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meta Directors

The meta directors are all strong deactivators and make the second SEAr very difficult. They hamper it because they drastically reduce the electron density of the ring they are attached to.

Electrostatic potential maps of benzene, benzoic acid and nitrobenzene: the substituted rings are poorer in electrons (blue)
Electrostatic potential maps, from left to right, of benzene, benzoic acid and nitrobenzene. Red indicates excess electron density and blue a deficit: the COOH and NO2 groups concentrate the negative charge on their oxygens and leave the ring poorer in electrons (bluer) than that of benzene.

The resonance forms of nitrobenzene and benzoic acid indicate that the ring must be very electron deficient, mainly at the ortho and para positions:

Resonance forms of nitrobenzene and benzoic acid: the positive charge appears at the ortho and para positions; rings deactivated towards SEAr

A meta director deactivates all ring positions, but especially the ortho and para ones, which are those with the lowest electron density. That's why it directs the (always difficult) second SEAr to the meta position.

Therefore, a strong deactivator directs the SEAr but does not favor it: strong deactivators heavily hamper the SEAr but, when it takes place, they direct it to the least disfavored position, the meta.

Please, never say that a deactivator favors meta SEAr. A deactivating group directs to meta while hindering the SEAr.

Bromination of nitrobenzene: the ortho and para benzenonium ions have a resonance form with the positive charge next to the nitro group, of negligible contribution; the meta ion does not
Energy diagram of the bromination of nitrobenzene: all its pathways lie above that of benzene, and the meta one is the least unfavorable