Question

Among the following amines, which one has the highest $p{K_b}$ value in aqueous solution?
A.Methanamine
B.N, N-dimethylaniline
C.Ethanamine
D.Benzenamine
E.None of the above

Answer 1

Electron density increases on the nitrogen atom due to +Inductive effect and therefore makes it more basic whereas +R effect involves the lone pair of Nitrogen in conjugation making it not available for further reaction and thereby reduces its basicity.

Complete step-by-step answer:
Amines are basic in nature as they react with acids to form salts. Amines have an unshared pair of electrons on nitrogen atoms and that is why they behave as Lewis base. We can understand this basic character of amines by $p{K_b}$ values.
$p{K_b} = - \log {K_b}$ just like we calculate pH ( $pH = - \log [{H^ + }]$ )
Larger the value of ${K_b}$ or smaller the value of $p{K_b}$ , stronger is the base. Aliphatic amines are stronger bases than ammonia ( $p{K_b}$ = 4.75) due to +I effect of alkyl groups resulting in high electron density on the nitrogen atom. $p{K_b}$ values lie in the range of 3 to 4.22.
On the contrary, aromatic amines are weaker bases than ammonia due to the electron withdrawing nature of the aryl group. Basicity of amines is related to their structure as it depends upon the ease of formation of the cation by accepting a proton from the acid. The more stable the cation is relative to the amine, more basic is the amine. So, greater the number of resonating structures, greater is the stability. It is observed that electron releasing groups like ether, methyl, etc increase the basic strength whereas electron withdrawing groups like nitrate, carboxylic acid, halogens decrease it.

The lone pair on nitrogen is localised in methanamine ($C{H_3} - N{H_2}$ ), ethanamine ( $C{H_3} - C{H_2} - N{H_2}$ ) and N, N-dimethylaniline ( ${({C_2}{H_5})_2} - N - {C_2}{H_5}$ ) resulting in their lower $p{K_b}$ value and higher basicity. Due to unavailability of lone pair on nitrogen in benzenamine (aniline ${C_6}{H_5} - N{H_2}$ ) as it takes part in resonance, it has less basicity as the lone pair is delocalised. Therefore, it has the highest $p{K_b}$ due to lowest basicity.

Hence, the correct option is (D).

Note: In aqueous phase, if R is a methyl group then the order of basicity is secondary amine> primary amine > tertiary amine but if R is any group other than methyl group, then order is secondary > tertiary > primary.