Electrophilic aromatic substitution of heteroaromatic compounds

Although heteroaromatic compounds generally have lower resonance energies than benzene (Table 14.1, p. 629), they also undergo electrophilic aromatic substitution rather than addition. 

8a Reactions of Pyridine and Pyrrole The heteroatom introduces a complication in aromatic substitution not present in benzene. Benzene has six equivalent positions, but substitution could occur at two positions in pyrrole and three in pyridine (Fig. 14.59). 

FIGURE 14.59 Possible positions for electrophilic aromatic substimtion in pyridine and pyrrole. 

FIGURE 14.60 In pyridine, substitution at the 3-position is favored, although the reaction is slower than with benzene. 

This rate-diminishing effect is accentuated if the nitrogen is protonated, as it very often is under the strongly acid conditions necessary to carry out substitution on pyridine. Therefore it is not surprising that substitution of pyridine takes place more slowly than analogous reactions invohdng benzene. Pyridine itself substitutes about as rapidly as nitrobenzene, 10 times more slowly than benzene. The pyri-dinium ion substitutes an impressive 10 times more slowly than benzene. 

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