Question

Aniline on direct nitration yields

• A. 51%-ortho, 47%-para, 2%-meta derivatives
• B. 51%-meta, 47%-ortho, 2%-para derivatives
• C. 51%-para, 47%-meta, 2%-ortho derivatives
• D. 51%-ortho, 47%-meta, 2%-para derivatives

Hint:

To get purely para-nitroaniline, protect the \( -NH_2 \) group by acetylation (\( -NHCOCH_3 \)) first, then nitrate and hydrolyze.

Answer 1

The Correct Option is C

Step 1: Conceptual Understanding:
Aniline is generally ortho-para directing due to its \( +M \) (electron-donating) effect from the amino group (-NH\(_2\)). This effect allows the aniline ring to easily undergo substitution reactions at the ortho and para positions. However, in the case of nitration, which is typically carried out in a strongly acidic medium containing a mixture of nitric acid (\( HNO_3 \)) and sulfuric acid (\( H_2SO_4 \)), the behavior of aniline changes.
Step 2: Detailed Explanation:
In the acidic nitrating mixture, aniline becomes protonated to form the anilinium ion (\( -NH_3^+ \)). The \( -NH_3^+ \) group is a strong electron-withdrawing group due to its \( -I \) (inductive electron-withdrawing) effect. This change in the electron density of the ring significantly alters the reactivity. The electron-withdrawing nature of the anilinium ion makes it meta-directing, meaning that electrophilic substitution will predominantly occur at the meta position of the aromatic ring.
As a result, when nitration is carried out under acidic conditions, a substantial amount (47%) of meta-nitroaniline is formed due to the meta-directing effect of the anilinium ion. However, the non-protonated form of aniline still reacts through its usual \( +M \) effect and undergoes para-substitution (51%) and a small amount of ortho-substitution (2%). This leads to a mixture of products: primarily para-nitroaniline, with a smaller portion of meta-nitroaniline and a trace of ortho-nitroaniline.
Step 3: Final Answer:
The final yield of products is 51% para-nitroaniline, 47% meta-nitroaniline, and 2% ortho-nitroaniline. The variation in these yields is due to the protonation of aniline in the acidic medium, which switches its directing effect from ortho-para to meta in the anilinium ion form.