Question

The correct order of basic strength of given aniline is:
(A) o-nitroaniline > p-nitroaniline > m-nitroaniline
(B) m-nitroaniline > p-nitroaniline > o-nitroaniline
(C) m-nitroaniline > o-nitroaniline > p-nitroaniline
(D) o-nitroaniline > m-nitroaniline > p-nitroaniline

Answer 1

Hint: In order to give the decreasing order of the basicity of the aniline, we must first know about the \[p{K_a}\]value and the inductive effect present in the given aniline groups such as o-nitroaniline, p-nitroaniline and m-nitroaniline.

Complete step by step solution:
The \[p{K_a}\] value of o-nitroaniline, p-nitroaniline and m-nitroaniline is -0.3, 1.0 and 2.5 respectively. First thing is that we have to see the group present in the Nitroaniline compound. The \[ - N{O_2}\] is the group that is present in the nitroaniline. \[ - N{O_2}\] group is the electron-withdrawing group. As the electron-withdrawing group is present, there will be $-I$ effect present. In the o-nitroaniline and p-nitroaniline compound, delocalization of the lone pair is observed. Therefore, both -I effect and -m effect is observed. But compared to p-nitroaniline, o-nitroaniline will be having a close inductive effect. Therefore, o-nitroaniline is less basic than p-nitroaniline compounds. In m-nitroaniline only inductive effect is observed and therefore, it is more basic compared to others.
Therefore, the decreasing order of basic strength of given aniline is:
m-nitroaniline > p-nitroaniline > o-nitroaniline

Hence, the correct answer is an option (B).

Note:
There are some important factors also which affects the basicity of amine, which are as follows-
(i) Any factor that makes a molecules’ conjugate base more stable will increase its acidity because they tend to stabilize negative charge. The more unstable an electron pair, the more will be the basicity. This is what Le-Chatelier’s principle states.

(ii) The conjugate base of an amine will always be a stronger base than amine itself. The basicity of the amine molecule increases with the increase in a negative charge. For example, in comparing amine, amide anion, and amide dianion, the amine having no negative charge is thus the least basic, and amide dianion bearing 2- charge is thus most basic.

(iii) Resonance stabilized amines are less basic, in other words, resonance stabilization decreases the basicity of amines. Amine lone pairs that are in conjugation with pi bonds and which can participate in resonance are stabilized to relative non-conjugated amines. Resonance decreases the basicity. This is the reason why aniline is less basic than cyclohexylamine.

(iv) Inductive effects decrease basicity, that is electron-withdrawing groups on amines decrease the basicity. The lower the charge density, the more stable the molecule is and lesser will be the basicity.

(v) When nitrogen acts as a pi-donor, the basicity is decreased. When nitrogen acts as a pi-acceptor, the basicity of nitrogen is increased. This is the reason why the basicity of amides is lower than amines.

(vi) Alkynes are usually more acidic than alkene and alkanes this is because basicity increases with decreasing s-character.

(vii) Pyridine is far more basic than pyrrole; this is because pyridine is an aromatic compound while pyrrole is a non-aromatic compound. So, aromaticity is also a factor that affects the basicity.