Question

Why does $N{{O}_{2}}$ group show its effect only at ortho?

Answer 1

Nitrogen dioxide ( $N{{O}_{2}}$ ) group does not only show its effect at ortho position, but it also shows its effect at para position. Various reasons can be considered for justifying the effect of $N{{O}_{2}}$ group at the positions ortho and para in a benzene ring.

Complete answer: When nitrogen dioxide ( $N{{O}_{2}}$ ) group attacks on ortho and para position of a benzene ring, the electron density of the benzene ring is withdrawn towards the Nitrogen dioxide ( $N{{O}_{2}}$ ) group.
This thing helps the attack of the nucleophile on the haloarene, as the Nitrogen dioxide ( $N{{O}_{2}}$ ) group is present at ortho or para position. The carbocation formed during the reaction when $N{{O}_{2}}$ is present at para or ortho position is more stable than the carbocation formed during the reaction when $N{{O}_{2}}$ is present at meta position.
The reason for the carbocation to be more stable when $N{{O}_{2}}$ group is at ortho and para positions is that there will resonance in the structure, which cannot happen when $N{{O}_{2}}$ group is present at meta position.
$N{{O}_{2}}$ group is an electrophilic group, thus it craves for electrons. In a nitrobenzene ring, resonance shows that ortho and para positions are positive and meta position is negative, thus if another electrophile attacks the nitrobenzene ring, it will be attached at meta position.
And, if a nucleophile attacks the nitrobenzene ring, it will be attached at either ortho or para position.

Note:
Each functional group shows its full impact on ortho and para positions as it is in the plane of the ring, meta position is not in the plane of the ring. As seen above that $N{{O}_{2}}$ group makes ortho and para positively charged and meta as negatively charge, we can say that during an attack of an electrophile, $N{{O}_{2}}$ group is a meta director.