Question

Match the major products (P) of the nitration of aniline using $Conc.~HNO_3/H_2SO_4$:

(Aromatic compound with an isobutyl side chain) $\xrightarrow[KOH, Heat]{KMnO_4}$ ? A. o-nitroaniline
B. p-nitroaniline
C. m-nitroaniline
D. 2,4-dinitroaniline
Choose the correct answer from the options given below:

• A. A Only
• B. B and C Only
• C. A and B Only
• D. C and D Only

Hint:

Nitration of aniline is an "exception" question! Usually, amines are ortho/para directors, but because of the acidic "Nitrating Mixture," you get a massive amount of the meta product alongside the para product.

Answer 1

The Correct Option is B

Concept: The nitration of aniline is a unique case in organic chemistry. While $-NH_2$ is an ortho/para directing group, the strongly acidic conditions of the reaction change the behavior of the molecule.

Step 1: Analyze the direct effect of the $-NH_2$ group.
Normally, $-NH_2$ is highly activating and directs nitration to the ortho and para positions. However, in the presence of $Conc.~HNO_3/H_2SO_4$, aniline is protonated to form the anilinium ion ($-NH_3^+$).

Step 2: Effect of the Anilinium Ion.
The $-NH_3^+$ group is strongly deactivating and meta-directing. Because of this equilibrium, a significant amount of m-nitroaniline (C) is formed (approx. 47%).

Step 3: Identify major products.
Despite the meta-direction of the ion, the remaining free aniline still undergoes nitration at the para position due to its high reactivity, producing p-nitroaniline (B) (approx. 51%). Ortho-nitroaniline (A) is formed in negligible amounts (2%) due to steric hindrance and the protonation effect.