Q.Which of the following reactions belong to electrophilic aromatic substitution?
(Two or more options may be correct.)

The key idea is Electrophilic Aromatic Substitution (EAS): an electrophile replaces a hydrogen on the aromatic ring itself, not a hydrogen on a side group.

Step 1: Bromination of acetanilide -- Br+ (from Br2/catalyst) attacks the activated ring; a ring hydrogen is replaced. This is EAS.

Step 2: Coupling reaction of aryldiazonium salts -- the diazonium ion acts as a weak electrophile and attacks an activated ring (e.g. phenol), again replacing a ring hydrogen. This is EAS too.

Step 3: Diazotisation of aniline converts -NH2 to -N2+ at the nitrogen using nitrous acid; the ring itself is untouched. Not EAS. …

Electrophilic aromatic substitution is the workhorse of aromatic chemistry. The pi-electron cloud above and below a benzene ring makes it nucleophilic -- it attacks an electron-deficient species (the electrophile), replacing a ring hydrogen with the electrophile.

Let's examine each reaction with this lens.

1. Bromination of acetanilide

   Acetanilide has an -NHCOCH3 group attached to the benzene ring. This group is activating and ortho/para-directing because the nitrogen lone pair (even though partly delocalised onto the carbonyl) still donates some electron density into the ring by resonance.

   Bromine (Br2) is polarised by a catalyst to generate Br+ (the electrophile). The ring attacks Br+, and after loss of H+, we get p-bromoacetanilide (major) and o-bromoacetanilide (minor).

   This is a textbook EAS reaction. Option (i) is correct.

2. Coupling reaction of aryldiazonium salts

   An aryldiazonium salt (Ar-N2+) is a weak electrophile. In coupling, it attacks an activated aromatic ring (like phenol or aniline) at the para position. The diazonium ion is the electrophile; the OTHER aromatic ring is the nucleophile that loses a ring hydrogen.

   The substitution happens on the phenol/aniline ring being coupled to -- so this is genuinely EAS. Option (ii) is correct.

3. Diazotisation of aniline

   Aniline (C6H5NH2) reacts with nitrous acid (HNO2, generated from NaNO2 + HCl) at 0-5 degC to form a diazonium salt:

   C6H5NH2 + HNO2 + HCl -> C6H5N2+Cl- + 2H2O

   The reaction occurs at the amino nitrogen; the aromatic ring's hydrogen count is unchanged. This is not a ring substitution. Option (iii) is incorrect.

4. Acylation of aniline …

Method: Identify the Reaction Type by Mechanism

The key is to check whether the electrophile attacks the aromatic ring directly, replacing a hydrogen atom.

Step 1: Recall the definition of Electrophilic Aromatic Substitution (EAS)

• An electrophile ($E^+$) attacks the aromatic ring.
• A hydrogen atom is substituted (not added).
• The aromaticity is restored after the reaction.

Step 2: Analyse each option

(A) Bromination of acetanilide

• Acetanilide has an aromatic ring.
• The $-\text{NHCOCH}_3$ group activates the ring.
• $Br_2$ in presence of a Lewis acid (or even without, due to activation) generates $Br^+$ as the electrophile.
• $Br^+$ attacks the ring → EAS.
• ✓ Correct

(B) Coupling reaction of aryldiazonium salts

• The aryldiazonium ion ($ArN_2^+$) acts as the electrophile.
• It attacks an activated aromatic ring (like phenol or aniline).
• This is an electrophilic aromatic substitution (azo coupling).
• ✓ Correct

(C) Diazotisation of aniline

• Aniline reacts with $NaNO_2 + HCl$ at $0\text{–}5^\circ\text{C}$.
• The $NO^+$ (nitrosyl cation) attacks the amino group ($-\text{NH}_2$), not the ring.
• This forms a diazonium salt — it is not substitution on the ring.
• ✗ Not EAS

(D) Acylation of aniline …

Here are the common mistakes students make on this question, along with the precise reasoning to avoid them.

Mistake 1: Confusing "Coupling" with "Diazotisation"

• The Error: Students often see "diazonium" in option (B) and (C) and assume both are the same type of reaction. They might mark (C) as EAS because it involves an aromatic ring.
• Why it’s wrong:
  • Diazotisation (C) is the formation of the diazonium salt. It is an electrophilic substitution on nitrogen (of the $NH_2$ group), not on the aromatic ring. The ring is untouched.
  • Coupling (B) is the reaction of the diazonium salt (acting as an electrophile) with an activated aromatic ring (like phenol or aniline). This is a classic electrophilic aromatic substitution.
• How to avoid: Memorize the definitions:
  • Diazotisation: $ArNH_2 \xrightarrow{NaNO_2/HCl} ArN_2^+$ (Formation of electrophile, not substitution on ring).
  • Coupling: $ArN_2^+ + Ar'H \rightarrow Ar-N=N-Ar'$ (The diazonium ion is the electrophile attacking the ring).

Mistake 2: Forgetting that Aniline must be "Protected" for Acylation

• The Error: Students see "Acylation of aniline" (D) and think, "Aniline has an amino group, so it activates the ring for Friedel-Crafts acylation." They mark it as EAS.
• Why it’s wrong: Aniline ($C_6H_5NH_2$) is so strongly activating that it reacts violently with the Lewis acid catalyst ($AlCl_3$) used in acylation. The $NH_2$ group forms a complex with $AlCl_3$, making the nitrogen positively charged and deactivating the ring. The reaction fails or gives very poor yields. In practice, aniline is first acylated (to acetanilide) and then subjected to Friedel-Crafts acylation.
• How to avoid: Remember the "Aniline Problem": Direct Friedel-Crafts on aniline does not work. The correct EAS reaction on aniline is bromination (which happens so fast it gives the tribromo product unless controlled). Acylation of aniline is not a standard EAS.

Mistake 3: Misidentifying the Electrophile in Bromination

• The Error: Students know bromination is EAS, but they might think option (A) "Bromination of acetanilide" is not EAS because acetanilide is an amide, not a simple benzene.
• Why it’s wrong: Acetanilide has an acetamido group ($-NHCOCH_3$) attached to the ring. This group is a strong activating and ortho/para-directing group. The reaction proceeds via the same mechanism: $Br_2/FeBr_3$ generates $Br^+$, which attacks the ring. …