strain and so retains aromaticity.

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