. Discuss Effect of Substituents on Reactivity & Orientation
When substituted benzenes undergo electrophilic attack, groups already on the ring affect both, the rate of the reaction and the site of attack. We say, therefore, that substituent groups affect both reactivity and orientation in electrophilic aromatic substitutions.
We can divide substituent groups into two classes according to their influence on the reactivity of the ring. Those that cause the ring to be more reactive than benzene itself are called activating groups. Those that cause the ring to be less reactive than benzene are called deactivating groups.
We also find that we can divide substituent groups into two classes according to the way they influence the orientation of attack by the incoming electrophile. Substituents in one class tend to bring about electrophilic substitution primarily at the positions ortho and para to themselves. We call these groups ortho - para directors because they tend to direct the incoming group into the ortho and para positions. Substituents in the second category tend to direct the incoming electrophile to the meta position. We call these groups meta directors.