18.20: 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.
