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16.29 a) Indole can be viewed as resulting from the fusion of a benzene ring and a heterocyclic pyrrole ring, so its heterocyclic ring is aromatic. b) The fused heterocyclic ring of benzimidazole is imidazole (see solution 16.8) which is aromatic. c) The fused heterocyclic ring of quinoline is pyridine, which is aromatic. 16.30 Adenine can be viewed as resulting from the N fusion of two aromatic rings, so it is aromatic. N + N N H 16.31 Both rings are aromatic in resonance structure B. Because of the importance of this resonance structure, electrons are transferred from the seven-membered ring to the five-membered ring creating a large dipole moment in the molecule. + A B 16.32 With six TT electrons, the cation formed in this reaction H fulfills the criteria for aromaticity. All the hydrogens on the cation are on the periphery the ring and are equivalent, so + they appear as a singlet. The signal is shifted downfield from the normal aromatic position because the lower electron density of the carbocation results in additional deshielding. 16.33 The dication species produced in this reaction is aromatic. The carbons of the methyl groups on the ring are all magnetically equivalent as are the four ring carbons. The + ring carbons are very far downfield because they are part of + an aromatic ring. They are deshielded even more due to the low electron density in the ring. 244