Question

Which of the following compound is the most basic?

• A.
• B.
• C.
• D. All have equal basic strength.

Answer 1

The correct option is A

Option (a): m-nitroaniline
Option (b): p-nitroaniline
Option (c): o-nitroaniline
$-N{O}_2$ is an electron withdrawing group so it has -R and -I effect. This will decrease the electron density on N and makes it less basic. -R effect is absent at meta position, so m-nitroaniline has more basic strength than p-nitroaniline.
The conjugate acid of o-nitroaniline formed after abstraction of $H^{+}$ will experience steric hindrance due to presence of $-N{O}_2$ at ortho position. This is called Steric inhibition of protonation. This will make the conjugate acid less stable so it is less basic than others.
Therefore, order of basic strength is
m-nitroaniline > p-nitroaniline > o-nitroaniline
Option (a) is correct.