Nitration of aniline in strong acidic medium also gives m-nitroaniline because
- In spite of substituents nitro group always goes to only m-position.
- In absence of substituents nitro group always goes to m-position.
- In electrophilic substitution reactions amino group is meta directive.
- In acidic (strong) medium aniline is present as anilinium ion.
The Correct Option is D
Solution and Explanation
To understand why the nitration of aniline in a strong acidic medium results in m-nitroaniline, we need to consider the behavior of aniline in such a medium, especially during electrophilic substitution reactions.
Aniline (\( \mathrm{C_6H_5NH_2} \)) is a primary amine with an amino group (-NH2) attached to the benzene ring. It is a reactive compound and typically undergoes electrophilic substitution reactions. The NH2 group is an activating group and is ortho/para directing under normal conditions. However, the scenario changes in a strong acidic medium.
In a strong acidic medium, such as concentrated sulfuric and nitric acid used for nitrations, aniline gets protonated to form an anilinium ion (\( \mathrm{C_6H_5NH_3^+} \)). The structure of anilinium ion is different compared to unprotonated aniline.
The positive charge on the nitrogen atom reduces the electron density on the benzene ring significantly. As a result, the directing effect of the NH2 group changes from ortho/para to meta. Therefore, the nitro group, being an electron-withdrawing group, tends to go into the meta position because the ortho and para positions are less favorable due to electronic factors.
Hence, the correct answer to the question is that in acidic (strong) medium aniline is present as anilinium ion.
Other options can be ruled out because:
- The nitro group does not always go to the meta position without influence; it depends on the directing groups present.
- The amino group in unprotonated form is not meta-directing; it is ortho/para directing.
This detailed explanation shows why under these conditions, anilinium ion promotes meta substitution during nitration.
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