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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. |
Answer» Solution : 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. |
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