Generation of electrophile

Step I      Formation of σ complex

Step II     Re-aromatisation

Brief description

• Interaction of sulphuric and nitric acids results in the electrophile nitronium ion NO₂⁺.
• The Pi-electrons of benzene ring attack the electrophile, forming a σ complex.
• Deprotonation of the σ complex, by the base yields the product.
• The σ complex is also known as wheland intermediate or arenium ion. (Advanced organic Chemistry 4th edn page 502, Jerry March )

• It is a two step process. (generation of electrophile is not a step of the mechanism. WHY?)
• The first step is slow and reversible. It is slow because bond breaking is involved; hence there is an activation energy barrier.
• A stable aromatic compound is becoming a fairly unstable product. It is reversible because the products in the step can easily recombine to go back to the reactants (very little activation energy barrier)
• The second is the fast step (formation of a highly stable product from an unstable reactant, rearomatisation)
• The base HSO₄^- deprotonates the complex in the second step.
• H₂SO₄ donates a proton to HNO₃ hence it is an acid and HNO₃ is a base in comparison in this specific reaction.