The presence of nitro 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. The negative charge appears at ortho- and para- positions with respect to the halogen substituent is stabilised by –NO2 group while in case of metanitrobenzene, none of the resonating structures bear the negative charge on carbon atom bearing the –NO2 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 –NO2 group at meta-position.
