Question

The strongest base among the following is:
A) ${C_6}{H_5}N{H_2}$
B) $p - N{H_2}{C_6}{H_4}N{H_2}$
C) $m - N{O_2}{C_6}{H_4}N{H_2}$
D) ${C_6}{H_5}C{H_2}N{H_2}$

Answer 1

We know Amines are basic because the nitrogen atom present in it possesses lone pairs. Due to the presence of the lone pair, the electron density around the nitrogen atom is very high. Thus, if an electron withdrawing group will be attached to amine the basicity will decrease and if an electron donating group will be attached to amine the basicity will increase.

Complete answer:
We know that if a group will donate electrons ( $+ I$ effect), it will increase the electron density and thus the basicity will also increase. Such groups are known as the electron donating group like alkyl groups $(C{H_3},{C_2}{H_5},etc)$ while the groups which will withdraw electrons( $- I$ effect) will decrease the electron density and thus will decrease the basicity of amines. Such groups are known as electron donating groups like $(N{O_2},etc)$.

Among the given options $p - N{H_2}{C_6}{H_4}N{H_2}$ and $m - N{O_2}{C_6}{H_4}N{H_2}$ are the least basic as in $p - N{H_2}{C_6}{H_4}N{H_2}$ the amine group is attached to the para position and in $m - N{O_2}{C_6}{H_4}N{H_2}$ an electron withdrawing group is present at the ortho position which will extract electrons and will lower the electron density and thus will decrease the basicity.

On the other hand we see that ${C_6}{H_5}C{H_2}N{H_2}$ is more basic than ${C_6}{H_5}N{H_2}$ as lone pair of electrons present on ${C_6}{H_5}N{H_2}$ are delocalized due to resonance as the amine group is directly attached to the benzene ring and as we know ${C_6}{H_5}C{H_2}N{H_2}$ is a primary amine so the amine group is not directly attached to the aromatic ring. Thus, it has the lone pair of electrons present on nitrogen available for protonation.

Also, we know that in ${C_6}{H_5}C{H_2}N{H_2}$ two electron donating groups are present so it will increase the electron density around nitrogen and hence will also increase its basicity. So, the most basic amine will be ${C_6}{H_5}C{H_2}N{H_2}$

Hence the correct answer is Option D.

Note: Aromatic amines are less basic than aliphatic amines and ammonia. Anything which is ortho to the amine, no matter whether it is an electron withdrawing group or an electron donating group will decrease the basicity of amines because of the ortho effect, which is basically the steric hindrance.