Q.Why is $-NH_2$ group of aniline acetylated before carrying out nitration?

The key idea is that the amino group ($-NH_2$) is a strongly activating and ortho/para-directing group, but it is also highly basic and gets protonated in the acidic nitrating mixture. Protonation converts $-NH_2$ into the deactivating, meta-directing $-NH_3^+$ group, drastically reducing the yield and giving unwanted meta-nitroaniline. …

The key to understanding this lies in Electrophilic Aromatic Substitution (EAS). Aniline has a free amino group ($-NH_2$) that is a very strong activating and ortho/para-directing group. This sounds good for nitration, but it creates two serious problems.

Problem 1: Over-activation and poly-substitution. The $-NH_2$ group donates electrons into the ring so powerfully that the ring becomes too reactive. Under typical nitrating conditions (conc. $HNO_3$ / conc. $H_2SO_4$), aniline would undergo multiple nitrations, giving a messy mixture of di- and tri-nitro products. You cannot stop at mono-nitration.

Problem 2: Oxidation by the nitrating agent. The amino group is also a strong reducing agent. Concentrated nitric acid ($HNO_3$) is a powerful oxidising agent. It will oxidise the $-NH_2$ group before it can nitrate the ring, destroying the starting material and forming tarry, black oxidation products. This is the more immediate disaster.

Acetylation solves both problems in one elegant step. Let's see how.

1. Protection from oxidation. The acetyl group ($-COCH_3$) is attached to the nitrogen, forming acetanilide ($C_6H_5NHCOCH_3$). The lone pair on nitrogen is now delocalised into the carbonyl group ($C=O$) of the acetyl group. This makes the nitrogen much less basic and much less nucleophilic — it is no longer easily oxidised by $HNO_3$. The ring is now safe to nitrate.

2. Taming the activating power. The $-NHCOCH_3$ group is still activating and ortho/para-directing, but it is much weaker than $-NH_2$. The delocalisation of the nitrogen's lone pair into the carbonyl group reduces the electron density it can push into the benzene ring. The ring is still reactive enough for mono-nitration, but not so reactive that it undergoes uncontrolled poly-substitution.

3. Steric control for para-selectivity. The $-NHCOCH_3$ group is a bulky group. The ortho positions are sterically hindered — the incoming nitronium ion ($NO_2^+$) finds it harder to attack there. The para position is wide open. As a result, the major product is p-nitroacetanilide, with only a small amount of the ortho isomer. This gives you excellent regioselectivity. …

Method: Protection-Deprotection Strategy in Electrophilic Aromatic Substitution

Why This Method is Needed

The $-\text{NH}_2$ group in aniline is a strongly activating and ortho/para-directing group. If we nitrate aniline directly, two problems arise:

1. Overactivation — The ring becomes so reactive that multiple substitutions occur, giving a messy mixture.
2. Oxidation — The $-\text{NH}_2$ group gets oxidised by the nitrating mixture ($\text{HNO}_3/\text{H}_2\text{SO}_4$), destroying the compound.

The Solution: Acetylation

We convert the $-\text{NH}_2$ group into an acetamido group ($-\text{NHCOCH}_3$) by treating aniline with acetic anhydride or acetyl chloride.

Step-by-Step Reasoning

1. Acetylation reaction

   Aniline + $(\text{CH}_3\text{CO})_2\text{O}$ → Acetanilide

   The $-\text{NHCOCH}_3$ group is still activating and ortho/para-directing, but less strongly than $-\text{NH}_2$.

2. Why this helps

   • The acetamido group reduces electron density on the ring compared to $-\text{NH}_2$ (due to resonance with the carbonyl group).
   • This slows down the reaction, allowing mononitration to occur cleanly.
   • The group protects the amino group from oxidation by the nitrating mixture.

3. Nitration step …

Here are the common mistakes students make on this exact question, along with how to avoid each — explained with the why first, then the correct answer.

✗ Mistake 1: Saying “Acetylation reduces the reactivity of aniline”

Why it’s wrong:

This is partially true but misses the real reason. Aniline is too reactive — it undergoes unwanted side reactions (like oxidation and polymerisation) under nitrating conditions. Acetylation doesn’t just “reduce reactivity”; it protects the amino group from being oxidised and controls the orientation of the incoming nitro group.

How to avoid:

Always mention both protection and regiocontrol. The correct answer is:

Acetylation converts the strong activating $-NH_2$ group into a moderate activating $-NHCOCH_3$ group, which:

• Prevents oxidation of the amino group by the nitrating mixture.
• Directs the nitro group to the para position (with some ortho), avoiding unwanted meta substitution.

✗ Mistake 2: Saying “Acetylation deactivates the ring”

Why it’s wrong:

The $-NHCOCH_3$ group is still activating (due to resonance donation from the nitrogen lone pair), just less strongly than $-NH_2$. It is not deactivating. Deactivating groups are electron-withdrawing (like $-NO_2$, $-CN$, $-COOH$).

How to avoid:

Remember the activating/deactivating hierarchy:

So acetylation moderates activation, not deactivates.

✗ Mistake 3: Forgetting to mention ortho/para directing change

Why it’s wrong:

Both $-NH_2$ and $-NHCOCH_3$ are ortho/para directors, but the extent differs. The free $-NH_2$ group gives significant ortho substitution (due to steric hindrance, ortho product is less, but still possible). The acetyl group bulkier and reduces ortho attack, favouring para product — which is easier to separate and more useful industrially.

How to avoid:

State clearly:

Acetylation increases the para selectivity because the $-NHCOCH_3$ group is bulkier than $-NH_2$, sterically hindering ortho positions.

✗ Mistake 4: Not mentioning reversibility of acetylation

Why it’s wrong: …