It is possible to place electrophiles in position 4 of the pyridine through the formation of N-oxides.
I detail the three stages of the reaction:
1. Formation of pyridine N-oxide
![electrophilic substitution position 4 pyridine 02](/images/stories/heterociclos/piridina/sustitucion-electrofila/sustitucion-electrofila-posicion-4-piridina-02.png)
2. Electrophilic substitution at position 4
![electrophilic substitution position 4 pyridine 03](/images/stories/heterociclos/piridina/sustitucion-electrofila/sustitucion-electrofila-posicion-4-piridina-03.png)
3. Reduction of pyridine N-oxide
![electrophilic substitution position 4 pyridine 04](/images/stories/heterociclos/piridina/sustitucion-electrofila/sustitucion-electrofila-posicion-4-piridina-04.png)
The release of the oxygen couple makes the reaction work under milder conditions, not requiring such drastic conditions as in pyridine. The electrophile can also enter position 2, although in a minority due to steric hindrance.