Conceptual Checkpoint 22.20 When aniline is treated with a mixture of nitric acid and sulfuric acid, the expected nitration product (para-nitroaniline) is obtained in poor yield. Instead, the major product from nitration is meta-nitroaniline. Apparently, the amino group is protonated under these acidic conditions, and the resulting ammonium group is a meta-director, rather than an ortho-para director. Propose a plausible method for converting aniline into para-nitroaniline.

See explanation below

Explanation:

The aniline is a very strong base, and also, a very strong activating molecule. IT's natural that the expected product would be the p-nitroaniline. However, as aniline is a strong base, when it reacts in acid medium with HNO3/H2SO4 instead of occuring the nitration of the molecule, it would occur an acid base reaction, and the NH2, becomes NH3+, which is a weak base, and a deactivating group of the molecule. Then the nitro goes into the meta position.

Now, how can we get the p-nitroaniline?, first we need to protect the aniline in the molecule. To do this, we first do an acylation of the amine group, using CH3COOCl. In this way, the aniline is protected from an acid base reaction, and also the ortho position of the ring is protected and a poor yield of orto nitroaniline forms. Now, when the acylation occurs, the mix of HNO3/H2SO4 can be used. Then, the nitro group will go to the para position due to steric hindrance of the CH3COONH group protecting the orto position. And finally, we make a reaction with acid or base to deprotect the molecule and turn the CH3COONH group into NH2 again. In that way we can get the p-nitroaniline.

See picture for method.