Pyridine is a "π-deficient" heterocyclic ring, and its electron cloud density is lower than benzene, so its electrophilic substitution reaction activity is also lower than benzene, and it is comparable to nitrobenzene. Due to the passivation of nitrogen atoms on the ring, the conditions of electrophilic substitution reaction are harsh and the yield is low, and the substituents mainly enter the 3( ) position.
Compared with benzene, the cyclic electrophilic substitution of pyridine becomes more difficult, and the substituents mainly enter the 3( ) position, which can be explained by the relative stability of the intermediate.
The electrophilic substitution of pyridine is more difficult than that of benzene because of the presence of electrically absorbing nitrogen atoms and the positive ions of the intermediates are not as stable as the corresponding intermediates substituted by benzene. Comparing the position of the electrophilic reagent attack, it can be seen that when attacking the 2( ) and 4( ) positions, the formed intermediate has a resonance limit formula that is positively charged on the electronegative nitrogen atom, which is extremely unstable, and the 3( ) substituted intermediate does not have this extremely unstable limit formula, and its intermediate is more stable than the intermediate attacking the 2 and 4 positions. Therefore, the substitution products of 3 positions are easily generated.