(A) In the direct nitration of aniline significant amount of m-nitro aniline is formed.
(R) In Anilinium ion NH₃⁺group shows only negative inductive effect but not positive mesomeric effect.

The nitration of aniline occurs in an acidic medium where nitric acid and sulfuric acid are typically used as reagents. The process involves the introduction of a nitro group (-NO₂) into the aromatic ring of aniline. However, under strongly acidic conditions, the amine group (-NH₂) of aniline gets protonated, forming an anilinium ion (-NH₃⁺).

This protonation significantly alters the electron-donating nature of the amine group. The anilinium ion exhibits a strong negative inductive effect and lacks the positive mesomeric (resonance) effect typically shown by the unprotonated -NH₂ group. As a result, the electron density on the ortho and para positions of the benzene ring decreases, making these positions less reactive toward electrophilic substitution. Consequently, the meta position becomes relatively more reactive, directing the nitro group to the meta position.

Hence, during the nitration of aniline, a significant amount of m-nitroaniline is formed. This meta-directing effect of the anilinium ion explains why the reaction does not primarily yield ortho or para products, as would be expected if the -NH₂ group remained unprotonated.

Key Points about the Nitration of Aniline:

• The reaction takes place in a strongly acidic medium, leading to the formation of the anilinium ion (-NH₃⁺).
• The anilinium ion exhibits a negative inductive effect and lacks a positive mesomeric effect, reducing electron density at the ortho and para positions.
• The nitro group (-NO₂) predominantly substitutes at the meta position, resulting in the formation of m-nitroaniline.

To summarize, the nitration of aniline in an acidic medium yields a substantial proportion of m-nitroaniline due to the meta-directing nature of the anilinium ion. Understanding this process highlights the influence of reaction conditions on the regioselectivity of electrophilic aromatic substitution reactions.