Question

Nitration of aniline in strong acidic medium gives significant amount of m-nitroaniline because

• A. In electrophilic substitution reaction, amino group is meta directing
• B. In strong acidic medium, aniline is present as anilinium ion
• C. -NH\(_2\) group always directs to meta position
• D. m-nitroaniline has higher molar mass than o\&p nitroanilines

Hint:

Always consider the effect of the reaction medium on the functional groups present. The directing influence of a group like -NH\(_2\) can be completely altered by pH. In acidic solution, it becomes -NH\(_3\)\(^+\) and its directing effect changes from o,p-directing to m-directing.

Answer 1

The Correct Option is B

Step 1: Understanding the Reaction Conditions:
The question describes the nitration of aniline, which is an electrophilic aromatic substitution. The reaction is carried out in a strong acidic medium (typically a mixture of concentrated nitric acid and concentrated sulfuric acid).
Step 2: Analyzing the Directing Influence of the Amino Group:
The amino group (-NH\(_2\)) is a powerful activating group due to the lone pair of electrons on the nitrogen, which can be delocalized into the benzene ring (+R effect). This increases the electron density at the ortho and para positions, making the -NH\(_2\) group an ortho, para-director. Therefore, options (A) and (C) are incorrect.
Step 3: Considering the Effect of the Strong Acidic Medium:
Aniline is a weak base due to the lone pair on the nitrogen atom. In a strongly acidic medium, the -NH\(_2\) group gets protonated to form the anilinium ion (-NH\(_3\)\(^+\)). \[ \text{C}_6\text{H}_5\text{-NH}_2 + \text{H}^+ \rightleftharpoons \text{C}_6\text{H}_5\text{-NH}_3^+ \] This protonation happens to a large extent.
Step 4: Analyzing the Directing Influence of the Anilinium Ion:
The anilinium ion (-NH\(_3\)\(^+\)) has a positive charge on the nitrogen atom adjacent to the ring. This group strongly withdraws electron density from the benzene ring through the inductive effect (-I effect). It is a strongly deactivating group. Deactivating groups direct incoming electrophiles to the meta position.
Because a significant portion of the aniline exists as the anilinium ion in the reaction mixture, electrophilic attack by the nitronium ion (NO\(_2\)\(^+\)) occurs at the meta position of the anilinium ion, leading to the formation of a substantial amount of m-nitroaniline (about 47%). The ortho and para products are also formed (about 2% and 51% respectively) from the small amount of unprotonated aniline that is still present.
Step 5: Evaluating the Options:
- (A) and (C) are incorrect as the -NH\(_2\) group is o,p-directing.
- (B) correctly states that aniline is present as the anilinium ion, which is the reason for meta-direction.
- (D) is incorrect; o, p, and m-nitroaniline are isomers and have the same molar mass.
Step 6: Final Answer:
The formation of a significant amount of m-nitroaniline is due to the presence of the meta-directing anilinium ion in the strong acidic medium.