18.16 : メタ指向性不活性化剤: -NO_2、-CN、-CHO、-⁠CO_2R、-COR、-CO_2H

The meta directors are also known as deactivating groups because they decrease the reactivity of an aromatic ring towards electrophilic substitution.

For example, nitration of nitrobenzene occurs more slowly than benzene. Therefore, nitro is a deactivating group.

Deactivation by meta directors can be understood by the formation of a resonance-stabilized carbocation, which is the rate-determining step in electrophilic substitution.

Meta directors destabilize the carbocation intermediates by withdrawing electrons from the ring. This results in increased transition state energy. The activation energy barrier is high, making the reaction slower than the corresponding reaction of benzene.

Consequently, meta directors deactivate the ring at all positions. However, the deactivation is stronger at the ortho and para positions than at the meta position, thereby forming the meta-substituted product as the major product.

Overall, electron-withdrawing groups deactivate the ring and are meta directors.