Q.Why does acetylation of $-NH_2$ group of aniline reduce its activating effect?

The key idea is that the lone pair on nitrogen, which is responsible for activating the benzene ring, gets delocalised into the carbonyl group after acetylation.

1. In aniline, the $-\text{NH}_2$ group strongly activates the ring via resonance — the lone pair on nitrogen is donated into the benzene ring, increasing electron density at ortho and para positions.
2. Acetylation converts $-\text{NH}_2$ to $-\text{NHCOCH}_3$. The lone pair on nitrogen now participates in resonance with the carbonyl ($\text{C=O}$) group of the acetyl moiety. …

The key to understanding this lies in the resonance effect — specifically, how the lone pair on nitrogen interacts with the rest of the molecule.

Aniline’s $-NH_2$ group is a powerful activating and ortho/para-directing group because the nitrogen lone pair is directly conjugated with the benzene ring. This lone pair can delocalise into the ring, increasing electron density at the ortho and para positions. The more freely this lone pair is available, the stronger the activation.

When you acetylate aniline, you replace one hydrogen on the $-NH_2$ with an acetyl group ($-COCH_3$), forming acetanilide. The product now has an amide linkage: $-NHCOCH_3$.

1. The critical structural change: The acetyl group contains a carbonyl ($C=O$) bond. The carbon of this carbonyl is $sp^2$ hybridised and is directly attached to the nitrogen. This creates a new, competing resonance interaction.

2. The competition for the lone pair: The nitrogen lone pair can now delocalise in two directions:

   • Into the benzene ring (as in aniline), activating the ring.
   • Into the carbonyl group of the acetyl group, forming a resonance structure where the nitrogen has a partial positive charge and the carbonyl oxygen has a partial negative charge.

   This second resonance is very significant. The carbonyl group acts as an electron sink, pulling electron density away from the nitrogen.

3. The net effect on activation: Because the lone pair is now partially tied up in resonance with the carbonyl, it is less available to donate into the benzene ring. The electron-donating capacity of the $-NHCOCH_3$ group is therefore much weaker than that of the free $-NH_2$ group.

   Resonance hybrid of acetanilide:

   $\text{Ph} - \overset{\ominus}{\text{N}} - \overset{\oplus}{\text{C}} = \overset{\ominus}{\text{O}} \quad \longleftrightarrow \quad \text{Ph} - \text{N} = \text{C} - \overset{\ominus}{\text{O}}$

   The lone pair is shared between the ring and the carbonyl. …

Concept: Acidity of Phenol

Method: Resonance Stabilisation Analysis of Conjugate Base

Why this method?

The acidity of any compound is determined by the stability of its conjugate base after losing a proton ($H^+$). For phenol, we compare the phenoxide ion (conjugate base) with the alkoxide ion (from alcohols) to understand why phenol is more acidic.

Steps:

1. Write the dissociation reaction Phenol loses $H^+$ from its $-OH$ group to form the phenoxide ion:

2. Draw resonance structures of the phenoxide ion

   The negative charge on oxygen can be delocalised into the benzene ring. Draw the contributing structures:

   • One structure with negative charge on oxygen.
   • Three structures where the negative charge moves to ortho and para positions of the ring (via π-bond shifts).

   Key result: The negative charge is spread over multiple atoms, making the ion more stable.

3. Compare with alcohol (e.g., ethanol)

   In alkoxide ion ($CH_3CH_2O^-$), the negative charge is localised only on oxygen — no resonance delocalisation possible. This makes it less stable.

4. Conclusion

   Greater stability of phenoxide ion → phenol loses $H^+$ more easily → phenol is more acidic than alcohols.

Why does acetylation of $-NH_2$ group of aniline reduce its activating effect?

Method: Resonance & Electron-Withdrawing Effect of Amide Group

Steps:

1. Recall the activating effect of $-NH_2$

   In aniline, the lone pair on nitrogen is delocalised into the benzene ring (resonance), making the ring electron-rich and highly reactive toward electrophilic substitution.

2. What happens during acetylation?

   The $-NH_2$ group reacts with acetyl chloride/acetic anhydride to form an amide:

3. Analyse the new group ($-NHCOCH_3$)
   • The lone pair on nitrogen is now shared with the carbonyl group ($C=O$) via resonance: …

Common Mistakes & How to Avoid Them

1. Confusing “Activating Effect” with “Basicity”

• Mistake: Students think acetylation increases activation because the $-\text{NHCOCH}_3$ group still donates electrons via resonance.
• Why it’s wrong: Activating effect in electrophilic aromatic substitution (EAS) depends on electron density on the ring. Acetylation converts a strong activator ($-\text{NH}_2$) into a moderate activator ($-\text{NHCOCH}_3$). The resonance donation is weaker because the lone pair on N is partially delocalised into the carbonyl group ($\text{C=O}$), reducing its availability to the ring.
• How to avoid: Always compare the resonance structures:
  • In aniline: lone pair on N directly donates into the ring → strong activation.
  • In acetanilide: lone pair is shared with the $\text{C=O}$ group → less electron density reaches the ring.

2. Forgetting the Role of the Carbonyl Group

• Mistake: Treating $-\text{NHCOCH}_3$ as if it were just $-\text{NH}_2$ with an extra carbon.
• Why it’s wrong: The carbonyl group is electron-withdrawing by induction ($-I$ effect) and also participates in resonance with the N lone pair. This reduces the net electron-donating ability of the nitrogen.
• How to avoid: Draw the resonance hybrid of acetanilide. Notice that the lone pair on N is delocalised into the $\text{C=O}$ bond, forming a partial double bond between N and C. This decreases the lone pair’s availability for donation to the benzene ring.

3. Misinterpreting “Reduced Activating Effect” as “Deactivating”

• Mistake: Saying acetylation makes the group deactivating.
• Why it’s wrong: $-\text{NHCOCH}_3$ is still activating (though weaker than $-\text{NH}_2$). It is an ortho/para director but less powerful.
• How to avoid: Remember the order: $-\text{NH}_2$ (strong activator) > $-\text{OH}$ > $-\text{NHCOCH}_3$ (moderate activator) > $-\text{OCH}_3$ (moderate). Acetylation reduces but does not reverse the activating nature.

4. Ignoring the Inductive Effect of the Acetyl Group

• Mistake: Only considering resonance, forgetting the $-I$ effect of the carbonyl.
• Why it’s wrong: The $\text{C=O}$ group pulls electron density through sigma bonds, further decreasing the electron density on the nitrogen and hence on the ring.
• How to avoid: Always evaluate both resonance and inductive effects. For $-\text{NHCOCH}_3$:
  • Resonance: $+M$ (donation to ring) but weaker than $-\text{NH}_2$.
  • Inductive: $-I$ (withdrawal) from the carbonyl → net activation is reduced.

5. Not Relating to Exam-Specific Examples …