Question

Why is \(-NH_2\) group of aniline acetylated before carrying out nitration?

Hint:

Acetylation is a standard "protection" strategy in organic chemistry whenever a highly reactive functional group interferes with a specific reaction.

Answer 1

The \(-NH_2\) (amino) group in aniline is a strongly activating group for electrophilic aromatic substitution reactions, such as nitration. The amino group donates electron density to the benzene ring through its lone pair of electrons, making the ring highly reactive towards electrophiles like the nitronium ion (\(NO_2^+\)) in the nitration reaction. This electron donation increases the electron density on the ortho and para positions of the benzene ring, making these positions more reactive and more likely to undergo substitution by electrophiles.
However, the presence of the \(-NH_2\) group makes aniline excessively reactive, which can lead to unwanted side reactions during nitration. The nitration of aniline typically results in a mixture of products, with excessive substitution occurring at the ortho and para positions. This is because the electron-donating nature of the amino group causes the nitration to occur at these positions, leading to a poor yield and a less selective product.
To control the reactivity of the amino group and improve the selectivity of the nitration, the \(-NH_2\) group in aniline is first acetylated. This is done by reacting aniline with an acylating agent like acetic anhydride or acetyl chloride, which introduces an acetyl group (\(-COCH_3\)) onto the nitrogen atom, forming aniline acetate (\(C_6H_5NHCOCH_3\)). The acetyl group is an electron-withdrawing group, meaning it reduces the electron density on the benzene ring by pulling electron density away from the ring through the inductive effect. This deactivates the ring towards electrophilic substitution, especially at the ortho and para positions.
As a result, the acetylation of the amino group decreases the overall reactivity of the aniline towards electrophilic substitution and directs the nitration to occur predominantly at the meta position. This selective meta substitution provides a more controlled and desired product, improving both the yield and purity of the nitration reaction.
Therefore, acetylating the \(-NH_2\) group of aniline before nitration serves to moderate the reactivity of the amino group, reduce unwanted side reactions, and provide better control over the position of nitration, ultimately leading to a more selective and efficient reaction.