An answer to a question
posted by aciel
The question was whether the nitro group, the amine group, or both would be protonated when you add sulfuric acid.
As you can see from the picture below, m-nitroaniline has two resonance contributors.
The electrons on the nitro group are shared between the two oxygens, making it kind of difficult for the proton to find the electrons. Also, the nitrogen maintains its plus charge, which is balanced by the negative charge on one of the oxygens. If you were to protonate one of the oxygens, the nitrogen would have a plus charge that is not offset by the oxygens.
The amine has an unshared pair of electrons that are concentrated on the nitrogen. Because the electrons are not delocalized, it is much easier for nitrogen to share its electrons than for the nitro group to share its electrons.
Sulfuric acid is one of our strong acids, so the first proton falls off pretty easily. It's much harder to pull off that second proton, and a nitro group that has resonance is certainly not strong enough to do it.
The salt, then, would be NH3+ and HSO4-, which is soluble in your aqueous layer. You actually won't have a precipitate until you add a base to remove the proton from the salt and neutralize the solution. m-Nitroaniline is not soluble in water, so once you take the proton off and return it to its original state, it would precipitate out of solution.
I hope that helps!