18.16:
meta-Directing Deactivators: –NO2, –CN, –CHO, –⁠CO2R, –COR, –CO2H
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.
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 benzene because of the deactivating 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 nitrobenzene show that 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 reaction slower. While meta-directing groups deactivate the ring at all positions, deactivation at the ortho and para positions is stronger, resulting in meta-substituted products.