Explain why the electron withdrawing groups such as `-NO_(2)` show its effect only at o - position and p - position and not at m - positions.

Explanation

As shown in figure, the presence `- NO_(2)` electron withdrawing group at ortho - and para - positions withdraws the electron density from the benzene ring and thus facilitates the attack of the nucleophile on haloarene. The carbanion thus formed is stabilised through resonance.
At ortho - and para - positions with respect to the halogen substituent, the negative charge appeared is stabilised by `- NO_(2)`, whereas in case of meta - nitrobenzene, none of the resonating structures bear the negative charge on carbon bearing the `- NO_(2)` group. Therefore, the presence of nitro group at metaposition does not stabilise the negative charge and no effect on reactivity is observed by the presence of `-NO_(2)` group at meta - position.