How do you count pi electrons in aromatic compounds?

Four rules to be said for a compound as aromatic.

  • First, it must be cyclic.
  • Second, every atom in the ring must be conjugated.
  • Third, the molecule must have [4n+2] pi electrons
  • Fourth, the molecule must be flat (if 1 and 3 conditions are obeyed)

Benzene and cyclooctatetraene are both cyclic and conjugated, but benzene is aromatic and cyclooctatetraene is not. The difference is that benzene has 6 pi electrons, and cyclooctatetraene has 8. The condition that aromatic molecules must have [4n+2] pi electrons is sometimes called “Hückel’s rule”.

Each p orbital contains a single electron. We can verify the total number of pi electrons in benzene by counting the pi bonds: 3 pi bonds times two electrons = 6 pi electrons total.

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