The chloronium ion

This ion is formed by reaction between aluminum chloride and chlorine: $AlCl_3+Cl_2\rightarrow AlCl_4^-+Cl^+$

The alkylium ions

These ions are formed by reaction between aluminum chloride and haloalkanes (often rearranging to form tertiary cations): $AlCl_3+RCl\rightarrow AlCl_4^-+R^+$

The acylium ions

These ions are formed by reaction between aluminum chloride and acid chlorides.

The nitronium ion

This ion is formed by reaction in the sulfonitric mixture : $H_2SO_4+HNO_3\rightarrow HSO_4^-+NO_2^++H_2O$

Sulfur trioxide

It is present in fuming sulfuric acid.

The benzenediazonium ion

It is formed by the action of nitrous acid on aniline in an acid medium.

Progress of the electrophilic substitution reaction on benzene or its derivatives

The electrophilic attack is followed by a nucleofugic departure of the $ H^+$ ion from the attack position and a reconstitution of the mesomerism in the benzene ring. The $H^+$ ion may be uptaken, depending on the case, by aluminum chloride or hydrogen sulfate ion or the sulfonate group attached to benzene