Question

Arrange the following amines in decreasing order of their basic nature:
i.Aniline, propan-1-amine, and N-methyl ethanamine
ii.Benzene-1,4-diamine, ammonia, and 4-aminobenzoic acid.
iii.N-methylaniline, phenylmethylamine, and N-phenylaniline.

Answer 1

According to the Lewis’s acid-base theory, a base is defined as a chemical compound that accepts lone pairs from another chemical compound. Among the different bases containing nitrogen, those bases that can donate the lone pair easily will be termed as more basic.

Complete answer:
i.Among the following amines, the order of basicity will be dependent upon the donating ability of the bases.
Aniline, propan-1-amine, and N-methyl ethanamine – The basicity of the amine is favoured by resonance and inductive effect. The $\text{s}{{\text{p}}^{\text{2}}}$ carbon atoms in the benzene structure of aniline are more electronegative than the $\text{s}{{\text{p}}^{3}}$ carbon atoms which makes it the least basic among the three while the N-methyl ethanamine and propane-1-amine are stabilized by the inductive effect. Hence the order is:
$N-methyl ethanamine> propane-1-amine >Aniline$.

ii.Benzene-1,4-diamine, ammonia, and 4-aminobenzoic acid – the carboxyl group present in 4-aminobenzoic acid is highly electron-withdrawing and hence it is the least basic in the three. Benzene-1,4-diamine is less basic than ammonia due to the presence of the aromatic benzene ring which is electron-withdrawing in nature. The order is:
$Ammonia>Benzene-1, 4-diamine>4-aminobenzoic acid$.

iii.N-methylaniline, phenylmethylamine, and N-phenylaniline – The order of basicity for these three compounds will be:
$N-methylaniline= phenylmethylamine> N-phenylaniline$
This is because the first and the second compounds are the same and the third compound has an aromatic electron-withdrawing phenyl group attached to the nitrogen atom of the aniline group which makes it less prone to donate the electron pair.

Note:
The basicity of the organic bases is dependent upon the electron-donating or the electron-withdrawing groups that are attached to the basic atoms, which is mainly nitrogen. The electron-donating groups are the methyl, ethyl, and other alkyl groups while the electron-withdrawing ones are the halogen group, carboxylic groups, aromatic rings, and others.