Question

Why does NO2 group show its effect only at ortho and para positions and not at meta
positions?

Answer 1

The presence of NO2 group at ortho and para positions with draws the electron
density form the benzene ring and thus facilitates the attact of the nucleophile on
halo arene. The carbanaion thus formed is stabilized through resonance. The
negative charge appeared at ortho and para positions with respect to the halogen
substituent is stabilized by NO2 group while in case of meta nitro benzene, none of
the resonating structures bear the negative charge on carbon atom bearing the NO2
group. There fore the presence of nitro group at meta position does not stabilize the negative charge and no effect on reactivity isobserved by the presence of NO2 group
at meta position.

Answer 2

negative charge and no effect on reactivity isobserved by the presence of NO2 group

at meta position.