Aromaticity and Hiickels Rule

Hiickel s rule (1931) for planar species states that if the number of tt 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 -hybridized to provide a p orbital for extended it 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 (a) l,3-cyclopentadienyl anion but not l,3-cyclopentadiene (b) l, 3,5-cycloheptatrienyl cation but not 1,3,5-cycloheptatriene (c) cyclopropenyl cation (d) the heterocycles pyrrole, furan and pyridine. M 

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

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

Pyrrole (6 7T electrons 2 unshared electrons on N overlap in the 7t system) 

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