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18.16 : meta-Directing Deactivators: –NO2, –CN, –CHO, –⁠CO2R, –COR, –CO2H

All meta-directing substituents are deactivating groups. These substituents withdraw electrons from the aromatic ring, making the ring less reactive toward electrophilic substitution. For example, the nitration of nitrobenzene is 100,000 times slower than that of benzenebecause of thedeactivating effect of the nitro group. The first step in an electrophilic aromatic substitution is the addition of an electrophile to form a resonance-stabilized carbocation. The energy diagrams for the nitration of benzene and nitrobenzeneshowthat the electron-withdrawing nitro group destabilizes the carbocation intermediate and increases the transition state energy. The higher activation energy barrier for nitrobenzene, compared to that for benzene,makes the reactionslower. Whilemeta-directing groups deactivatethe ring at all positions,deactivation at the ortho and para positions is stronger, resulting inmeta-substituted products.

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Meta directingDeactivatingNitro GroupElectrophilic Aromatic SubstitutionCarbocationActivation EnergyMeta substitution

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