Why direct nitration of aniline yields small amounts of meta-nitroaniline but no ortho or para nitrnoaniline?
Answer:
Because of -NH2 group, resonace, nitrating agent is ortho & para oriented so we get less of meta nitro-aniline.
The nitro group in ring deactivating and a meta director. This is because it is very electron withdrawing. Draw the resonance structures and you will see what I mean. Hope this helps.
Oops. I misread the question. Kremar is correct. Sorry.
Becouse in a direct nitration, the highly acid media (from the nitric/sulfuric acid mixture used) forces the aniline to be almost completely protonated. A -NH3+ group is deactivating and meta-directing when it comes to aromatic electrophillic substitutions, thus yielding only small amounts of m-nitroaniline.
You can draw the resonance structures for the addition intermediary. You'll see that for ortho- and para- additions there's one resonance structure with a positive charge on C1, adjacent to the positive charge of the nitrogen. The meta- addition intermediary doesnt have that kind of resonance structures.
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