Huckels Rule and Charged Molecules

In Summary Cyclic conjugated polyenes are aromatic if their tt electron count is 4n + 2. This number corresponds to a completely filled set of bonding molecular orbitals. Conversely, 4n TT systems have open-shell, antiaromatic structures that are unstable, are reactive, and lack aromatic ring-current effects in NMR. Finally, when steric constraints impose nonplanarity, cyclic polyenes behave as nonaromatic alkenes. 

Hiickel s rule also applies to charged molecules, as long as cyclic delocalization can occur. Their aromaticity is reflected in relative thermodynamic and kinetic stability, the observation of ring currents in the NMR experiment, and the absence of bond alternation in crystal structures. This section shows how charged aromatic systems can be prepared. 

Cyclopentadienyl anion is aromatic six tt electrons cyclically delocalized over five carbon atoms. 

In contrast, the cyclopentadienyl cation, a system of four tt electrons, can be produced only at low temperature and is extremely reactive. 

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