Substituents already attached to benzene exert an influence on additional atoms or groups attempting to bond to the benzene ring via electrophilic aromatic substitution reactions. An atom or group already attached to a benzene ring may direct an incoming electrophile to either the ortho‐para positions or the meta position. Atoms or groups that make the benzene molecule more reactive by increasing the ring's electron density are called activating groups. Activating groups serve as ortho‐para directors when they are attached to a benzene ring, meaning that they direct an incoming electrophile to the ortho or para positions. An atom or group that makes the benzene molecule less reactive by removing electron density from the ring acts as a deactivating group. Deactivating groups direct incoming electrophiles to the meta position. You can further classify activating and deactivating groups or atoms as strong, moderate, or weak in their directing influence. This table lists some typical activating and deactivating groups by the order of their strength.
Halogen atom influence
Halogen atoms show both activating and deactivating characteristics. Because they have three pairs of unshared electrons, halogen atoms can supply electrons toward the ring. However, because of their high electronegativities, halogen atoms also tend to remove electrons from the benzene ring. These conflicting properties make halogens a weak ortho‐para director and also a ring deactivator. This means that the presence of a halogen atom on a benzene causes an incoming electrophile to attach at an ortho or para position. However, these positions are not very electron rich, so the reaction proceeds poorly under ordinary electrophilic aromatic substitution conditions, leading to poor yields of the disubstituted product.
Predicting second group position
Using the previous information in this section, predicting the position that a second substituent group will take on a benzene ring is easy. For example, the nitration of methyl benzene (toluene) will produce ortho and para nitrotoluene as the main product because the methyl group is an ortho‐para director.
However, methylating nitrobenzene leads to the formation of meta nitrotoluene because the nitro group is meta directed.