Question

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

• A. In spite of substituents nitro group always goes to only m-position.
• B. In absence of substituents nitro group always goes to m-position.
• C. In electrophilic substitution reactions amino group is meta directive.
• D. In acidic (strong) medium aniline is present as anilinium ion.

Answer 1

The Correct Option is D

The nitration of aniline in a strong acidic medium leads to the formation of m-nitroaniline because, in such medium, aniline primarily exists as an anilinium ion.

1. Aniline (\( \mathrm{C_6H_5NH_2} \)) is a basic compound due to the availability of a lone pair of electrons on the nitrogen atom. This lone pair can be easily protonated in the presence of strong acids.
2. When aniline is placed in a strong acidic medium, the lone pair on the nitrogen accepts a proton (\( \mathrm{H^+} \)) to form an anilinium ion (\( \mathrm{C_6H_5NH_3^+} \)). The reaction can be represented as:

\( \mathrm{C_6H_5NH_2 + H^+ \rightarrow C_6H_5NH_3^+} \)

3. In the anilinium ion, the lone pair on the nitrogen is no longer available for resonance with the benzene ring. This changes the electronic structure of the ring and diminishes the activating and ortho/para-directing influence of the amino group.
4. The anilinium ion is less activating toward electrophilic substitution reactions than the unprotonated aniline. Moreover, without the activating effect of the lone pair, the ring becomes less reactive and directs incoming electrophiles to the meta position.
5. Consequently, when nitration is carried out (usually by using a nitrating mixture of \( \mathrm{HNO_3} \) and \( \mathrm{H_2SO_4} \)), the nitro group is introduced at the meta position, forming m-nitroaniline.

Therefore, the correct option is: In acidic (strong) medium aniline is present as anilinium ion.