Question

Why is acetylation performed before nitration of aniline?

Answer 1

Hint: We have to remember that the nitration of aromatic compounds is usually done employing a nitration mixture which consists of conc.\[HN{O_3}\] and conc. Sulphuric acid at low temperature. \[ - N{H_2}\] group present in aniline may be a strong activating group and is ortho and para directing.

Complete step by step answer:
The activation effect is often defined because the increase in the reactivity of a compound by the increase within the availability of the number of electrons within the compound which are readily transferred within the solution and there occurs a progress within the reaction.
The electron withdrawing group decreases the activation effect of the compounds whereas the electron donating groups increase the activation effect by increasing the availability of electrons within the compound.
We can write the chemical equation for the nitration of amines:

\[C{H_3} - C{H_2} - N{H_2} + HO - N{O_2}\xrightarrow{{423K - 698K}}C{H_3} - C{H_2} - N{O_2}\]

Acetylation is performed before nitration of aniline. Direct nitration isn’t administered because NH2 may be a highly activating group and it’ll result in the reaction between NH2 and HNO3.
In the above reaction, aniline may be a moderate base as it’s an electron rich compound and acts as a good nucleophile. The electron cloud of aniline remains stable unless it’s being reacted with some acetyl halide to supply an acetic anhydride. The acetyl group that later gets attached with aniline deactivates the aniline due to its electron withdrawing tendency. Thus, we will say that acetylation of aniline reduces its activation effect.

Note: We have to remember that the acetylation is that the reaction which involves the replacement of the hydrogen atom of a hydroxyl with an acetyl group yields a selected ester which, during this case is referred as acetate. Acetic anhydride is usually used as an acetylating agent reacting with free hydroxyl groups. For example, it’s utilized in the synthesis of aspirin, heroin, etc