SolutionsHeteroaromatic Compounds

The more stable the cation formed on protonating the base, the more the equilibrium is shifted to the right, the more hydroxide is produced and the stronger the base.

The positive charge of the conjugate acid of pyridine, the pyridinium cation, can be delocalized over the ring, as the resonance forms show.

In the case of imidazole there are fewer resonance forms, but two of them are completely equivalent: the positive charge is shared equally by the two nitrogens.

The equivalence of resonance forms is a sign of exceptional stability of the resonating species. That is why the imidazolium cation is more stable than pyridinium and imidazole is more basic than pyridine. The acidity of carboxylic acids is explained in the same way: their conjugate bases, the carboxylate anions, are described by two equivalent resonance forms.

• Furan, more reactive than benzene, is nitrated under mild conditions (acetyl nitrate, formed from HNO₃ and Ac₂O) at position 2.
• In pyrrole, furan and thiophene, attack at position 2 gives an intermediate with three resonance forms and at position 3 only two: position 2 is preferred.
• Pyridine reacts with difficulty, and at position 3: attack at 2 or 4 places the positive charge on the nitrogen (forms in red), which is very unfavorable.
• 4-Phenylpyridine is nitrated on the benzene ring, which is richer in electrons, at the para position.
• 3-Phenylpyrrole is alkylated on the pyrrole ring, much more reactive than benzene, at position 2.

Pyrrole > thiophene > benzene > pyridine. Pyrrole and thiophene release electron density to the ring through the heteroatom lone pair (nitrogen does it better than sulfur); pyridine, with an electronegative nitrogen that does not contribute its lone pair, is poorer in electrons than benzene.