Question

What is the active electrophile in the nitration of toluene experiment?

Answer 1

Toluene is treated with concentrated sulphuric acid solution and concentrated nitric acid at a temperature less than ${50^ \circ }C$ . Here, the electrophile formed by the sulphuric and nitric acid reaction is the nitryl cation or nitronium ion $NO_2^ +$ .

Complete answer:
The mixture of sulphuric acid and concentrated nitric acid forms the nitronium ion which is the active species. The primary electrophile in aromatic nitration is the nitronium ion. A weak base such as water or the $HSO_4^ -$ ion left after protonation of nitric acid, removes a proton, thus splitting the $C - H$ and reforming the $C - C$ pi from the carbon atom bearing the nitro group. Some nitrating agents are: Concentrated, fuming and aqueous nitric acid, nitrogen tetroxide, nitrogen pentoxide and mixtures of nitric acid with the sulphuric acid, acetic anhydride, acetic acid, chloroform and phosphoric acid. The nitration reaction is highly exothermic. The nitration reaction should be controlled by the systematic cooling designed to withdraw the evolved energy. Nitration reactions are used in the production of explosives such as the conversion of guanidine to nitroguanidine and converting toluene to trinitrotoluene.
Therefore, the active electrophile in the nitration of toluene experiment is the nitronium ion.

Note:
Nitration can add nitrogen atoms to a benzene ring which can be used in substitution reactions further. The nitro group acts as a ring deactivator. Having a nitrogen atom in a ring is very useful because it can be used as a directing group and a masked amino group.