(i) Aniline does not undergo Friedal-Crafts reaction because aniline being a Lewis base forms a complex with AlCl3 which is a Lewis acid. The amino group is not in a position to activate the benzene ring towards electrophilic substitution. Therefore the reaction is not possible.
(ii) Aromatic primary amines can not be prepared by Gabriel's phthalimide synthesis because haloarenes have to react with potassium phthalimide and they are little reactive. So the bond cleavage does not take place.
(iii) Aliphatic amines are stronger bases than ammonia because the alkyl group in aliphatic amines has +I effect. So the alkyl group tends to increase the electron density on the nitrogen atom whereas the electron releasing tendency of amines becomes more than that of ammonia.