Aromaticity and Huckels Rule

Hiickel s rule (1931) for planar species states that if the number of ir electrons is equal to 2 + 4n, where n equals zero or a whole number, the species is aromatic. The rule was first applied to carbon-containing monocyclics in which each C is capable of being 5p -hybridized to provide a p orbital for extended v bonding it has been extended to unsaturated heterocyclic compounds and fused-ring compounds. Note that benzene corresponds to n = 1. 

Problem 10.6 Account for aromaticity observed in (o) 1,3-cyclopentadienyl anion but not 1,3-cyclopen-tadiene (b) 1,3,5-cycloheptatrienyl cation but not 1,3,5-cycloheptatriene (c) cyclopropenyl cation (d) the heterocycles pyrrole, furan and pyridine. 

Generation of a carbocation by ionization permits cyclic overlap of p orbitals on each C. With six w electrons, the cation is aromatic ( = 1). 

The ions in parts (a), (b), and (c) are reactive but they are much more stable than the corresponding open-chain ions. 

Pyridine (6 w electrons the unshared pair of electrons on N does not participate in the ir overlap) 

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