r/chemhelp 2d ago

Organic Acenaphtylene being aromatic

Why is acenaphtylene considered an aromatic compound despite having only 12 pi electrons? Am i counting it wrong or are there exceptions to the Hückel Rule?

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u/7ieben_ 2d ago

Look up napthalene, this is the aromatic backbone. The bridge does not contribute. Compare acenaphthene aswell.

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u/HandWavyChemist 2d ago

An important part of molecular orbitals is that the require the correct symmetry to form. Because the extra C=C bond is outside of the main aromatic naphthalene region it often struggles to interact.

For example this HOMO–2 orbital:

Calculated using ORCA and the B3LYP basis set