Explain why the aromatic amine are less basic than ammonia and aliphatic amines .

Aromatic amines are less basic than ammonia and aliphatic amines due to following reasons:

 (i) Due of the presence of resonance in aromatic amines

For example, aniline may be regarded as resonance hybrid of the following structures:

Basicity is the tendency to gain a proton or to donate a pair of electrons.

As a result of resonance, the lone pair of electrons on the nitrogen atom gets delocalised over the benzene ring and thus is less easily available for protonation or donation making it a weak base.

(ii) Due to lower stability of anilinium ion than aniline: When aniline gains a proton then it forms anilinium ion. Aniline is resonance hybrid of five structures while anilinium ion is a resonance hybrid of only two structures. Therefore, aniline is more stable than anilinium ion. Hence aniline has very less tendency to accept a proton to form anilinium ion.

 On the other hand, in case of ammonia and aliphatic amines, delocalisation of lone pair of electrons on the nitrogen is not possible. Furthermore, in aliphatic amines, the electron density on the nitrogen atom is increased by electron donating inductive effect. Therefore, lone pair on nitrogen atom is easily available for donation or protonation. As a result, aromatic  amines are weaker bases than aliphatic amines and ammonia.