Question
Why electron withdrawing groups such as $- NO_2$ show their effect only at $O$ and $P -$ position and not at $m -$ position.
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Answer
$\rightarrow$ As shown, 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.
$\rightarrow$ The carbanion thus, formed is stabilised through resonance.
$\rightarrow$ The negative charge appeared at ortho $-$ and para positions with respect to the halogen substituent is stabilised by $-NO_2$ group while in case of meta$-$nitrobenzene, none of the resonating structures bear the negative charge on carbon atom bearing the $-NO_2$ group.
$\rightarrow$ Therefore, the presence of nitro group at meta $-$ position does not stabilise the negative charge and no effect on reactivity is observed by the presence of $-NO_2$ group at meta$-$position.
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