Heterobenzenes and Other Heterocyclic Aromatic Compounds

9a Contrasting Structures of Pyridine and the Five-Membered Rings Pyrrole, Furan, and Thiophene If a CH unit of benzene is replaced with a nitrogen, the result is pyridine, another aromatic six-membered ring (Fig. 13.46). From the provincial view of organic chemistry all atoms except 

The electron counting for pyridine can he confusing at first because of the extra pair of electrons on nitrogen. Doesn t pyridine have eight n electrons and thus violate the 4n + 2 rule Put a better way, we have to worry whether antibonding orbitals are occupied in pyridine. A good orbital drawing shows that the extra two electrons are not in the n system, and pyridine is not in violation of the Hiickel rule (Fig. 13.47). 

The bonds made from spVsp overlap are not in the ic system 

FIGURE 13.47 Pyridine has only 6 n electrons. The lone-pair electrons on nitrogen are in an orbital perpendicular to the n system, not in the n system of the ring. 

FIGURE 13.48 Oxabenzene must be charged and is not very stable. 

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