Benzene is a nucleophile due to its π electrons, but its reactions are slow relative to those of alkenes with the same electrophile (Br2 for example). Thus reagents for the aromatic electrophilic substitutions shown below are a combination of the source of the electrophile (E+) and a catalyst which activates the reagent so that it gives rise to the electrophile. The new bond formed is between the electrophile and the benzene ring.
| forming a benzene-halogen bond | ||||
 | Br2  FeBr3 |  | Cl2 FeCl3 |  |
| forming a benzene-nitrogen bond | ||||
| | conc HNO3 conc H2SO4 |  | ||