Benzene ring heteroatoms introduced into

The replacement of two CH groups in benzene by a neutral NR, O or S introduces into the new ring an electron-donating heteroatom. This electron-donor character is accentuated in the pyrrole anion where N is introduced. Thus the five-membered rings with one heteroatom are electron rich (rr-excessive), and the chemistry of pyrrole, furan and thiophene is dominated by this effect and is again considered together as a whole in Part 3. 

Thiepin, as a seven-membered conjugated system with sulfur as heteroatom, is a member of the 8 7t-electron heteroannulenes which are antiaroinatic according to Hiickel s rule. In contrast to oxepin, thiepin is not stable at room temperature and no valence isomerism with an arene sulfide has been observed. Stable thiepins are obtained only when two bulky substituents, e.g. /ert-butyl, are introduced into positions 2 and 7. In benzothiepins the annellation effect of the aromatic rings contributes decisively to the stability of these compounds stability increases with an increasing number of fused benzene rings. 

The replacement of one CH group in benzene by —N = to give pyridine introduces an electron-withdrawing heteroatom into the ring. The electron-withdrawing effect is accentuated when a CH in benzene is replaced by a positively charged atom (NR+, 0+ or S+). Thus, the six-membered... 

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