Aromatic character (Huckel Rule)

Benzene's characteristic preference for substitution reactions over addition reactions is not unique to benzene alone -- it is a general property, called aromaticity or aromatic character, shared by a whole family of compounds, and it arises from extensive CYCLIC delocalisation of pi electrons within a planar ring. Three conditions together define whether a given cyclic compound qualifies as aromatic. First, the compound must be cyclic and planar, meaning every atom in the ring must be sp2-hybridised (so that each one has an available, unhybridised p-orbital). Second, each ring atom's p-orbital must be able to line up parallel to its neighbours, so that continuous, unbroken overlap is possible all the way around the ring -- if even one ring atom is sp3-hybridised and therefore has no p-orbital at all, the overlap chain is broken and the compound cannot be aromatic no matter how many pi electrons it has. Third, and only once the first two structural conditions are satisfied, Huckel's rule applies: the cyclic pi molecular orbital system formed by this overlap must contain (4n+2) pi electrons, where n is any whole number (0, 1, 2, 3, and so on). Applying these three rules to worked examples clarifies how they interact: benzene (cyclic, planar, a p-orbital on every carbon, 6 pi electrons from 3 double bonds, satisfying 4n+2=6 with n=1) is aromatic; naphthalene (the same three structural conditions met, 10 pi electrons from 5 double bonds, satisfying 4n+2=10 with n=2) is aromatic; pyridine (a six-membered ring with one nitrogen in place of a CH, still 6 pi electrons in the ring system, with nitrogen's lone pair sitting in a separate, non-participating sp2 orbital rather than contributing to the count) is likewise aromatic; but cycloheptatriene, despite technically having 6 pi electrons from its three double bonds (a number that WOULD satisfy Huckel's rule), is non-aromat …