Basic Character of Amines

Amines are basic in nature due to the presence of a lone pair of electrons on nitrogen.

Basicities of amines can be compared with respect to ammonia, by comparing the availability of pair of electrons on nitrogen. Ammonia and amines, both when dissolved in water, attract a proton from water to form an ammonium or alkylammonium ion, respectively, and a hydroxide ion.

Aliphatic amines contain one or more alkyl groups in place of hydrogen atoms of ammonia. Since alkyl groups are electron releasing groups, they increase the electron density on nitrogen. This makes the lone pair of electrons on nitrogen atom to be easily available for sharing and hence, this increases the basicity of the amine. So, we expect that the basicities of the amines would increase as we move from primary to secondary to tertiary amines.

But the order of the basicities has been found to be:

Primary < Tertiary > Secondary

The tertiary amines are less basic than secondary amines. The reason is that a tertiary amine, though has three alkyl groups which can donate electrons to the nitrogen atom but
they also cause crowding (steric hinderance) around nitrogen. This hinders the protonation at nitrogen atom and hence, reduces the basicity.

The aromatic amines are weaker bases than ammonia because the aromatic ring is electron withdrawing. It reduces the electron density at nitrogen and makes the aromatic amines less basic. 

Aromatic amines < Ammonia < Aliphatic amines