Chemistry · Amines · NEET
The -NH2 group is a very powerful activating and ortho/para directing group. It makes the ring so reactive that substitution happens at many positions at once, so you cannot cleanly get one product. There is also a second problem: the strong nitrating acid mixture oxidises aniline and protonates it. So direct nitration is messy and low-yielding.
Acetic anhydride converts -NH2 into -NHCOCH3 (the amide group of acetanilide). Now the nitrogen lone pair is shared with the carbonyl oxygen by resonance, so less of it goes into the ring. The group is still ortho/para directing but its activating power is reduced, so the ring reacts in a controlled way and gives mainly the para product.
The -NHCOCH3 group is bulky. The ortho positions are blocked by this size (steric hindrance), so the incoming nitro group prefers the free para position. That is why nitration of acetanilide gives mainly p-nitroacetanilide.
After the nitration is done, the amide is hydrolysed (acid or base, with water). Hydrolysis removes the acetyl cover and converts -NHCOCH3 back to -NH2. So p-nitroacetanilide gives p-nitroaniline. This is the 'cover, react, uncover' sequence.
Yes. In strong acidic medium, un-protected aniline becomes the anilinium ion (-NH3+), which is deactivating and meta directing, so some m-nitroaniline forms. The acetylation route is a deliberate protection strategy done to obtain the para product cleanly, not the accidental meta product from protonation.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Acetic anhydride ((CH3CO)2O), usually in the presence of a base like pyridine to neutralise the acid formed. This converts aniline into acetanilide (C6H5NHCOCH3).
p-Nitroaniline. The steps are: aniline to acetanilide (acetylation), then nitration to p-nitroacetanilide, then hydrolysis to p-nitroaniline.
In acetanilide the nitrogen lone pair is pulled towards the carbonyl oxygen by resonance, so it is less available to donate into the ring. Less electron donation means a less activated, less reactive ring.
No. It is only a temporary protecting group. After the desired substitution, hydrolysis removes it and the -NH2 group is restored.
Yes. NCERT Class 12 Amines states that the high reactivity of aromatic amines is controlled by protecting the -NH2 group by acetylation with acetic anhydride, doing the substitution, then hydrolysing back to the amine.
