Aromaticity -- Hückel's Rule

Hückel proposed that aromaticity is a function of a molecule's electronic structure: a compound is aromatic only if it satisfies ALL three conditions -- (i) the molecule (or at least the ring system) is co-planar; (ii) there is complete delocalisation of the pi electrons around the ring; (iii) the ring contains (4n+2) pi electrons, where n = 0, 1, 2, ... (Hückel's rule). Worked examples: BENZENE is planar, has 6 delocalised pi electrons, and 6 = 4(1)+2 (n=1) -- aromatic. NAPHTHALENE is planar, has 10 delocalised pi electrons, and 10 = 4(2)+2 (n=2) -- aromatic. ANTHRACENE is planar, has 14 delocalised pi electrons, and 14 = 4(3)+2 (n=3) -- aromatic. CYCLOPENTADIENE is planar and has 4 pi electrons, but those electrons are NOT fully delocalised (one ring carbon is sp3, CH2, breaking the conjugated system) -- NOT aromatic. CYCLOOCTATETRAENE is non-planar (tub-shaped) -- NOT …