If electrophilic aromatic substitution of a monosubstituted benzene is slower than that of benzene under identical conditions, the substituent in the monosubstituted benzene is called a deactivating group.
Under identical conditions, Reaction 2 is slower than Reaction 1. Thus, the nitro group is a deactivating group.
All deactivating groups are electron-withdrawing groups.
Common deactivating groups: