All the three classes of aliphatic amines, i.e.,
1∘,2∘ and
3∘ amines are stronger bases than ammonia, this is due to the reason that alkyl groups are electron donating groups. As a result, the electron density on the nitrogen atom increases and thus they can donate the lone pair of electrons more easily than ammonia. Thus, the basicity of amines should decrease in the order
3∘ amine
>2∘ amine
>1∘ amine
> ammonia On the other hand, aromatic amines are far less basic than ammonia. This is because due to resonance in aniline, the lone pair of electrons on the nitrogen atom gets delocaliged over the benzene ring and thus is less easily available for protonation.
But when the hydrogen atom of the amino group in aniline are replaced by electron donating alkyl groups, the basicity of resultant arylamines increases. e.g.,
N-methylaniline is a stronger base than aniline and N, N-dimethyl aniline is even stronger than N-methylaniline.
However, they are not stronger bases than ammonia. Thus, the basicity of N-substituted anilines relative to aniline follows the sequence