Question

Choose the correct order of basic strength of given anilines.

• 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

The correct option is B m-nitroaniline > p-nitroaniline > 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