18.3 Activating and Deactivating Groups | Ortho/Para vs Meta Directors
|π donors||σ donors||Halogens||Carbonyls||Other|
Activating and Deactivating Groups
Activating groups are substituents that increase the rate of a reaction (by lowering the activation energy).
Deactivating groups are substituents that decrease the rate of a reaction (by increasing the activation energy).
For EAS Reactions
Electron-Donating Groups = Activating Groups
Electron-Withdrawing Groups = Deactivating Groups
In Electrophilic Aromatic Substitution reactions (EAS reactions) benzene acts as a nucleophile (electron-pair donor). Electron-donating groups increase the reactivity of nucleophiles. Therefore, in EAS reactions electron-donating groups are referred to as Activating Groups, and a benzene with one of the electron-donating groups shown in the table above will perform an EAS reaction faster than benzene alone.
In contrast, electron-withdrawing groups decrease the reactivity of nucleophiles. Therefore, in EAS reactions electron-withdrawing groups are referred to as Deactivating Groups, and a benzene with one of the electron-withdrawing groups shown in the table above will perform an EAS reaction slower than benzene alone.
Ortho / Para Directors and Meta Directors
The table above allows us to summarize the following regarding Ortho/Para directors and Meta directors:
-ALL Electron-Donating Groups are Ortho/Para Directors.
-MOST Electron-Withdrawing Groups are Meta Directors.
-The halogens are the only Electron-Withdrawing Groups that are Ortho/Para Directors.
Electron-Donating Groups lower the activation energy for all positions on the benzene ring, but it lowers it more for the ortho and para positions than the meta position (see image below for EAS involving aniline). Ortho and para substitution is significantly faster and the majority of the products will be ortho and para substituted. (For large substituents the para tends to be favored due to steric effects.)
Electron-withdrawing groups increase the activation energy for all positions on the benzene ring, but it increases it more for the ortho and para positions than for the meta position (see image below for EAS involving nitrobenzene). Meta substitution is significantly faster (though still slower than benzene alone) and the majority of the products will be meta substituted.