Reactions of Aromatic Compounds (overall)
Inductive and Mesomeric Effects of Selected Substituents
Aside from hydrogen, all substituents have a positive or negative inductive effect (+I or -I effect), as the inductive effect's sign is determined by the substituent's influence on the aromatic compounds reactivity in electrophilic aromatic substitutions compared to benzene. Hydrogen's inductive effect is therefore zero, because hydrogen is the substituent of benzene! However, even if every substituent displays an inductive effect, it need not necessarily have a mesomeric effect, too. A substituent's mesomeric effect may have the same sign as its inductive effect though the signs may also be opposite to each other. The nitro group, for instance, is characteristic of a -I and a -M effect, while chlorine (-) and the methoxy group (-) have a -I and a +M effect. The -I effect of chlorine exceeds its +M effect. Thus, chlorine is a deactivating substituent. In contrast, the methoxy group's +M effect is considerably stronger than its -I effect. The methoxy group is, therefore, an activating substituent.
In the following illustration, a selection of important substituents in electrophilic aromatic substitution is ordered according to the substituent's activating or deactivating qualities. The activating or deactivating influence of a substituent results from the sum of its inductive and - if extant - mesomeric effect.
The exact order of the substituents also depends on the applied electrophile. Due to strong electrostatic repulsion, the group, for instance, is particularly deactivating if the electrophile is a cation, as well.