SolutionsAromatic Nitration
1. Can benzene be aminated?
Can an NH2 group be introduced directly into an aromatic ring by SEAr? Can you think of an indirect method?
No: ammonia is a nucleophile, not an electrophile, and the benzene ring is also electron rich. The indirect route is nitration followed by reduction of the nitro group (with H2/Pd, Fe/HCl or Sn/HCl).
2. The nitronium ion
Write the reaction that forms the nitronium ion. What role does nitric acid play in it?
HNO3 + 2 H2SO4 → NO2+ + H3O+ + 2 HSO4−. Towards the stronger sulfuric acid, nitric acid acts as a base: it is protonated on an OH and loses a molecule of water.
3. Dinitration
Why does the nitration of benzene easily stop at nitrobenzene?
The NO2 group withdraws electron density from the ring and deactivates it: the second nitration is about 107 times slower and requires much harsher conditions; when it occurs it gives m-dinitrobenzene.